0306-301617910701
l Radiation
Sensitivity:
10491%02.0010
Facts and Models
CHEMICAL PROCESSES INDUCED RADIOLYTICALLY WELL DEFINED AQUEOUS SYSTEMSt
IN
J. KERRY THOMAS, Ph.D. Chemistry
Department
and
Radiation
Laboratory,
University U.S.A.
of Notre
Dame,
Notre
Dame,
IN 46556,
The radiation chemistry of dilute aqueous systems is discussed in terms of the simple primary radicals and ions produced, and also in terms of reactions of secondary radicals produced via attack of the primary species on organic solutes. These simple systems are extended to 3 more complex systems: (a) solutions of polymers, (b) micelles and vesicles, and (c) inverted micelles containing water bubbles. These latter systems all contain new and interesting features not exhibited by dilute solutions of simple molecules, and are of particular importance with respect to bio-systems. Aqueous systems,
Primary radicals, Secondary radicals.
INTRODUCTION The past 20 years have seen significant advances in understanding the radiation chemistry of water and several reviews or texts are now available on the conceded that the primary subject.23V2’ It is generally consequence of energy loss by ionizing radiation is the production of ions, namely electrons and H,O+. Neither of these species has actually been observed as they rapidly form hydrated electrons, e& hydrated protons and OH radicals:
Reactions of OH radicals. The rates of reactions of OH radicals with several solutes dissolved in water have been measured” by pulse radiolytic and comparative techniques. It is convenient to divide the reactions into 3 classes: electron transfer, H atom abstraction, and OH addition. The rates of the 3 classes of reactions can vary from close to the diffusion controlled limit of 10” L mol-‘s-’ to about lo’- lo6 L mol-Is-‘. Little is known of the actual mechanisms for most reactions apart from the correlation with Hammett constants which provide an indication of the electrophilicity of these reactions4 by comparison with standard thermal reactions. Electron transfer tends to dominate in the reactions of inorganic materials. In general, these reactions can be written as:
nHzO * H,O+ + e- + H30+ + OH + eag As a result of heterogenous energy loss in the medium, locally high concentrations of ions and radicals are formed. In these “spur” regions radicalradical reactions lead to additional products namely H atoms, HzOz and HZ. The latter 2 products are not of immediate interest and this paper will focus attention on the reactive species, i.e. H, OH and e&, which for the present discussion are formed with the following G yalues (No. of entities/100 ev energy absorbed): G(eJ = 2.8, G(OH) = 2.8, G(H) = 0.6. It is customary to suggest that the observed reactions of these species with solutes dissolved in water may be related to the chemical processes induced by the irradiation of living cells.3’ This paper will also follow this path for some distance, but then some new concepts will be introduced.
tThe research Office of Basic
described herein Energy Sciences
OH + MC”-‘)+ + M”+ + OHA classic example is the oxidation of FeZi to Fe3’ in dosimeter (K = 3.0 x 10sL mol-‘s-l). Fricke the Some intriguing examples of electron transfer have also come to light in organic systems. For example, the reaction of OH radicals with poly-arenes in water normally gives the OH radical adduct; but in acidic solution electron transfer can occur and an intermediate poly-arene cation is observed, which subsequently hydrolyses to give the OH adduct.”
was supported by the of the Department of
Energy. This is Document Dame 1049
Radiation
Laboratory.
No. NDRL-1916
from the Notre
l Biology 0 Physics
Radiation Oncology
IO50
OH + RH -
RH(OH). H’
\ RH++
OH-
It is quite difficult to observe such transitory intermediates in OH radical reactions, but they may be more prevalent than was originally thought. The absolute rate constants for the reactions of OH radicals with benzene” and benzene derivatives were among the first to be measured. The product in the case of benzene was identified as the cyclohexanedienyl radical:
OH
Reactions of the hydrated electron, e,, have gained much prominence during the past 15 years and hundreds of rate constants for reaction are available.2.‘4a20 Again, the fine details of these reactions are relatively unknown. As expected, electrons readily reduce such solutes as Cu++ to Cu’, etc. In some cases discerniible negative ions are formed, e.g. the ketyl ion by reaction with benzophenone:
ph
e, +
ph ‘GO+
/ ph
‘c-o / ph’
The intermediate ion may have an extremely short lifetime and may never be observed; e.g. with organic halides the halide ion and organic radical are the only observed reaction products:
+
It is thought that OH adds readily to unsaturated bonds, e.g. ethylenic and acetylenic bonds and arene compounds; but the possibility of intermediate electron transfer always has to be considered.22 The hydrogen atom abstraction by OH radicals from organic compounds has received some detailed attention: OH + RH + R. + Hz0 Aliphatic alcohols are quite reactive, with k > lo9 L mol-‘s-l; the reactivity increases with increasing chain length.4 On the other hand, both carboxylic acids such as acetic or formic acid, or sulphate esters such as methyl and ethyl hydrogen sulphate, are quite unreactive with k - lo8 L mol-‘s-’ or less. This results from the electron withdrawing power of the acid groups, which reduces the electron density on the (Y and p carbon atoms of the acids and thus the reactivity toward the electrophilic OH radical. Increasing the alkane chain increases the reactivity and Cl2 arenes are quite reactive. The site of H atom abstraction in a large molecule is not known with certainty, but in a long C12-chain the probability of OH attack is similar at all positions, except those adjacent to groups such as -COOH or -S04H. Retrctions
July 1979, Volume 5, No. 7
of H utoms
und hydrutrd
electrons
Hydrogen atoms abstract other H atoms from organic molecules and generate H2 gas. The reactivity is some 100 fold smaller than corresponding reactions of OH radicals.’ H atom addition to unsaturated compounds is rapid, however, and often approaches the diffusion controlled limit. Electron transfer of H atoms is not as prevealent, but reduction of inorganic ions probably proceeds via electron transfer; e.g.: H + Fe7’ + Fe” + H’.
e,+RX-+R’+X-
(X = Cl; Br; I)
The data obtained by pulse radiolysis may be extrapolated, with care, to more complex systems. At high solute concentrations the electron may react prior to solvation and the observed yield of e& is reduced.‘3,‘6 This is true for acetone where a 0.31.0 M concentration greatly reduces G(e;J, while the anion of acetone is formed. Of particular interest is the case of benzene, which is relatively unreactive with solvated electrons, but readily reacts with presolvated electrons at a concentration near 1 M. Such reactions are of interest in biological systems where high concentrations of solute molecules exist. Reactions
of secondary
radicals
The reactions of H, e, and OH are rapid and tend to be non-selective. Many attempts have been made to produce secondary radicals which have more selectivity, or to react the orgainic radicals formed in the events described above with reactive solutes. Some of these systems are listed below and have been reviewed recently.” The sulphate radical SO,- is produced by the reaction of e;iq with persu1phate:‘R*‘9
eaq+ S20Rz~+
So:- + so4
This radical tends to react via electron transfer and forms the cation of several solutes. H atom abstraction and addition are not important reactions with this species and hence unique electron transfer reactions can be initiated by S04-. A similar reaction of e;, with peroxodiphosphate ion leads to the 3 acid-base forms of the phosphate radical,‘” H2Pb4, HP04~, PO:--. These radicals abstract H atoms from aliphatic compounds only slowly (k - 10’ for acetic
Chemical
processes
induced
radiolytically
acid); addition to a double bond is also slow. The reactivities of the phosphate radicals and S04- are quite similar. It is suggested that this may give rise to a greater selectivity than that displayed by the more reactive OH radical. Other radicals of the type X2-, such as Br2-, formed by the reactions
OH + Bra-Br
+ OH’?Br I COOH
Hz0
R. + Fe(CN)ior the formation
/ + R’ + Fe(CN)z-
of the quinone
anion: 0-
0 -R++
R’+
H2O
0
\
0 ROH
+ H+
The nature of radical R., controls the rate of the reaction. For example .CH20H is very reactive with Fe(CN)z-, k being > lo9 L mol-‘so’; however, .CH2CHzOH is quite unreactive. Such reactions are useful in determining the nature of the radicals formed in the radiolysis of an aqueous system. This short discussion has left out details of the reactions shown, and also several reactions of interest to physical chemists. It strives, however, to present a strong case that the aqueous radiation chemist has a firm grip on the fundamental physical properties of irradiated systems, so that he can, within limits, control the outcome of the reaction.
CH> OHS II + R,-C + CH2-C-R2 I I COOH COOH Both processes are impared by 02, which reacts with the intermediate radical. Thio alcohols and RHS repair the intermediate radical, RI, by the reaction R; + RSH + R,H + RS.; The RS radical is inert. Similarly ferricyanide reacts with the radicals and prevents crosslinking or degradation by oxidizing the intermediate radical to an alcohol. The latter process has been used to measure the rate of decay of the intermediate polymethacrylic acid radical to give the degradation products; the rate of this process at 25°C is 10-4s-‘. An interesting and important kinetic concept appears in such experiments. For example, the rate of reaction of OH radicals with constituents of the polymer is readily estimated from available data; the next stage is to relate these data to the attack of OH on the polymer containing such monomer units. The
1052
rate constants
Radiation
for the polymer
of the rate constants
Oncology
0 Biology
is not merely
for (OH + monomer),
0 Physics
July 1979, Volume 5, No. 7
the sum
responding increase in the rate of formation of Pm, but the rate tends to a maximum. This is interpreted as being because the rate controlling feature of the kinetics is the approach of e, to the vesicle; once at the vesicle the reaction with pyrene is rapid. This type of behavior was indicated earlier for polymers’5 and is a feature of the clustering of the system. It also occurs with OH radical reactions, as will be discussed below. The hydrated electron does not readily react with lecithin vesicles. However, some reactivity is noted with the OH radical,‘**’ and it is suggested that the point of attack is towards the surface of the vesicle in the choline group rather than at the fatty acid region below the glycerol unit. With pyrene located in the bilayer, OH attack produces the spectrum of the OH-pyrene adduct. In homegeneous solution the rate constant for this reaction is 1.34 x 10” L molK’s_‘, the rate varies linearly with pyrene concentration and it is diffusion controlled. Figure 1 shows how the rate of appearance of the OH-pyrene adduct and the yield vary with increasing pyrene concentration in the bilayers; the rate of reaction increases slightly and the yield of product approaches a maximum. This is because the rate limiting step is that at which the OH radical reaches the vesicle. The rate constant for a diffusion controlled process can be written as k = 4nrD x 6 x 10zo L mol-Is-‘, where r is the sum of the reactant radii (rOH + rpyrene= 6 A), and D is the sum of the diffusion constants (in the present system, D = 4 x 10e5 cm*s-‘); k is then calculated as 1.8 x 10” L mol-‘s-l. This is typical of
ko, but it 0
depends critically on the size of the polymer coil and may be calculated from diffusion theory.’ This will be dealt with in the next section, that concerns vesicles and micelles. Radiolysis in micelles Surfactant molecules, such as sodium lauryl sulphate (NaLS) and cetyltrimethylammonium bromide (CTAB), have lipid, hydrophobic, and hydrophilic parts. In water above certain concentrations these molecules cluster to form micelles. The micelles are spherical, with radii of 15-20 A, and have a lipid core with the hydrophilic end groups towards the water phase. The system may be looked upon as a host of small oil droplets solubilized in water. Water insoluble materials such as pyrene are solubilized by micelles. Radiolysis of these systems leads to e,+ OH; the OH radicals react with the surfactant and the eiq with the solubilized molecule. With NaLS micelles the surface charge is negative and e& is repelled, thereby decreasing the rate of (e,+ pyrene) from k = 10” L molss’ in homogeneous solutions to k micelles. With CTAB in anionic IO6L mol-‘ss’ micelles the positive charge attracts e, and the reaction rate of (e, + pyrene) is catalyzed to k > IO” L mol~‘s~‘.24 Similar effects are observed with bovine serum albumin where the rate of reaction of eLq with the protein may be controlled by addition of a suitable surfactant.“’ These systems provide the first step in advancing from the radiolysis of dilute aqueous solutions to more complicated bio-systems. Increased selectivity of reaction is noted immediately. Radiolysis in vesicles Even more complicated structures, such as vesicles have been investigated recently.5.9*2’ These systems are constructed by the sonication of lecithin in water and consist of closed bilayers of radius 300-500 A. The bilayer encloses and traps a volume of water in the core. Hydrophobic molecules such as pyrene may be solubilized in the bilayer and used as a monitor for chemical reactions of eag’,” and OH’ radicals produced in the surrounding bulk water. Hydrated electrons react with pyrene in the bilayer producing the pyrene anion, Pm, which decays slowly via second order kinetics. It is suggested*’ that since the anion is more hydrophilic than pyrene, it exists into the water and is annihilated there. The rate of reaction of (e, + pyrene) increases above the temperature of the phase change in the vesicle, when the bilayer becomes more fluid. However, increasing pyrene concentration does not produce a cor-
I
/
I
I
[DSL]
6
1
I
,
= 2 mM
I
OO
I1
21 [PYRENE]
3I
41
5I
, M (do4)
Fig. 1. Effect of pyrene concentration on the rate of growth and yield of the OH- pyrene adduct in vesicles of distearyl and dipalmitoyl lecithin.
Chemical
processes
induced
radiolytically
many OH radical reactions and close to that measured for (OH + pyrene). The radius of a vesicle is 250 A, and the diffusion of the vesicle is slow, so D = 2 x 10~scm2s-‘. The calculated rate constant is now 3.9 X 10” L mol-‘s-’ and is a measure of the rate at which OH encounters the vesicle. There are some 1000 lecithin molecules in the vesicle, so k (OH + lecithin) is 3.9 x 10’ and close to that measured. Addition of pyrene introduces competition between the vesicle and the pyrene for OH. The yield of the OH - pyrene adduct increases with increasing pyrene concentration. However, the rate of reaction remains relatively unchanged as the rate dependent step is that at which OH encounters the vesicle. The rate tends to a limit of 5 x 10’s_’ and a vesicle concentration of 2 x 10m6M. The rate constant for (OH + vesicle) is then 2.5 x 10” L mol-Is-‘, in agreement with the above discussion. These aggregated systems, which are reminiscent of true bio-systems, introduce new features into the conventional kinetic patterns. However, these are now understood and may be utilized to explain more complicated systems. Electron capture by water bubbles It is customary, when discussing the application of data from the radiolysis of dilute solutions to biosystems, to start with the reactive species in water. However, in actual bio-systems the fraction of water in certain regions may be quite low, and‘model systems should be designed which mimic this more complex situation. One such model system is to create inverted micelles utilizing the surfactant sodium di-isooctyl sulphosuccinate, and to prepare water bubbles of different sizes in these micelles. The bulk solvent is now heptane containing 3% surfactant and up to 6% water. These solutions are optically clear and quite convenient for photochemical or radiolytic studies.*’ The nature of the water in the clusters has been examined by spectroscopic*’ and NMR techniques.” The small water bubbles do not exhibit the properties of bulk water and at least 1% water is required in order for the bubbles to exhibit physical properties reminiscent of bulk water. At this stage, the water bubbles can capture electrons liberated radiolytically in the bulk hydrocarbon phase; this event is detected by observing the hydrated electron in the water bubbIe.28 Figure 2 shows how the yield of electrons captured by the bubble varies with the fraction of water in the system.29 At 6% water G(e&) is only 16%
in well defined
0 J. H. THOMAS
aqueous
systems
061
I
I
I
2
1053
1
I
I
,
3
4
5
6
05 t -
0.4 i CT
z ”
03020.1 -
0
%H,O
7
-
Fig. 2. Yield of hydrated electrons in reversed micellar systems of 3% sodium diisooctyl sulphosuccinate in heptane with various percentages of dissolved water, saturated with Nz or SF+
that of pure water, as the back neutralization of the electron and solvent cation dominates over electron capture by the bubble. The rate of capture of the electrons is high and the bubbles show an attractive force on the electron presumably because of induced dipole in the water bubble.29 Figure 2 also shows the effect of SF6 on the electron yield; it decreases as the SF, captures electrons in the alkane bulk prior to capture by the water. The remaining yield results from electrons produced directly in the bubble by direct energy loss. This follows the electron fraction of the water bubbles in the system. In certain microemulsion systems containing 10% water G(eJ equals that of bulk water (unpublished data). These models emphasize the importance of a smaller percentage of water in attracting and captuiing electrons from the bulk phase. The resulting chemistry is then that of the aqueous phase involving hydrated electrons. CONCLUSION Understanding of the physical processes that take place in dilute aqueous solutions has developed considerably over the past 20 years, especially after the development of pulse radiolysis. More complex systems are now under study, and the initial results are promising in the sense that they are readily understood in terms of the earlier simpler systems, while introducing new features of interest and selectivity. These systems bring us one step closer to the radiolysis of genuine bio-systems.
REFERENCES 1. Adams, G.E., Redpath, J.L., Bisby, R.H., Cundall, R.B.: Use of free radical probes in the study of
mechanisms of enzyme 1073-93, 1972.
inactivation.
Zsr. J. Chem.
10:
IO54
Radiation
Oncology
0 Biology 0 Physics
2. Anbar, M., Bambenek, M., Ross, A.B.: Selected specific rates of reactions of transients from water in aqueous solution-l. Hydrated electron. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.) 43: 61, 1973. 3. Anbar, M., Farhatazis, Ross, A.B.: Selected specific rates of reactions of transients from water in aqueous solution-H. Hydrogen atoms. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.) 51: 56, 1975. 4. Anbar, M., Meyerstein, D., Neta, P.: The reactivity of aromatic compounds toward hydroxyl radicals. J. Phys. Chem. 70: 2660-2662, 1966. 5. Barber, D.J.W., Thomas, J.K.: Reactions of radicals with lecithin bilayers. Radiat. Res. 74: 51-65, 1978. J.H., Thomas, J.K.: The degradation of 6. Baxendale, polymethacrylic acid by U.V. and x-irradiation. Truns. Faraday Sot. 54: 1515-1525, 1958. 7. Behzadi, A., Borgwardt, U., Henglein, A., Schamberg, E., Schnable, W.: Pulse radiolytic study of the kinetics of diffusion controlled reactions of OH-radicals with polymers and oligomers in aqueous solution. Ber. Bunsenges. Phys. Chem. 74: 453-469, 1970. 8. Bisby, R.H., Cundall, R.B., Wardman, P.: Pulse radiolysis study of some free radical reactions with Biochim. Biophys. Acta 389: erythrocyte membranes. 137-144, 1975. 9. Cheng, S., Thomas, J.K., Kulpa, C.F.: Dynamics of pyrene fluorescence in Escherichia cofi membrane vesicles. Biochemistry 13: 1135-l 139, 1974. 10. Cooper, M., Thomas, J.K.: A pulsed laser study of excited states of aromatic molecules absorbed in globular proteins. Radiat. Res. 70: 312-324, 1977. II. Dorfman, L.M., Adams, G.E.: Reactivity of the hydroxyl radical in aqueous solutions. Nat. Stand. Ref. Datu Ser., Nat. Bur. Stand. (U.S.) 46: 72 pp. 1973. 12. Dorfman, L.M., Taub, I.A., Buhler, R.E.: Pulse radiolysis studies--I. Transient spectra and reactionrate constants in irradiated aqueous solutions of benzene. J. Chem. Phys. 36: 3051-3061, 1%2. 13. Hamill, W.H.: A model for the radiolysis of water. J. Phys. Chem. 73: 1341-1347, 1%9. 14. Hart, E.J., Anbar, M.: The Hydrated Electron, New York, Wiley, 1970. 15. Henglein, A., Schnable, W.: Ein Fiihrung in die Strahlenchemie, Weinhelm/Bergstr., Verlag Chemie, 1969. 16. Maruthamuthu, P., Neta, P.: Reaction of phosphate radicals with organic compounds. J. Phys. Chem. 81: 1622-1625, 1977. 17. Neta, P.: Application of radiation techniques to the study of organic radicals. Ado. Phys. Org. Chem. 12: 233-297, 1976. 18. Neta, $P., Madhavan, V., Zemel, H., Fessenden, R.W.: Rate constant and mechanism of reaction of sulfate
July 1979, Volume 5, No. 7
19.
20.
21.
22.
23. 24.
25.
26.
27.
28.
29.
30.
31.
radical anion with aromatic compounds. J. Am. Chem. Sot. 99: 163-164, 1977. O’Neill, P., Steenken, S., Schulte-Frohlinde, D.: Formation of radical cations of methoxylated benzenes by reaction with OH radicals, Tl”, Ag”, and SO? . in aqueous solution. An optical and conductometric pulse radiolysis and in situ electron spin resonance study. J. Phys. Chem. 79: 2773-2779, 1975. Ross, A.B.: Selected specific rates of reactions of transients from water in aqueous solution. Hydrated electron, supplementary data. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), Suppl. 43, 1975, 43 pp. Schnecke, W., Gratzel, M., Henglein, A.: Reactions of the hydrated electron with pyrene in lipid bilayer vesicles. Ber. Bunsenges. Phys. Chem. 81: 821-826, 1977. Sehested, K., Corfitzen, H., Christenson, H.C., Hart, E.J.: Rates of reaction of oxygen(1 -) ions, hydroxyl radicals, and atomic hydrogen with methylated benzenes in aqueous solution. Optical spectra of radicals. J. Phys. Chem. 79: 310-315, 1975. Swallow, A.J.: Introduction to Radiation Chemistry, London, Oxford Univ. Press, 1973. Thomas, J.K.: Effect of structure and charge on radiation-induced reactions in micellar systems. Act. Chem. Res. 10: 133-138, 1977. Thomas, J.K.: Elementary processes and reactions in in Radiation the radiolysis of water. In Advances Chemistry, Vol. 1, Ed. by Magee, J.L., Burton, M., New York, Wiley, 1970, pp. 103-198. Wolff, R.K., Bronskill, M.J., Aldrich, J.E., Hunt, J.W.: Picosecond pulse radiolysis--IV. Yield of the solvated electron at 30 picoseconds. J. Chem. Phys. 77: 13501355, 1973. Wong, M., GrCtzel, M., Thomas, J.K.: Fluorescence probing of inverted micelles. The state of solubilized water clusters in alkaneldiisooctyl sulfosuccinate (Aerosol OT) solution. J. Am. Chem. Sot. 98: 23912397, 1976. Wong, M., Gratzel, M., Thomas, J.K.: On the nature of solubilized water clusters in Aerosol AT/alkane solutions. A study of the formation of hydrated electrons and 1,8-anilinonaphthalene sulfonate fluorescence. Chem. Phys. Lett. 30: 329-337, 1975. Wong, M., Grieser, F., Thomas, J.K.: Berichte Der Bunsen Geseuschaff Fur Physikalische Chemie, Schloss Emlau Meeting, W. Germany 82: 950, 1978. Wong, M., Thomas, J.K., Nowak, T.: Structure and state of Hz0 in reversed micelles-3. J. Am. Chem. sot. 99: 4370-4375, 1977. Wardman, P.: The use of nitro aromatic compounds as hypoxic cell radiosensitizers. Curr. Topics in Radiat. Res. Quarterly. 11: 347-398, 1977.
l Radiation
Sensitivity:
10491%02.0010
Facts and Models
CHEMICAL PROCESSES INDUCED RADIOLYTICALLY WELL DEFINED AQUEOUS SYSTEMSt
IN
J. KERRY THOMAS, Ph.D. Chemistry
Department
and
Radiation
Laboratory,
University U.S.A.
of Notre
Dame,
Notre
Dame,
IN 46556,
The radiation chemistry of dilute aqueous systems is discussed in terms of the simple primary radicals and ions produced, and also in terms of reactions of secondary radicals produced via attack of the primary species on organic solutes. These simple systems are extended to 3 more complex systems: (a) solutions of polymers, (b) micelles and vesicles, and (c) inverted micelles containing water bubbles. These latter systems all contain new and interesting features not exhibited by dilute solutions of simple molecules, and are of particular importance with respect to bio-systems. Aqueous systems,
Primary radicals, Secondary radicals.
INTRODUCTION The past 20 years have seen significant advances in understanding the radiation chemistry of water and several reviews or texts are now available on the conceded that the primary subject.23V2’ It is generally consequence of energy loss by ionizing radiation is the production of ions, namely electrons and H,O+. Neither of these species has actually been observed as they rapidly form hydrated electrons, e& hydrated protons and OH radicals:
Reactions of OH radicals. The rates of reactions of OH radicals with several solutes dissolved in water have been measured” by pulse radiolytic and comparative techniques. It is convenient to divide the reactions into 3 classes: electron transfer, H atom abstraction, and OH addition. The rates of the 3 classes of reactions can vary from close to the diffusion controlled limit of 10” L mol-‘s-’ to about lo’- lo6 L mol-Is-‘. Little is known of the actual mechanisms for most reactions apart from the correlation with Hammett constants which provide an indication of the electrophilicity of these reactions4 by comparison with standard thermal reactions. Electron transfer tends to dominate in the reactions of inorganic materials. In general, these reactions can be written as:
nHzO * H,O+ + e- + H30+ + OH + eag As a result of heterogenous energy loss in the medium, locally high concentrations of ions and radicals are formed. In these “spur” regions radicalradical reactions lead to additional products namely H atoms, HzOz and HZ. The latter 2 products are not of immediate interest and this paper will focus attention on the reactive species, i.e. H, OH and e&, which for the present discussion are formed with the following G yalues (No. of entities/100 ev energy absorbed): G(eJ = 2.8, G(OH) = 2.8, G(H) = 0.6. It is customary to suggest that the observed reactions of these species with solutes dissolved in water may be related to the chemical processes induced by the irradiation of living cells.3’ This paper will also follow this path for some distance, but then some new concepts will be introduced.
tThe research Office of Basic
described herein Energy Sciences
OH + MC”-‘)+ + M”+ + OHA classic example is the oxidation of FeZi to Fe3’ in dosimeter (K = 3.0 x 10sL mol-‘s-l). Fricke the Some intriguing examples of electron transfer have also come to light in organic systems. For example, the reaction of OH radicals with poly-arenes in water normally gives the OH radical adduct; but in acidic solution electron transfer can occur and an intermediate poly-arene cation is observed, which subsequently hydrolyses to give the OH adduct.”
was supported by the of the Department of
Energy. This is Document Dame 1049
Radiation
Laboratory.
No. NDRL-1916
from the Notre
l Biology 0 Physics
Radiation Oncology
IO50
OH + RH -
RH(OH). H’
\ RH++
OH-
It is quite difficult to observe such transitory intermediates in OH radical reactions, but they may be more prevalent than was originally thought. The absolute rate constants for the reactions of OH radicals with benzene” and benzene derivatives were among the first to be measured. The product in the case of benzene was identified as the cyclohexanedienyl radical:
OH
Reactions of the hydrated electron, e,, have gained much prominence during the past 15 years and hundreds of rate constants for reaction are available.2.‘4a20 Again, the fine details of these reactions are relatively unknown. As expected, electrons readily reduce such solutes as Cu++ to Cu’, etc. In some cases discerniible negative ions are formed, e.g. the ketyl ion by reaction with benzophenone:
ph
e, +
ph ‘GO+
/ ph
‘c-o / ph’
The intermediate ion may have an extremely short lifetime and may never be observed; e.g. with organic halides the halide ion and organic radical are the only observed reaction products:
+
It is thought that OH adds readily to unsaturated bonds, e.g. ethylenic and acetylenic bonds and arene compounds; but the possibility of intermediate electron transfer always has to be considered.22 The hydrogen atom abstraction by OH radicals from organic compounds has received some detailed attention: OH + RH + R. + Hz0 Aliphatic alcohols are quite reactive, with k > lo9 L mol-‘s-l; the reactivity increases with increasing chain length.4 On the other hand, both carboxylic acids such as acetic or formic acid, or sulphate esters such as methyl and ethyl hydrogen sulphate, are quite unreactive with k - lo8 L mol-‘s-’ or less. This results from the electron withdrawing power of the acid groups, which reduces the electron density on the (Y and p carbon atoms of the acids and thus the reactivity toward the electrophilic OH radical. Increasing the alkane chain increases the reactivity and Cl2 arenes are quite reactive. The site of H atom abstraction in a large molecule is not known with certainty, but in a long C12-chain the probability of OH attack is similar at all positions, except those adjacent to groups such as -COOH or -S04H. Retrctions
July 1979, Volume 5, No. 7
of H utoms
und hydrutrd
electrons
Hydrogen atoms abstract other H atoms from organic molecules and generate H2 gas. The reactivity is some 100 fold smaller than corresponding reactions of OH radicals.’ H atom addition to unsaturated compounds is rapid, however, and often approaches the diffusion controlled limit. Electron transfer of H atoms is not as prevealent, but reduction of inorganic ions probably proceeds via electron transfer; e.g.: H + Fe7’ + Fe” + H’.
e,+RX-+R’+X-
(X = Cl; Br; I)
The data obtained by pulse radiolysis may be extrapolated, with care, to more complex systems. At high solute concentrations the electron may react prior to solvation and the observed yield of e& is reduced.‘3,‘6 This is true for acetone where a 0.31.0 M concentration greatly reduces G(e;J, while the anion of acetone is formed. Of particular interest is the case of benzene, which is relatively unreactive with solvated electrons, but readily reacts with presolvated electrons at a concentration near 1 M. Such reactions are of interest in biological systems where high concentrations of solute molecules exist. Reactions
of secondary
radicals
The reactions of H, e, and OH are rapid and tend to be non-selective. Many attempts have been made to produce secondary radicals which have more selectivity, or to react the orgainic radicals formed in the events described above with reactive solutes. Some of these systems are listed below and have been reviewed recently.” The sulphate radical SO,- is produced by the reaction of e;iq with persu1phate:‘R*‘9
eaq+ S20Rz~+
So:- + so4
This radical tends to react via electron transfer and forms the cation of several solutes. H atom abstraction and addition are not important reactions with this species and hence unique electron transfer reactions can be initiated by S04-. A similar reaction of e;, with peroxodiphosphate ion leads to the 3 acid-base forms of the phosphate radical,‘” H2Pb4, HP04~, PO:--. These radicals abstract H atoms from aliphatic compounds only slowly (k - 10’ for acetic
Chemical
processes
induced
radiolytically
acid); addition to a double bond is also slow. The reactivities of the phosphate radicals and S04- are quite similar. It is suggested that this may give rise to a greater selectivity than that displayed by the more reactive OH radical. Other radicals of the type X2-, such as Br2-, formed by the reactions
OH + Bra-Br
+ OH’?Br I COOH
Hz0
R. + Fe(CN)ior the formation
/ + R’ + Fe(CN)z-
of the quinone
anion: 0-
0 -R++
R’+
H2O
0
\
0 ROH
+ H+
The nature of radical R., controls the rate of the reaction. For example .CH20H is very reactive with Fe(CN)z-, k being > lo9 L mol-‘so’; however, .CH2CHzOH is quite unreactive. Such reactions are useful in determining the nature of the radicals formed in the radiolysis of an aqueous system. This short discussion has left out details of the reactions shown, and also several reactions of interest to physical chemists. It strives, however, to present a strong case that the aqueous radiation chemist has a firm grip on the fundamental physical properties of irradiated systems, so that he can, within limits, control the outcome of the reaction.
CH> OHS II + R,-C + CH2-C-R2 I I COOH COOH Both processes are impared by 02, which reacts with the intermediate radical. Thio alcohols and RHS repair the intermediate radical, RI, by the reaction R; + RSH + R,H + RS.; The RS radical is inert. Similarly ferricyanide reacts with the radicals and prevents crosslinking or degradation by oxidizing the intermediate radical to an alcohol. The latter process has been used to measure the rate of decay of the intermediate polymethacrylic acid radical to give the degradation products; the rate of this process at 25°C is 10-4s-‘. An interesting and important kinetic concept appears in such experiments. For example, the rate of reaction of OH radicals with constituents of the polymer is readily estimated from available data; the next stage is to relate these data to the attack of OH on the polymer containing such monomer units. The
1052
rate constants
Radiation
for the polymer
of the rate constants
Oncology
0 Biology
is not merely
for (OH + monomer),
0 Physics
July 1979, Volume 5, No. 7
the sum
responding increase in the rate of formation of Pm, but the rate tends to a maximum. This is interpreted as being because the rate controlling feature of the kinetics is the approach of e, to the vesicle; once at the vesicle the reaction with pyrene is rapid. This type of behavior was indicated earlier for polymers’5 and is a feature of the clustering of the system. It also occurs with OH radical reactions, as will be discussed below. The hydrated electron does not readily react with lecithin vesicles. However, some reactivity is noted with the OH radical,‘**’ and it is suggested that the point of attack is towards the surface of the vesicle in the choline group rather than at the fatty acid region below the glycerol unit. With pyrene located in the bilayer, OH attack produces the spectrum of the OH-pyrene adduct. In homegeneous solution the rate constant for this reaction is 1.34 x 10” L molK’s_‘, the rate varies linearly with pyrene concentration and it is diffusion controlled. Figure 1 shows how the rate of appearance of the OH-pyrene adduct and the yield vary with increasing pyrene concentration in the bilayers; the rate of reaction increases slightly and the yield of product approaches a maximum. This is because the rate limiting step is that at which the OH radical reaches the vesicle. The rate constant for a diffusion controlled process can be written as k = 4nrD x 6 x 10zo L mol-Is-‘, where r is the sum of the reactant radii (rOH + rpyrene= 6 A), and D is the sum of the diffusion constants (in the present system, D = 4 x 10e5 cm*s-‘); k is then calculated as 1.8 x 10” L mol-‘s-l. This is typical of
ko, but it 0
depends critically on the size of the polymer coil and may be calculated from diffusion theory.’ This will be dealt with in the next section, that concerns vesicles and micelles. Radiolysis in micelles Surfactant molecules, such as sodium lauryl sulphate (NaLS) and cetyltrimethylammonium bromide (CTAB), have lipid, hydrophobic, and hydrophilic parts. In water above certain concentrations these molecules cluster to form micelles. The micelles are spherical, with radii of 15-20 A, and have a lipid core with the hydrophilic end groups towards the water phase. The system may be looked upon as a host of small oil droplets solubilized in water. Water insoluble materials such as pyrene are solubilized by micelles. Radiolysis of these systems leads to e,+ OH; the OH radicals react with the surfactant and the eiq with the solubilized molecule. With NaLS micelles the surface charge is negative and e& is repelled, thereby decreasing the rate of (e,+ pyrene) from k = 10” L molss’ in homogeneous solutions to k micelles. With CTAB in anionic IO6L mol-‘ss’ micelles the positive charge attracts e, and the reaction rate of (e, + pyrene) is catalyzed to k > IO” L mol~‘s~‘.24 Similar effects are observed with bovine serum albumin where the rate of reaction of eLq with the protein may be controlled by addition of a suitable surfactant.“’ These systems provide the first step in advancing from the radiolysis of dilute aqueous solutions to more complicated bio-systems. Increased selectivity of reaction is noted immediately. Radiolysis in vesicles Even more complicated structures, such as vesicles have been investigated recently.5.9*2’ These systems are constructed by the sonication of lecithin in water and consist of closed bilayers of radius 300-500 A. The bilayer encloses and traps a volume of water in the core. Hydrophobic molecules such as pyrene may be solubilized in the bilayer and used as a monitor for chemical reactions of eag’,” and OH’ radicals produced in the surrounding bulk water. Hydrated electrons react with pyrene in the bilayer producing the pyrene anion, Pm, which decays slowly via second order kinetics. It is suggested*’ that since the anion is more hydrophilic than pyrene, it exists into the water and is annihilated there. The rate of reaction of (e, + pyrene) increases above the temperature of the phase change in the vesicle, when the bilayer becomes more fluid. However, increasing pyrene concentration does not produce a cor-
I
/
I
I
[DSL]
6
1
I
,
= 2 mM
I
OO
I1
21 [PYRENE]
3I
41
5I
, M (do4)
Fig. 1. Effect of pyrene concentration on the rate of growth and yield of the OH- pyrene adduct in vesicles of distearyl and dipalmitoyl lecithin.
Chemical
processes
induced
radiolytically
many OH radical reactions and close to that measured for (OH + pyrene). The radius of a vesicle is 250 A, and the diffusion of the vesicle is slow, so D = 2 x 10~scm2s-‘. The calculated rate constant is now 3.9 X 10” L mol-‘s-’ and is a measure of the rate at which OH encounters the vesicle. There are some 1000 lecithin molecules in the vesicle, so k (OH + lecithin) is 3.9 x 10’ and close to that measured. Addition of pyrene introduces competition between the vesicle and the pyrene for OH. The yield of the OH - pyrene adduct increases with increasing pyrene concentration. However, the rate of reaction remains relatively unchanged as the rate dependent step is that at which OH encounters the vesicle. The rate tends to a limit of 5 x 10’s_’ and a vesicle concentration of 2 x 10m6M. The rate constant for (OH + vesicle) is then 2.5 x 10” L mol-Is-‘, in agreement with the above discussion. These aggregated systems, which are reminiscent of true bio-systems, introduce new features into the conventional kinetic patterns. However, these are now understood and may be utilized to explain more complicated systems. Electron capture by water bubbles It is customary, when discussing the application of data from the radiolysis of dilute solutions to biosystems, to start with the reactive species in water. However, in actual bio-systems the fraction of water in certain regions may be quite low, and‘model systems should be designed which mimic this more complex situation. One such model system is to create inverted micelles utilizing the surfactant sodium di-isooctyl sulphosuccinate, and to prepare water bubbles of different sizes in these micelles. The bulk solvent is now heptane containing 3% surfactant and up to 6% water. These solutions are optically clear and quite convenient for photochemical or radiolytic studies.*’ The nature of the water in the clusters has been examined by spectroscopic*’ and NMR techniques.” The small water bubbles do not exhibit the properties of bulk water and at least 1% water is required in order for the bubbles to exhibit physical properties reminiscent of bulk water. At this stage, the water bubbles can capture electrons liberated radiolytically in the bulk hydrocarbon phase; this event is detected by observing the hydrated electron in the water bubbIe.28 Figure 2 shows how the yield of electrons captured by the bubble varies with the fraction of water in the system.29 At 6% water G(e&) is only 16%
in well defined
0 J. H. THOMAS
aqueous
systems
061
I
I
I
2
1053
1
I
I
,
3
4
5
6
05 t -
0.4 i CT
z ”
03020.1 -
0
%H,O
7
-
Fig. 2. Yield of hydrated electrons in reversed micellar systems of 3% sodium diisooctyl sulphosuccinate in heptane with various percentages of dissolved water, saturated with Nz or SF+
that of pure water, as the back neutralization of the electron and solvent cation dominates over electron capture by the bubble. The rate of capture of the electrons is high and the bubbles show an attractive force on the electron presumably because of induced dipole in the water bubble.29 Figure 2 also shows the effect of SF6 on the electron yield; it decreases as the SF, captures electrons in the alkane bulk prior to capture by the water. The remaining yield results from electrons produced directly in the bubble by direct energy loss. This follows the electron fraction of the water bubbles in the system. In certain microemulsion systems containing 10% water G(eJ equals that of bulk water (unpublished data). These models emphasize the importance of a smaller percentage of water in attracting and captuiing electrons from the bulk phase. The resulting chemistry is then that of the aqueous phase involving hydrated electrons. CONCLUSION Understanding of the physical processes that take place in dilute aqueous solutions has developed considerably over the past 20 years, especially after the development of pulse radiolysis. More complex systems are now under study, and the initial results are promising in the sense that they are readily understood in terms of the earlier simpler systems, while introducing new features of interest and selectivity. These systems bring us one step closer to the radiolysis of genuine bio-systems.
REFERENCES 1. Adams, G.E., Redpath, J.L., Bisby, R.H., Cundall, R.B.: Use of free radical probes in the study of
mechanisms of enzyme 1073-93, 1972.
inactivation.
Zsr. J. Chem.
10:
IO54
Radiation
Oncology
0 Biology 0 Physics
2. Anbar, M., Bambenek, M., Ross, A.B.: Selected specific rates of reactions of transients from water in aqueous solution-l. Hydrated electron. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.) 43: 61, 1973. 3. Anbar, M., Farhatazis, Ross, A.B.: Selected specific rates of reactions of transients from water in aqueous solution-H. Hydrogen atoms. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.) 51: 56, 1975. 4. Anbar, M., Meyerstein, D., Neta, P.: The reactivity of aromatic compounds toward hydroxyl radicals. J. Phys. Chem. 70: 2660-2662, 1966. 5. Barber, D.J.W., Thomas, J.K.: Reactions of radicals with lecithin bilayers. Radiat. Res. 74: 51-65, 1978. J.H., Thomas, J.K.: The degradation of 6. Baxendale, polymethacrylic acid by U.V. and x-irradiation. Truns. Faraday Sot. 54: 1515-1525, 1958. 7. Behzadi, A., Borgwardt, U., Henglein, A., Schamberg, E., Schnable, W.: Pulse radiolytic study of the kinetics of diffusion controlled reactions of OH-radicals with polymers and oligomers in aqueous solution. Ber. Bunsenges. Phys. Chem. 74: 453-469, 1970. 8. Bisby, R.H., Cundall, R.B., Wardman, P.: Pulse radiolysis study of some free radical reactions with Biochim. Biophys. Acta 389: erythrocyte membranes. 137-144, 1975. 9. Cheng, S., Thomas, J.K., Kulpa, C.F.: Dynamics of pyrene fluorescence in Escherichia cofi membrane vesicles. Biochemistry 13: 1135-l 139, 1974. 10. Cooper, M., Thomas, J.K.: A pulsed laser study of excited states of aromatic molecules absorbed in globular proteins. Radiat. Res. 70: 312-324, 1977. II. Dorfman, L.M., Adams, G.E.: Reactivity of the hydroxyl radical in aqueous solutions. Nat. Stand. Ref. Datu Ser., Nat. Bur. Stand. (U.S.) 46: 72 pp. 1973. 12. Dorfman, L.M., Taub, I.A., Buhler, R.E.: Pulse radiolysis studies--I. Transient spectra and reactionrate constants in irradiated aqueous solutions of benzene. J. Chem. Phys. 36: 3051-3061, 1%2. 13. Hamill, W.H.: A model for the radiolysis of water. J. Phys. Chem. 73: 1341-1347, 1%9. 14. Hart, E.J., Anbar, M.: The Hydrated Electron, New York, Wiley, 1970. 15. Henglein, A., Schnable, W.: Ein Fiihrung in die Strahlenchemie, Weinhelm/Bergstr., Verlag Chemie, 1969. 16. Maruthamuthu, P., Neta, P.: Reaction of phosphate radicals with organic compounds. J. Phys. Chem. 81: 1622-1625, 1977. 17. Neta, P.: Application of radiation techniques to the study of organic radicals. Ado. Phys. Org. Chem. 12: 233-297, 1976. 18. Neta, $P., Madhavan, V., Zemel, H., Fessenden, R.W.: Rate constant and mechanism of reaction of sulfate
July 1979, Volume 5, No. 7
19.
20.
21.
22.
23. 24.
25.
26.
27.
28.
29.
30.
31.
radical anion with aromatic compounds. J. Am. Chem. Sot. 99: 163-164, 1977. O’Neill, P., Steenken, S., Schulte-Frohlinde, D.: Formation of radical cations of methoxylated benzenes by reaction with OH radicals, Tl”, Ag”, and SO? . in aqueous solution. An optical and conductometric pulse radiolysis and in situ electron spin resonance study. J. Phys. Chem. 79: 2773-2779, 1975. Ross, A.B.: Selected specific rates of reactions of transients from water in aqueous solution. Hydrated electron, supplementary data. Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), Suppl. 43, 1975, 43 pp. Schnecke, W., Gratzel, M., Henglein, A.: Reactions of the hydrated electron with pyrene in lipid bilayer vesicles. Ber. Bunsenges. Phys. Chem. 81: 821-826, 1977. Sehested, K., Corfitzen, H., Christenson, H.C., Hart, E.J.: Rates of reaction of oxygen(1 -) ions, hydroxyl radicals, and atomic hydrogen with methylated benzenes in aqueous solution. Optical spectra of radicals. J. Phys. Chem. 79: 310-315, 1975. Swallow, A.J.: Introduction to Radiation Chemistry, London, Oxford Univ. Press, 1973. Thomas, J.K.: Effect of structure and charge on radiation-induced reactions in micellar systems. Act. Chem. Res. 10: 133-138, 1977. Thomas, J.K.: Elementary processes and reactions in in Radiation the radiolysis of water. In Advances Chemistry, Vol. 1, Ed. by Magee, J.L., Burton, M., New York, Wiley, 1970, pp. 103-198. Wolff, R.K., Bronskill, M.J., Aldrich, J.E., Hunt, J.W.: Picosecond pulse radiolysis--IV. Yield of the solvated electron at 30 picoseconds. J. Chem. Phys. 77: 13501355, 1973. Wong, M., GrCtzel, M., Thomas, J.K.: Fluorescence probing of inverted micelles. The state of solubilized water clusters in alkaneldiisooctyl sulfosuccinate (Aerosol OT) solution. J. Am. Chem. Sot. 98: 23912397, 1976. Wong, M., Gratzel, M., Thomas, J.K.: On the nature of solubilized water clusters in Aerosol AT/alkane solutions. A study of the formation of hydrated electrons and 1,8-anilinonaphthalene sulfonate fluorescence. Chem. Phys. Lett. 30: 329-337, 1975. Wong, M., Grieser, F., Thomas, J.K.: Berichte Der Bunsen Geseuschaff Fur Physikalische Chemie, Schloss Emlau Meeting, W. Germany 82: 950, 1978. Wong, M., Thomas, J.K., Nowak, T.: Structure and state of Hz0 in reversed micelles-3. J. Am. Chem. sot. 99: 4370-4375, 1977. Wardman, P.: The use of nitro aromatic compounds as hypoxic cell radiosensitizers. Curr. Topics in Radiat. Res. Quarterly. 11: 347-398, 1977.
