Mksbauer Effect Studies on Some Bioinorganic Complexes of Europium C. J. Niu, Y. Q. Jia”, M. L. Liu, X. W. Liu, and M. 2. Jin CJN, YQJ. Changchun Institute of Applied Chemistry, Academia Sinica. Changchun, Jilin, China.-MLL, XWL, MZJ. Department of Physics, Jilin University Changchun, Jilin, China
ABSTRACT The bioinorganic complexes of europium with N-ace@-DL-alanine, N-acetyl-DL-valine, and DLalanyl-DL-alanine have been synthesized and the Miissbauer spectra at room temperature have been measured for these solid state complexes. The Miissbauer parameters indicate that the water molecules in these complexes are not directly linked to the central europium ion and are outside the coordination sphere of europium and biological ligands, and that the chemical bond between the europium ion and the ligands may be predominantly ionic in character, with the possibility of partial covalent contribution.
INTRODUCTION Recently, bioinorganic complexes have been a very attractive subject in inorganic chemistry. Some authors have also paid attention to the preparations and properties of the rare earth bioinorganic complexes because the rare earth ions can be used as a probe in research on biochemistry. The parameters, structures, and properties of some rare earth bioinorganic complexes have been investigated and published. Understanding the properties of the chemical bond between the rare earth atom and the biological ligand is very significant for exploring the biological effect of the bioinorganic complexes. MGssbauer spectroscopy can reveal some comparatively weak interaction between the Mlissbauer atom and the surrounding coordination atoms, so it has widely been applied to inorganic and bioinorganic chemistry. For the study of the properties of chemical bonds in the bioinorganic complexes of the rare earths and the amino acids with different substituent groups and of the rare earths and dipeptide, we have synthesized the solid state complexes of europium with N-acetyl-DL-alanine, N-acetyl-DL-valine, and DL-alanyl-DL-alanine and measured
Address reprint requests and correspondence to: Dr. Y. Q. Jia, Changchun Chemistry, Academia Sinica, Changchun, Jilin, China.
Institute
of Applied
151 Journal of Inorganic Biochemistry, 45, 15 I- 157 (1992) 0 1992 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, NY, NY 10010 0162-0134/92/$5.00
152 C. J. Niu et al.
their Miissbauer spectra. From the experimental Miissbauer spectra, some Mijssbauer parameters, such as the isomer shift (IS), the electric quadrupole splitting (eQV,,), and the asymmetric parameter (_rl) of the electric field gradient at the Mijssbauer nucleus have been derived. The reseults indicate that the water molecules in these solid state complexes are not directly coordinated to the central cm-opium ion, and that the chemical bonding in these bioinorganic complexes of europium with different biological ligands is mainly ionic.
EXPERIMENTAL All the solid state complexes arc synthesized by using a conventional method described previously [ 1. 21. All the chemicals used in the experiments arc analytical reagent grade. The freshed prepared europium carbonate or europium chloride is dissolved at 40°C by adding an aqueous solution containing the corresponding biological ligand (10mm2M or 10 ’ M) dropwise slowly; the biological ligands keep slightly in excess of the stoichiometric amount. Then. the resultant solution is filtered and the cleal filtrate is concentrated in a thermostated bath at 40°C. On cooling. the corresponding solid state complex is obtained and collected by tiltration. The solid statz- complex is repeatedly washed with acelone and recrystallized from the distilled waler solution. and finally dried in a vacuum desiccator over phosphori&‘! oxides. The results of elemental analyses for these solid state complexes are listed in Table 1. The results indicate that the composition of the europium bioinorganic complcu is .2H,O, and Eu(C,,H,,N,0,)i.411,0. Eu(C,H,NO,), . 1.5H,O, Eu(CiH,,NO,), respectively. The thermogravimetric investigatik also support these results. Besides, the thermogravirnetric curves and derivative thermogravimetric cur\t”i of these complexes show that all the water molecules in thcsc ~oiitl state z~~plex~‘s can be completely lost at 100°C. The sample is prepared in a thin absorber containing about 10 n-rg cm ’ europium. The Miissbauer spectra are recorded using an OXFORD MS-500 model constant acceleration Miissbauer spectrometer with a 1024 multichannel analyzer. The radiation source is SmF,(‘5’Sn~: about 100 rnC1 15’Eu); 2 Xenc)n (methane) proportional counter is used as a detector. ‘The penetration Miisshaucr spectra of these complexes at room temperature are shown in Figure 1. After the Miissbauer spectra of these samples arc measured. the absorbers of the samples mentioned above are placed in an oven and kept at about 100°C for 24 hr. The anh? drous complex can be obtained this way. Then, we measured the Miissbauer spectra for the samples of the anhydrous complexes. The experimental penetration Miissbauer spectra of the anhydrous complexes are shown in Figure 1.
TABLE 1. Analytical Data for Some Bioinorganic Complexes of Europium Formula Eu(C,HxNO,),-
Eu (7%)
I.SHzO
FOWld C’alc
Eu(C,H,>NO,),~2H:O Eu(C,H
,,N>O,),Cl,
.4H,O
Found Calc Found Calc.
26.61 26.69 ‘1-.X1 22.94 1X.98 1x.73
C (7%) 3? 65 .3i.64 37.41 3X.07 2h 37 2h.66
H ( %)
N (cir)
3.48 4 7’! 6.83 b 10 4.07 s.42
7 40 : ix 6.75 h.33 9.89 IO.25
Cl (%‘)
13.03 I3 Ii
MijSSBAUER EFFECT STUDIES
-6 -7 -6 -5 -4 -3 -2 -1
s
1
I
0
1 2
Velocity
81
3
I1
153
I
4 5 6 7
E
(mm/s)
FIGURE 1.
Mijssbauer spectra of “‘Eu in some bioinorganic complexes of europium at room temperature. 1,2,3: Mijssbauer spectra of europium with N-acetyl-DL-alanine, with N-ace@-DL-valine, and with DL-alanyl-DL-alanine complexes, respectively. a and b are the corresponding hydrous and anhydrous complexes, respectively. The relative positions and the relative intensities of the 12 transition lines in the complex of europium with N-acetyl-DLalanine are shown at top. (The relative intensities of the six weaker lines are enlarged.) The six main subspectra for the Mijssbauer spectrum of the hydrous complex of europium with
N-ace@-DL-alanine
are also shown in la.
RESULTS AND DISCUSSION It has been shown that in many cases the broad lines obtained for europium compounds result from unresolved quadrupole splitting of the Eu nucleus. Glentworth et al. have taken into account the quadrupole splitting of the “‘Eu nucleus in
their
studies
organic
of the
ligands
Miissbauer “‘Eu
used
effect
relatively
a modified
the complexes
spectra
method
complexes
with
et a.
the \ix
main
Mtissbauer
Lorentzian
the
experimental
111 fitting of europium
16, 71. We
1 However. practically.
and the asymmetric derived
directly
From both
parameter
from
fitting
in the complex with
dehydration europium
cannot
of the electric
\t;ItcL
atom or ion. Therefore,
IWO solid state complex
may not he directly
may be outside
the coordination
can also be supported
complexes
are easily
that the difference europium errors
between
and cannot
and from
a chemical
lead tO the variation the
europium dehydration water
TABLE --
.--..-
Why
shift
of thcrmogravimetric the DL-alanyl-DI.-alanine. of be
this
of the of the
might
complex
conipletellj
in the two
k\ 1’ ha\i
~/SO noted
i\ iust rlutsldc
detectable
spcctr;~ of
ik, m;Gnly ccrmposed of
lines
spectra,
fit IO
that ali the Miissbauer
the eicctric
field
some
tits of the
an exc.cilent
Miissbauer
2, we can find that the isomer
give
with
~TherefOrc:, wc have
Ixnes. These line\
shift,
to the experimental
of europiurn
of the solid
[“I.
have found
six of the twelvE
the N-,acetyI -I>Lvaline,
molecules
lines
each spectrum
SIJ~!: as the isomer
the results in Tabk
europium water
k~ausc
s&spectra.
parameters.
have obtained
eight
spectra can be very well tittctl u ith 12 Lorentzian on the top of Figure
of curopium
,O: x with a singic Lorentzian
nucleus in the EuBa,Cu
poor fits (41. Lippmaa
Miissbauer
bioinorganic
for
et al. have also found that the least-syuareh
spectra of the “‘ELI
line provide the
Miissbauer
131. Wortmann
rhi5 iiocx ytatt’
not agree
~~xnp1e.x
dernc~nstrate
40--5!1’
c’ and
%.ici~ ;i i0z1
thsr
of
that the ail
the
tempcraturc
2. The Miissbauer Parameters of Some Complexes of kiuropium and Biological Ligantit, - -----..--...-_ --.. -...-.._._-._-
--.
_~._-.--__--._.________. Electric QuaJrupole
Isomer shift IS iEuF,kmr Complex
( Lt0.02)
c
Splittinp
’
ecy,,* Jllll! \ i t i:o..!i
.__.. As> mmetric Parameter
rf! r O.lZi
01
MGSSBAUER
EFFECT
STUDIES
155
dehydration must imply that the water molecule is not directly coordinated to the central rare earth ion [8]. So, we believe that, like the water molecules in the complexes of europium with N-acetyl-DL-alanine and with N-acetyl-DL-valine, the water molecules in the complex of europium with DL-alanyl-DL-alanine are also the interstitial water molecules. In this case, we can assume that these water molecules may occupy the interstices between the biological ligands and are not directly linked to the europium ion. Therefore, we can assume that the second possible cause, which is responsible for the change in the isomer shift before and after dehydration of the complex of europium and DL-alanyl-DL-alanine, might be because of the structural and composition change in the coordination sphere of europium and biological ligands caused by the loss of the interstitial water molecules. On the basis of the molecular structure of the DL-alanyl-DL-alanine, those interstitial water molecules in the complex of europium and DL-alanyl-DL-alanine may be much closer to the central europium ion than the chlorine anion, because the water molecule can form the weak hydrogen bond with the oxygen atom of the carbonyl group or with the nitrogen atom of the amide in the biological ligands; on the contrary, the chlorine anion can associate only with the terminal amino group in the peptide chain. But, after dehydration, the chlorine anion may become closer to the europium ion due to elimination of the sterically hindered water molecules. This must lead to the variation of the charge distribution of the central europium ion and cause the change in the isomer shift of the 15’Eu nucleus. This can be supported by the considerable increase of the electric quadrupole splitting of the “‘Eu nucleus after dehydration. As the results in Table 2 show, the difference between the electric quadrupole splitting of the “‘Eu nucleus in the hydrous and anhydrous complex of europium with N-acetyl-DL-alanine and with N-acetyl-DL-valine is just inside the errors. This can indicate that the presence or the absence of the water molecules in the two complexes cannot give any detectable effect of the electric quadrupole splitting of the 15’Eu nucleus. As for the complex of europium with DL-alanyl-DL-alanine, the difference between the electric quadrupole splitting of the “‘Eu nucleus before and after dehydration of the complex is appreciably outside the errors. The difference can imply that the variation of the electric field gradient at the “‘Eu nucleus created by the surrounding atoms occurs in the dehydration process of the complex. It can be understood that the presence of an anion with a negative charge instead of a molecule without any charge near the Mossbauer atom, must lead to the increase of the electric field gradient at the Mossbauer nucleus. Besides, the asymmetric parameter of the electric field gradient at the “‘Eu nucleus in the hydrous complex and the dehydrous complex are also very different from each other. This can also indicate the change in the coordination environment of the europium ion before and after dehydration. Of course, the dissociation of the hydrogen chloride from the complex can also cause the variation of the isomer shift and the electric quadrupole splitting of the “‘Eu nucleus in the complex. However, the thermal decomposition experiments show that the dissociation of the hydrogen chloride in the complex will happen at higher temperature. Glentworth et al. have performed the studies of Mossbauer effects for many complexes of europium with organic ligands and determined that the chemical bond of europium ion with these organic ligands is predominantly ionic in character, with the possibility of a small covalent contribution based on the values of the isomer shifts of the “‘Eu nucleus in these complexes [3].
156 C. J. Niu et al.
Glentworth et al. compared the values of the isomer shifts obtained from the Miissbauer spectra of the complexes of europium with the organic ligands; the values of the isomer shifts of the 15’Eu nucleus in the bioinorganic complexes mentioned above will fall within the same range. Therefore, we can also suggest that the chemical bond between the europium ion and the amino acid or dipeptide may also be mainly ionic and possess some partial covalent. The isomer shift of the ‘“‘Eu nucleus in the complex of europium with the N-acetyl-DL-alanine and with the N-acetyl-DL-valine is the same. but the electric quadrupole splitting of the “‘Eu nucleus in the two complexes are different from each other. This shows that the effect of different substituent groups in the amino acid on the chemical bond between the rare earth ion and the amino acid IS not very large. However, the different substituent group in the ligands can give an appreciable effect on the electric field gradient at the 15’Eu nucleus. The iatticc hummation calculation of the electric field gradient has demonstrated that the electric field gradient of the atom in the crystal not only depends on the charge distribution at most neighbor atoms, and but also depends on the charge distribution at the far atoms. The considerable difference of the isomer shift and the electric quadrupole splitting of the “’ Eu nucleus in the complex of europium and DI..-alanyl-DL-alanine frotn that in the complex of europium and N-acetyl-DL-alanine and of eurvpium and N-acetyl-DL-valine can aixo demonstrate that the presence of the Inorganic anion with a more negative charge will give a likely effect both on the electric quadrupole splitting of the “‘Eu nucleub and on the chemical bonding of the rare earth ion with the biological ligands. Perhaps the chemical bond between the rare earth ion and the dipeptide possesses some different covalent character from that between the rare earth ion and the amino acid. CONCLUSION The Miissbauer effect studies of the hydrous and anhydrous complexes of europium with the N-acetyl-DL-alanine. the N-acetyl-DL-valine and the DL-alanyl-DL-alanine can demonstrate that the water molecules in these solid state complexes are not directly coordinated to the central europium ion and are outside the coordination sphere of the europium and the biological ligands. The chemical bond between the europium ion and these biological ligands is mainly ionic, and possesses a partial covalent character. The effect of the substituent group, for example, the methyl in the side chain of the amino acid, on the property of the chemical bond in these complexes is not very large. However, the presence of the inorganic anion in the solid state complex, for example, the chlorine anion in the complex elf europium and the dipeptide, will give a detectable effect on the chemical bonding of the rare earth ion and the biological ligand.
REFERENCES 1. O.E. Zvyagintsev and E. V. Goncharov, Zh. Neorgan. Khim. 8, 349 (1963). 2. 0. Farooq. A. U. Malik, and N. Ahmad, Acta Chim. Acad. Sci. Hung. 83, 343 (1974). 3. P. Glentworth, A. L. Nicholas, D. A. Newton, N. R. Large, and R. J. Bullock, J. C. S. Dalton 5, 546 (1973).
MijSSBAUER
EFFECT STUDIES
157
4. G. Wortmann, S. Blumenroder, A. Freimuth, and D. Riegel, P&s. Lett. A 126, 434 (1988). 5. M. Lippmaa, E. Realo, and K. Realo, Phys. Lett. A 139, 353 (1989). 6. M. Z. Jin and Y. Q. Jia, Science in China (series A), 33(4), 430 (lm). 7. M. Z. Jin, X. W. Liu, and Y. Q. Jia, Zl Nuovo Cimento 13D, N2, 157 (1991). 8. U. Sharma and N. Chandra, Thermochim. Actu. 65, 387 (1983).
Received July 29, 1991; accepted August 13, 1991
ABSTRACT The bioinorganic complexes of europium with N-ace@-DL-alanine, N-acetyl-DL-valine, and DLalanyl-DL-alanine have been synthesized and the Miissbauer spectra at room temperature have been measured for these solid state complexes. The Miissbauer parameters indicate that the water molecules in these complexes are not directly linked to the central europium ion and are outside the coordination sphere of europium and biological ligands, and that the chemical bond between the europium ion and the ligands may be predominantly ionic in character, with the possibility of partial covalent contribution.
INTRODUCTION Recently, bioinorganic complexes have been a very attractive subject in inorganic chemistry. Some authors have also paid attention to the preparations and properties of the rare earth bioinorganic complexes because the rare earth ions can be used as a probe in research on biochemistry. The parameters, structures, and properties of some rare earth bioinorganic complexes have been investigated and published. Understanding the properties of the chemical bond between the rare earth atom and the biological ligand is very significant for exploring the biological effect of the bioinorganic complexes. MGssbauer spectroscopy can reveal some comparatively weak interaction between the Mlissbauer atom and the surrounding coordination atoms, so it has widely been applied to inorganic and bioinorganic chemistry. For the study of the properties of chemical bonds in the bioinorganic complexes of the rare earths and the amino acids with different substituent groups and of the rare earths and dipeptide, we have synthesized the solid state complexes of europium with N-acetyl-DL-alanine, N-acetyl-DL-valine, and DL-alanyl-DL-alanine and measured
Address reprint requests and correspondence to: Dr. Y. Q. Jia, Changchun Chemistry, Academia Sinica, Changchun, Jilin, China.
Institute
of Applied
151 Journal of Inorganic Biochemistry, 45, 15 I- 157 (1992) 0 1992 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas, NY, NY 10010 0162-0134/92/$5.00
152 C. J. Niu et al.
their Miissbauer spectra. From the experimental Miissbauer spectra, some Mijssbauer parameters, such as the isomer shift (IS), the electric quadrupole splitting (eQV,,), and the asymmetric parameter (_rl) of the electric field gradient at the Mijssbauer nucleus have been derived. The reseults indicate that the water molecules in these solid state complexes are not directly coordinated to the central cm-opium ion, and that the chemical bonding in these bioinorganic complexes of europium with different biological ligands is mainly ionic.
EXPERIMENTAL All the solid state complexes arc synthesized by using a conventional method described previously [ 1. 21. All the chemicals used in the experiments arc analytical reagent grade. The freshed prepared europium carbonate or europium chloride is dissolved at 40°C by adding an aqueous solution containing the corresponding biological ligand (10mm2M or 10 ’ M) dropwise slowly; the biological ligands keep slightly in excess of the stoichiometric amount. Then. the resultant solution is filtered and the cleal filtrate is concentrated in a thermostated bath at 40°C. On cooling. the corresponding solid state complex is obtained and collected by tiltration. The solid statz- complex is repeatedly washed with acelone and recrystallized from the distilled waler solution. and finally dried in a vacuum desiccator over phosphori&‘! oxides. The results of elemental analyses for these solid state complexes are listed in Table 1. The results indicate that the composition of the europium bioinorganic complcu is .2H,O, and Eu(C,,H,,N,0,)i.411,0. Eu(C,H,NO,), . 1.5H,O, Eu(CiH,,NO,), respectively. The thermogravimetric investigatik also support these results. Besides, the thermogravirnetric curves and derivative thermogravimetric cur\t”i of these complexes show that all the water molecules in thcsc ~oiitl state z~~plex~‘s can be completely lost at 100°C. The sample is prepared in a thin absorber containing about 10 n-rg cm ’ europium. The Miissbauer spectra are recorded using an OXFORD MS-500 model constant acceleration Miissbauer spectrometer with a 1024 multichannel analyzer. The radiation source is SmF,(‘5’Sn~: about 100 rnC1 15’Eu); 2 Xenc)n (methane) proportional counter is used as a detector. ‘The penetration Miisshaucr spectra of these complexes at room temperature are shown in Figure 1. After the Miissbauer spectra of these samples arc measured. the absorbers of the samples mentioned above are placed in an oven and kept at about 100°C for 24 hr. The anh? drous complex can be obtained this way. Then, we measured the Miissbauer spectra for the samples of the anhydrous complexes. The experimental penetration Miissbauer spectra of the anhydrous complexes are shown in Figure 1.
TABLE 1. Analytical Data for Some Bioinorganic Complexes of Europium Formula Eu(C,HxNO,),-
Eu (7%)
I.SHzO
FOWld C’alc
Eu(C,H,>NO,),~2H:O Eu(C,H
,,N>O,),Cl,
.4H,O
Found Calc Found Calc.
26.61 26.69 ‘1-.X1 22.94 1X.98 1x.73
C (7%) 3? 65 .3i.64 37.41 3X.07 2h 37 2h.66
H ( %)
N (cir)
3.48 4 7’! 6.83 b 10 4.07 s.42
7 40 : ix 6.75 h.33 9.89 IO.25
Cl (%‘)
13.03 I3 Ii
MijSSBAUER EFFECT STUDIES
-6 -7 -6 -5 -4 -3 -2 -1
s
1
I
0
1 2
Velocity
81
3
I1
153
I
4 5 6 7
E
(mm/s)
FIGURE 1.
Mijssbauer spectra of “‘Eu in some bioinorganic complexes of europium at room temperature. 1,2,3: Mijssbauer spectra of europium with N-acetyl-DL-alanine, with N-ace@-DL-valine, and with DL-alanyl-DL-alanine complexes, respectively. a and b are the corresponding hydrous and anhydrous complexes, respectively. The relative positions and the relative intensities of the 12 transition lines in the complex of europium with N-acetyl-DLalanine are shown at top. (The relative intensities of the six weaker lines are enlarged.) The six main subspectra for the Mijssbauer spectrum of the hydrous complex of europium with
N-ace@-DL-alanine
are also shown in la.
RESULTS AND DISCUSSION It has been shown that in many cases the broad lines obtained for europium compounds result from unresolved quadrupole splitting of the Eu nucleus. Glentworth et al. have taken into account the quadrupole splitting of the “‘Eu nucleus in
their
studies
organic
of the
ligands
Miissbauer “‘Eu
used
effect
relatively
a modified
the complexes
spectra
method
complexes
with
et a.
the \ix
main
Mtissbauer
Lorentzian
the
experimental
111 fitting of europium
16, 71. We
1 However. practically.
and the asymmetric derived
directly
From both
parameter
from
fitting
in the complex with
dehydration europium
cannot
of the electric
\t;ItcL
atom or ion. Therefore,
IWO solid state complex
may not he directly
may be outside
the coordination
can also be supported
complexes
are easily
that the difference europium errors
between
and cannot
and from
a chemical
lead tO the variation the
europium dehydration water
TABLE --
.--..-
Why
shift
of thcrmogravimetric the DL-alanyl-DI.-alanine. of be
this
of the of the
might
complex
conipletellj
in the two
k\ 1’ ha\i
~/SO noted
i\ iust rlutsldc
detectable
spcctr;~ of
ik, m;Gnly ccrmposed of
lines
spectra,
fit IO
that ali the Miissbauer
the eicctric
field
some
tits of the
an exc.cilent
Miissbauer
2, we can find that the isomer
give
with
~TherefOrc:, wc have
Ixnes. These line\
shift,
to the experimental
of europiurn
of the solid
[“I.
have found
six of the twelvE
the N-,acetyI -I>Lvaline,
molecules
lines
each spectrum
SIJ~!: as the isomer
the results in Tabk
europium water
k~ausc
s&spectra.
parameters.
have obtained
eight
spectra can be very well tittctl u ith 12 Lorentzian on the top of Figure
of curopium
,O: x with a singic Lorentzian
nucleus in the EuBa,Cu
poor fits (41. Lippmaa
Miissbauer
bioinorganic
for
et al. have also found that the least-syuareh
spectra of the “‘ELI
line provide the
Miissbauer
131. Wortmann
rhi5 iiocx ytatt’
not agree
~~xnp1e.x
dernc~nstrate
40--5!1’
c’ and
%.ici~ ;i i0z1
thsr
of
that the ail
the
tempcraturc
2. The Miissbauer Parameters of Some Complexes of kiuropium and Biological Ligantit, - -----..--...-_ --.. -...-.._._-._-
--.
_~._-.--__--._.________. Electric QuaJrupole
Isomer shift IS iEuF,kmr Complex
( Lt0.02)
c
Splittinp
’
ecy,,* Jllll! \ i t i:o..!i
.__.. As> mmetric Parameter
rf! r O.lZi
01
MGSSBAUER
EFFECT
STUDIES
155
dehydration must imply that the water molecule is not directly coordinated to the central rare earth ion [8]. So, we believe that, like the water molecules in the complexes of europium with N-acetyl-DL-alanine and with N-acetyl-DL-valine, the water molecules in the complex of europium with DL-alanyl-DL-alanine are also the interstitial water molecules. In this case, we can assume that these water molecules may occupy the interstices between the biological ligands and are not directly linked to the europium ion. Therefore, we can assume that the second possible cause, which is responsible for the change in the isomer shift before and after dehydration of the complex of europium and DL-alanyl-DL-alanine, might be because of the structural and composition change in the coordination sphere of europium and biological ligands caused by the loss of the interstitial water molecules. On the basis of the molecular structure of the DL-alanyl-DL-alanine, those interstitial water molecules in the complex of europium and DL-alanyl-DL-alanine may be much closer to the central europium ion than the chlorine anion, because the water molecule can form the weak hydrogen bond with the oxygen atom of the carbonyl group or with the nitrogen atom of the amide in the biological ligands; on the contrary, the chlorine anion can associate only with the terminal amino group in the peptide chain. But, after dehydration, the chlorine anion may become closer to the europium ion due to elimination of the sterically hindered water molecules. This must lead to the variation of the charge distribution of the central europium ion and cause the change in the isomer shift of the 15’Eu nucleus. This can be supported by the considerable increase of the electric quadrupole splitting of the “‘Eu nucleus after dehydration. As the results in Table 2 show, the difference between the electric quadrupole splitting of the “‘Eu nucleus in the hydrous and anhydrous complex of europium with N-acetyl-DL-alanine and with N-acetyl-DL-valine is just inside the errors. This can indicate that the presence or the absence of the water molecules in the two complexes cannot give any detectable effect of the electric quadrupole splitting of the 15’Eu nucleus. As for the complex of europium with DL-alanyl-DL-alanine, the difference between the electric quadrupole splitting of the “‘Eu nucleus before and after dehydration of the complex is appreciably outside the errors. The difference can imply that the variation of the electric field gradient at the “‘Eu nucleus created by the surrounding atoms occurs in the dehydration process of the complex. It can be understood that the presence of an anion with a negative charge instead of a molecule without any charge near the Mossbauer atom, must lead to the increase of the electric field gradient at the Mossbauer nucleus. Besides, the asymmetric parameter of the electric field gradient at the “‘Eu nucleus in the hydrous complex and the dehydrous complex are also very different from each other. This can also indicate the change in the coordination environment of the europium ion before and after dehydration. Of course, the dissociation of the hydrogen chloride from the complex can also cause the variation of the isomer shift and the electric quadrupole splitting of the “‘Eu nucleus in the complex. However, the thermal decomposition experiments show that the dissociation of the hydrogen chloride in the complex will happen at higher temperature. Glentworth et al. have performed the studies of Mossbauer effects for many complexes of europium with organic ligands and determined that the chemical bond of europium ion with these organic ligands is predominantly ionic in character, with the possibility of a small covalent contribution based on the values of the isomer shifts of the “‘Eu nucleus in these complexes [3].
156 C. J. Niu et al.
Glentworth et al. compared the values of the isomer shifts obtained from the Miissbauer spectra of the complexes of europium with the organic ligands; the values of the isomer shifts of the 15’Eu nucleus in the bioinorganic complexes mentioned above will fall within the same range. Therefore, we can also suggest that the chemical bond between the europium ion and the amino acid or dipeptide may also be mainly ionic and possess some partial covalent. The isomer shift of the ‘“‘Eu nucleus in the complex of europium with the N-acetyl-DL-alanine and with the N-acetyl-DL-valine is the same. but the electric quadrupole splitting of the “‘Eu nucleus in the two complexes are different from each other. This shows that the effect of different substituent groups in the amino acid on the chemical bond between the rare earth ion and the amino acid IS not very large. However, the different substituent group in the ligands can give an appreciable effect on the electric field gradient at the 15’Eu nucleus. The iatticc hummation calculation of the electric field gradient has demonstrated that the electric field gradient of the atom in the crystal not only depends on the charge distribution at most neighbor atoms, and but also depends on the charge distribution at the far atoms. The considerable difference of the isomer shift and the electric quadrupole splitting of the “’ Eu nucleus in the complex of europium and DI..-alanyl-DL-alanine frotn that in the complex of europium and N-acetyl-DL-alanine and of eurvpium and N-acetyl-DL-valine can aixo demonstrate that the presence of the Inorganic anion with a more negative charge will give a likely effect both on the electric quadrupole splitting of the “‘Eu nucleub and on the chemical bonding of the rare earth ion with the biological ligands. Perhaps the chemical bond between the rare earth ion and the dipeptide possesses some different covalent character from that between the rare earth ion and the amino acid. CONCLUSION The Miissbauer effect studies of the hydrous and anhydrous complexes of europium with the N-acetyl-DL-alanine. the N-acetyl-DL-valine and the DL-alanyl-DL-alanine can demonstrate that the water molecules in these solid state complexes are not directly coordinated to the central europium ion and are outside the coordination sphere of the europium and the biological ligands. The chemical bond between the europium ion and these biological ligands is mainly ionic, and possesses a partial covalent character. The effect of the substituent group, for example, the methyl in the side chain of the amino acid, on the property of the chemical bond in these complexes is not very large. However, the presence of the inorganic anion in the solid state complex, for example, the chlorine anion in the complex elf europium and the dipeptide, will give a detectable effect on the chemical bonding of the rare earth ion and the biological ligand.
REFERENCES 1. O.E. Zvyagintsev and E. V. Goncharov, Zh. Neorgan. Khim. 8, 349 (1963). 2. 0. Farooq. A. U. Malik, and N. Ahmad, Acta Chim. Acad. Sci. Hung. 83, 343 (1974). 3. P. Glentworth, A. L. Nicholas, D. A. Newton, N. R. Large, and R. J. Bullock, J. C. S. Dalton 5, 546 (1973).
MijSSBAUER
EFFECT STUDIES
157
4. G. Wortmann, S. Blumenroder, A. Freimuth, and D. Riegel, P&s. Lett. A 126, 434 (1988). 5. M. Lippmaa, E. Realo, and K. Realo, Phys. Lett. A 139, 353 (1989). 6. M. Z. Jin and Y. Q. Jia, Science in China (series A), 33(4), 430 (lm). 7. M. Z. Jin, X. W. Liu, and Y. Q. Jia, Zl Nuovo Cimento 13D, N2, 157 (1991). 8. U. Sharma and N. Chandra, Thermochim. Actu. 65, 387 (1983).
Received July 29, 1991; accepted August 13, 1991
