Volume 4 no.1 January 1977
Nucleic Acids Research
Theoretical study on the proton chemical shifts of hydrogen bonded nucleic acid bases
Claude Giessner-Prettre, Bernard Pullman and Jacqueline Caillet Institut de Biologie Physico-Chimique, Laboratoire de Biochimie Theorique associe au C.N.R.S., 13, rue P.et M.Curie, 75005 Paris, France
Received 20 September 1976 ABSTRACT The variation of the proton chemical shifts due to the formation of intermolecular hydrogen bonds is computed for a number of complexes which can be formed between the bases of the nucleic acids. The shifts expected for the isolated base pairs, in particular for the G-N H, T(or U)-N3H protons and the protons of the amino groups of A, G and &, when combined with previous computations on the shifts to be expected upon base stacking, may enable a refined analysis of the high resolution NMR spectra of self complementary polynucleotides or tRNAs. Two examples are presented of a direct computation of proton shifts associated with helix-coil transitions, helpful for deducing the helical structure in solution.
INTRODUCTION
The formation of helical structures which occurs in appropriate expe-
rimental conditions between different units of complementary polynucleotides or different parts of single stranded nucleic acids (ex. tRNA) involves the establishment of hydrogen bonds between pairs of bases as well as of stacks of bases or base-pairs between the neighbouring units of the strands. In proton magnetic resonance spectra, hydrogen bond formation induces a large downfield shift for the protons participating in the intermolecular bond while stacking of conjugated molecules such as the nucleic acid bases is accompanied by an upfield shift of their protons. From the proton NMR study of helix-coil transitions in solution, only the sum of the two effects
experimentally. In the numerous studies by this technique of the secondary structure of complementary polynucleotides (1-5) or of tRNAs (6-9) the experimentalists have made ample use of the theoretical predictions (10) concerning the upfield shifts to be expected upon base stacking is accessible
the ring current effect of the bases located above and below the base or base-pair under investigation. The complete analysis of the experi-
due to
C Information Retrieval Limited 1 Falconberg Court London Wl V 5FG England
99
Nucleic Acids Research mental results and the assignment
of
resonances
to specific base-pairs
necessitates, however, the knowledge of the chemical shifts associated with the "free" hydrogen-bonded base-pairs, unperturbed by stacking with their neighbors. There have been
few NMR studies
a
the association by hydrogen bon-
on
ding of complementary base-pairs (11), nucleosides (12-15)
and nucleotides
(16). These results indicate variations of proton chemical shifts attributable to
hydrogen bonding but do not permit to ascertain that the chemical
shifts measured correspond to the intermolecular arrangements encountered in the helical structure of nucleic acids
other hand,
are
of polynucleotides. On the
the low-field exchangeable NH protons (G-N 1H and
as concerns
T(or U)-N3H) which
or
directly engaged in base pairing, Shulman, Kearns,
Patel and their
coworkers (3)(6-9)(17) have deduced values of their che-
mical shifts in
isolated A-U, G-C and A-T
experimental data
on
pairs from
a
small complementary polynucleotides
combination of or
tRNA fragments
and the theoretical isohielding values produced by ring currents (10). Besides these studies which
were
centered
Watson-Crick complementary bases
(19)
on
the association of
mono-
a
on
the association of the classical
number of data have been produced (18)
and oligonucleotides which refer to
non
classical pairings. In
a
preliminary note published
some
time
ago
(20)
we
have computed
the proton shifts due to hydrogen bonding for the AU and GC pairs and have shown that theory is
able to predict
a
reasonable order of magnitude for
this quantity. Because of the growing interest in the determination of the
contribution of hydrogen bonding to the variation of chemical shifts which are
measured when two
or
poly-stranded helices
ried out the evaluation of triplets and
a
base quartet.
are
formed,
we have now car-
this quantity for a variety of base pairs, base For each type of association we have examined
different geometrical arrangements which
can be
formed with the hydrogen
donating and accepting groups available in the molecules entering the com-
plex. Moreover, in two
cases we have
computed the global chemical shift
variation associated with the formation of the helical structure. The direct introduction of the hydrogen bond effect in this type of calculation which has notbeen done in the previous calculations on this problem (2) (3) (7) (21) (22), should allow a more straightforward and reliable comparison between
theory and experiment.
100
Nucleic Acids Research METHOD AND INPUT DATA
All the values of the variations of the proton chemical shift (Ad) are reported in p.p.m. with respect to the chemical shift of the proton in the isolated molecule. Negative values refer to downfield shifts (deshiel-
ding) and positive ones to
upfield shifts (increased shielding).
The present calculations are made with the approximations presented in a recent study (23) and include three different terms :
A) The ring current effect calculated from quantum mechanical ring current intensities, the spatial dependence of this effect being evaluated by the Johnson-Bovey procedure; B) The contribution of the anisotropy of the atomic diamagnetic sus-
ceptibility tensors, computed within the dipolar approximation; C) The polarization effect which introduces the influence on the polarization of the CH and NH bonds of one molecule by the electric field created by the atomic charges of the other molecules of the complex studied. In our above mentioned preliminary work (20) on the chemical shift va-
riations due to hydrogen bonding term B has not been included; recent results (23)(24) show, however, that this type of contribution may not be
negligeable. four nucleic acid bases considered here we have taken the standard geometries used in our laboratory (25) with CH bonds taken equal 0 0 to 1.08 A and NH bonds to 1.00 A. Unless otherwise mentioned the lengths For the
hydrogen bonds are in all cases taken equal to 2.95 A. Within the approximations utilized in the present work thymine and uracil produce identical results.
of the
RESULTS AND DISCUSSION A. Proton chemical shift variations due to hydrogen bonding. In Table I we
report the
calculated variations
of the proton chemical shifts for a num-
ber of hydrogen bonded complexes between two or more nucleic acid bases. The geometrical arrangements of the different complexes studied are indi-
cated in Fig. 1. As expected, we observe in Table I that the chemical shift variations and large for the protons engaged in the intermolecular downfield are bonds. For the other protons, the calculated variations are much smaller
101
Nucleic Acids Research
(9
Z-X --Z
O=
I,)=( Z..v-=-Z
0
z\ I
T
+
O U
U1
xN
(9
O
z
I
\IZ a-z
7/
\\
\
x
z;\-
>.
C3
z
C)
I z
'. v\1 cc
IJ
I
8\
I z-
/ ar:
102
z
fm =
x
z
I_
^Z-Zc
1.5 p.p.m. in all cases) upfield shift upon helix formation.
The calculations of Patel and Canuel (21) of the sole ring current contribution to the A6 which occurs for the helix-coil transition of poly
preference for a B-helix gave values which in reasonable agreement with the experimental ones for A-C2H but too
(dAdT) while also leading to were
small for
the
a
other CH protons of the AT pair. Our results show that this
discrepancy is due to the fact that for A-C2H the ring current provides the major contribution to A6 while for the other protons of the pair the
112
Nucleic Acids Research TABLE VII Proton chemical shift variation due to helix coil transition in poly (A-U) as a function of the value of the translation and of the rotation angle for position 2 of the helix axis (Fig. 2). Trans- Rotation lation angle 0 A
3.4 3.4
3.0
400 300 400
Uracil
Adenine C H
~~2H
1.32 1.07 1.78
NH (b) NH (a) ~2 ~2
0.77 0.73 0.88
-0.57 -0.58 -0.58
C C8HH
N3H N3H
C5H
C6H
0.37 0.46 0.41
-0.49 -0.15 -0.11
1.36 1.17
0.48 0.57
1.70
0.47
contributions of the local diamagnetic anisotropy and the polarization effects are the important terms in the overall chemical shift variations as
shown in Table VIII for
one of the helices considered.
present set of results which shows a reasonable agreement with the experimental data for poly (dAdT) if we suppose that in solution the double helix formed has a B DNA like structure (33) demonstrates that it The
is possible to calculate directly the A6 due to helix coil transition, within the underlying assumption that the values of the proton chemical shifts for the random coil are close to those of the isolated monomers. As a second example we have chosen the helix coil transition of the four strand helix of Fig. 1 proposed to exist in solution of 5'-guanosine monophosphate by analogy with the structure which has been observed in the crystal of poly(rG) (38). For this type of structure Pinnavaia et al. (19) have calculated that the A6 on G-C 8H due to the ring current effect should
TABLE VI I I Values of the different contributions to the proton chemical shift variation due to helix coil transition in poly (A-U) (translation 3.4 A, rotation 35°, position 2 of the axis). Uracil
Adenine
C2H Total
1.20
C5H
C6H
-0.31
1.28
0.52
NH2(a)
NH2 (b)
C8H
N3H
-0.58
0.75
0.41
Ring current 1.15 contribution of Contribution the atomic diamagnetic sus0.35 ceptibility.
0.48
0.32
0.12
0.87
0.86
0.23
-0.23
0.30
0.14
-0.06
0.27
0.08
Polarization contribution
-0.76
0.14
0.15
-1.12
0.19
0.22
-0.30
113
Nucleic Acids Research not be greater than 0.6
p.p.m..
tions predict for this
proton
For the an
same
helical structure
upfield shift of 0.89
ciably larger value in spite of the fact that the three bases coplanar with the (Table I) which
one
was
studied induce
a
our
p.p.m.,
an
calculaappre-
which
downfield shift of -0.12
are
p.p.m
neglected by Pinnavaia et al.. This result clearly
shows the possible importance of contributions other than the sole
ring
current in the chemical shift variation observed for helix coil transition.
CONCLUSION
The
present work has
tes the feasability of
a
double meaning. On the
hand, it indica-
one
theoretical evaluation of the proton chemical
a
shifts associated with hydrogen bonding between purine and pyrimidine bases,
whether complementary
sents the results of such
or
an
not in the Watson-Crick
evaluation for
a
sense,
and it
number of essential such
pairs. On the other hand, it indicates also the possibility of evaluation of
pre-
a
direct
the proton chemical shifts associated with the formation of
helical structures from random coils without the necessity of decomposing
their value into "intrinsic" (descriptive of isolated base pairs) and due stacking components.
to the
It must, of course, be realized that the computations presented here
pertain to the interactions between free bases, in they produce values of
vacuum.
The fact that
chemical shifts,in particular for the imino protons,
of the order of magnitude of those observed in solution is that the solvent has possibly only
a
moderate effect
on
an
indication
these shifts. The
fact remains to be verified, however, and work is in progress presently in our
of
laboratory in this our
direction. The exceedingly successful utilization
previous computations
complex
as
on
the "ring current" effect in problems
as
the determination of the solution conformation of tRNA's, in
spite of the fact that these computations have also been performed for molecules in due to
a
vacuum,
points to their practical significance. This may be
large extent to their correct estimation of the relative trends
or in solution. The present computation complefor the interpretation of NMR spectra involof data necessary te the set
in A6's, whether in vacuum
ving purine-pyrimidine interactions in
a
homogenous way.
REFERENCES 1 2
Cross, A.D. and Crothers, D. M. (1971) Biochemistry 10, 4015-4023 Kroon, P. A., Kreishman, G. P., Nelson, J. H. and Chan, S. I. (1974)
Biopolymers, 13, 114
2571-2592.
Nucleic Acids Research 3 4 5
6
7 8 9 10
11 12 13 14
15 16 17
18 19
20 21 22 23
Patel, D. J. and Tonelli, A. E. (1974) Biopolymers, 13, 1943-1964. Young, M. A. and Krugh, T. R. (1975) Biochemistry, 14, 4841-4847. Borer, P. N., KAn, L. S. and Ts'o, P. 0. P. (1975) Biochemistry, 14, 4847-4863. Shulman, R.G., Hilbers,C. W., Kearns, D. R., Reid, B. R. and Wong, Y. P. (1973) J. Mol. Biol. 78, 57-89. Lightfoot, D. R., Wong, K. L., Kearns, D. R., Reid, B. R. and Shulman, R. G. (1973) J. Mol. Biol. 78, 71-89. Rordorf, B. F., Kearns, D. R., Hawkins, E. and Chang, S. H. (1976) Biopolymers, 15, 325-336. Robillard, G. T., Hilbers, C. W., Reid, B. R., Gangloff, J., Dirheimer, G. and Shulman, R. G. (1976) Biochemistry, 15, 1883-1888. Giessner-Prettre, C. and Pullman, B. (1970) J. Theoret. Biol. 27, 8795. Shoup, R. R., Miles, H. T. and Becker, E. D. (1966) Biochem. Biophys. Res. Comm. 23, 194-200. Katz, L. and Penman, S. (1966) J. Mol. Biol. 15, 220-231. Katz, L. (1969) J. Mol. Biol. 44, 279-296. Newmark, R.A. and Cantor, C. R. (1968) J. Amer. Chem. Soc. 90, 50105017. Wang, S. M. and Lin, C. (1968) J. Amer. Chem. Soc.90, 5069-5074. Raszka, M. and Kaplan, N. 0. (1972) Proc. Natl. Acad. Sci. U.S.A., 69, 2025-2029. Wong, K. L., Wong, Y. P. and Kearns, D. R. (1975) Biopolymers, 14, 749-762. Kallenbach, N. R., Daniel Jr. W. E. and Kaminker, M. A. (1976) Biochemistry, 15, 1218-1224. Pinnavaia T. J., Miles, H. T. and Becker, E. D. (1975) J. Amer. Chem. Soc. 97, 7198-7200. Giessner-Prettre, C. and Pullman, B. (1970) Compt. Rend. Acad. Sci., 270, 866-868. Patel, D. J. and Canuel, L. (1976) Proc. Natl. Acad. Sci. 73, 674-678. Arter, D. B. and Schmidt, P. G. (1976) Nucleic Acid Res. 3, 1437-1448. Giessner-Prettre, C. and Pullman, B. (1976) J. Theoret. Biol., in press.
24
25 26
27 28 29 30 31 32
33 34 35
Giessner-Prettre, C. and Pullman, B. (1976) Biochem. Biophys. Res. Comm. 70, 578-581. Berthod, H., Giessner-Prettre, C. and Pullman, A. (1965) Theoret. Chim. Acta, 5, 53-68. Robillard, G. T., Tarr, C.E., Vosman, F. and Berendsen, H. J. C. (1976) Nature, 262, 363-369. Engel, J. D. and Von Hippel, P. H. (1974) Biochemistry, 13, 4143-4158. Howard, F. B., Frazier, J., Singer, M. F. and Miles, H. T. (1966) J. Mol. Biol. 16, 415-439. Scruggs, R. L. and Ross, P. D. (1970) J. Mol. Biol. 47, 29-40. Kim, S. H. (1976) in "Progress in Nucleic Acid Research and Molecular Biology" (Cohn, W. E. ed.) Academic Press, 17, 181-216. Paetkau, V., Coulter, M.B., Flintoff, W. F. and Morgan, A. R. (1972) J. Mol. Biol. 71, 293-306. Quigley, G. J., Seeman, N. C., Wang, A. H. J., Suddath, F. L. and Rich A. (1975) Nucleic Acid Res. 2, 2329-2341. Arnott, S. and Hukins, D. W. L. (1972) Biochem. Biophys. Res. Comm. 48, 1392-1399. Arnott, S. and Hukins, D. W. L. (1972) Biochem. Biophys. Res. Comm. 47, 1504-1520. Marvin, D. W., Spencer, M., Wilkins, M. H. F. and Hamilton, L. C. (1961) J. Mol. Biol. 3, 547-565.
115
Nucleic Acids Research 36 37 38
Arnott, S., Chandrasekaran, R., Hukins, D. W. L., Smith, P. J. and Watts, L. (1974) J. Mol. Biol. 88, 523-533. Rosenberg, J. M., Seeman, N. C., Day, R. and Rich, A. (1976) Biochem. Biophys. Res. Comm. 69, 979-987. Zimmerman, S. B., Cohen, G. H. and Davies, D. R. (1975) J. Mol. Biol.
92, 181-192.
116
Nucleic Acids Research
Theoretical study on the proton chemical shifts of hydrogen bonded nucleic acid bases
Claude Giessner-Prettre, Bernard Pullman and Jacqueline Caillet Institut de Biologie Physico-Chimique, Laboratoire de Biochimie Theorique associe au C.N.R.S., 13, rue P.et M.Curie, 75005 Paris, France
Received 20 September 1976 ABSTRACT The variation of the proton chemical shifts due to the formation of intermolecular hydrogen bonds is computed for a number of complexes which can be formed between the bases of the nucleic acids. The shifts expected for the isolated base pairs, in particular for the G-N H, T(or U)-N3H protons and the protons of the amino groups of A, G and &, when combined with previous computations on the shifts to be expected upon base stacking, may enable a refined analysis of the high resolution NMR spectra of self complementary polynucleotides or tRNAs. Two examples are presented of a direct computation of proton shifts associated with helix-coil transitions, helpful for deducing the helical structure in solution.
INTRODUCTION
The formation of helical structures which occurs in appropriate expe-
rimental conditions between different units of complementary polynucleotides or different parts of single stranded nucleic acids (ex. tRNA) involves the establishment of hydrogen bonds between pairs of bases as well as of stacks of bases or base-pairs between the neighbouring units of the strands. In proton magnetic resonance spectra, hydrogen bond formation induces a large downfield shift for the protons participating in the intermolecular bond while stacking of conjugated molecules such as the nucleic acid bases is accompanied by an upfield shift of their protons. From the proton NMR study of helix-coil transitions in solution, only the sum of the two effects
experimentally. In the numerous studies by this technique of the secondary structure of complementary polynucleotides (1-5) or of tRNAs (6-9) the experimentalists have made ample use of the theoretical predictions (10) concerning the upfield shifts to be expected upon base stacking is accessible
the ring current effect of the bases located above and below the base or base-pair under investigation. The complete analysis of the experi-
due to
C Information Retrieval Limited 1 Falconberg Court London Wl V 5FG England
99
Nucleic Acids Research mental results and the assignment
of
resonances
to specific base-pairs
necessitates, however, the knowledge of the chemical shifts associated with the "free" hydrogen-bonded base-pairs, unperturbed by stacking with their neighbors. There have been
few NMR studies
a
the association by hydrogen bon-
on
ding of complementary base-pairs (11), nucleosides (12-15)
and nucleotides
(16). These results indicate variations of proton chemical shifts attributable to
hydrogen bonding but do not permit to ascertain that the chemical
shifts measured correspond to the intermolecular arrangements encountered in the helical structure of nucleic acids
other hand,
are
of polynucleotides. On the
the low-field exchangeable NH protons (G-N 1H and
as concerns
T(or U)-N3H) which
or
directly engaged in base pairing, Shulman, Kearns,
Patel and their
coworkers (3)(6-9)(17) have deduced values of their che-
mical shifts in
isolated A-U, G-C and A-T
experimental data
on
pairs from
a
small complementary polynucleotides
combination of or
tRNA fragments
and the theoretical isohielding values produced by ring currents (10). Besides these studies which
were
centered
Watson-Crick complementary bases
(19)
on
the association of
mono-
a
on
the association of the classical
number of data have been produced (18)
and oligonucleotides which refer to
non
classical pairings. In
a
preliminary note published
some
time
ago
(20)
we
have computed
the proton shifts due to hydrogen bonding for the AU and GC pairs and have shown that theory is
able to predict
a
reasonable order of magnitude for
this quantity. Because of the growing interest in the determination of the
contribution of hydrogen bonding to the variation of chemical shifts which are
measured when two
or
poly-stranded helices
ried out the evaluation of triplets and
a
base quartet.
are
formed,
we have now car-
this quantity for a variety of base pairs, base For each type of association we have examined
different geometrical arrangements which
can be
formed with the hydrogen
donating and accepting groups available in the molecules entering the com-
plex. Moreover, in two
cases we have
computed the global chemical shift
variation associated with the formation of the helical structure. The direct introduction of the hydrogen bond effect in this type of calculation which has notbeen done in the previous calculations on this problem (2) (3) (7) (21) (22), should allow a more straightforward and reliable comparison between
theory and experiment.
100
Nucleic Acids Research METHOD AND INPUT DATA
All the values of the variations of the proton chemical shift (Ad) are reported in p.p.m. with respect to the chemical shift of the proton in the isolated molecule. Negative values refer to downfield shifts (deshiel-
ding) and positive ones to
upfield shifts (increased shielding).
The present calculations are made with the approximations presented in a recent study (23) and include three different terms :
A) The ring current effect calculated from quantum mechanical ring current intensities, the spatial dependence of this effect being evaluated by the Johnson-Bovey procedure; B) The contribution of the anisotropy of the atomic diamagnetic sus-
ceptibility tensors, computed within the dipolar approximation; C) The polarization effect which introduces the influence on the polarization of the CH and NH bonds of one molecule by the electric field created by the atomic charges of the other molecules of the complex studied. In our above mentioned preliminary work (20) on the chemical shift va-
riations due to hydrogen bonding term B has not been included; recent results (23)(24) show, however, that this type of contribution may not be
negligeable. four nucleic acid bases considered here we have taken the standard geometries used in our laboratory (25) with CH bonds taken equal 0 0 to 1.08 A and NH bonds to 1.00 A. Unless otherwise mentioned the lengths For the
hydrogen bonds are in all cases taken equal to 2.95 A. Within the approximations utilized in the present work thymine and uracil produce identical results.
of the
RESULTS AND DISCUSSION A. Proton chemical shift variations due to hydrogen bonding. In Table I we
report the
calculated variations
of the proton chemical shifts for a num-
ber of hydrogen bonded complexes between two or more nucleic acid bases. The geometrical arrangements of the different complexes studied are indi-
cated in Fig. 1. As expected, we observe in Table I that the chemical shift variations and large for the protons engaged in the intermolecular downfield are bonds. For the other protons, the calculated variations are much smaller
101
Nucleic Acids Research
(9
Z-X --Z
O=
I,)=( Z..v-=-Z
0
z\ I
T
+
O U
U1
xN
(9
O
z
I
\IZ a-z
7/
\\
\
x
z;\-
>.
C3
z
C)
I z
'. v\1 cc
IJ
I
8\
I z-
/ ar:
102
z
fm =
x
z
I_
^Z-Zc
1.5 p.p.m. in all cases) upfield shift upon helix formation.
The calculations of Patel and Canuel (21) of the sole ring current contribution to the A6 which occurs for the helix-coil transition of poly
preference for a B-helix gave values which in reasonable agreement with the experimental ones for A-C2H but too
(dAdT) while also leading to were
small for
the
a
other CH protons of the AT pair. Our results show that this
discrepancy is due to the fact that for A-C2H the ring current provides the major contribution to A6 while for the other protons of the pair the
112
Nucleic Acids Research TABLE VII Proton chemical shift variation due to helix coil transition in poly (A-U) as a function of the value of the translation and of the rotation angle for position 2 of the helix axis (Fig. 2). Trans- Rotation lation angle 0 A
3.4 3.4
3.0
400 300 400
Uracil
Adenine C H
~~2H
1.32 1.07 1.78
NH (b) NH (a) ~2 ~2
0.77 0.73 0.88
-0.57 -0.58 -0.58
C C8HH
N3H N3H
C5H
C6H
0.37 0.46 0.41
-0.49 -0.15 -0.11
1.36 1.17
0.48 0.57
1.70
0.47
contributions of the local diamagnetic anisotropy and the polarization effects are the important terms in the overall chemical shift variations as
shown in Table VIII for
one of the helices considered.
present set of results which shows a reasonable agreement with the experimental data for poly (dAdT) if we suppose that in solution the double helix formed has a B DNA like structure (33) demonstrates that it The
is possible to calculate directly the A6 due to helix coil transition, within the underlying assumption that the values of the proton chemical shifts for the random coil are close to those of the isolated monomers. As a second example we have chosen the helix coil transition of the four strand helix of Fig. 1 proposed to exist in solution of 5'-guanosine monophosphate by analogy with the structure which has been observed in the crystal of poly(rG) (38). For this type of structure Pinnavaia et al. (19) have calculated that the A6 on G-C 8H due to the ring current effect should
TABLE VI I I Values of the different contributions to the proton chemical shift variation due to helix coil transition in poly (A-U) (translation 3.4 A, rotation 35°, position 2 of the axis). Uracil
Adenine
C2H Total
1.20
C5H
C6H
-0.31
1.28
0.52
NH2(a)
NH2 (b)
C8H
N3H
-0.58
0.75
0.41
Ring current 1.15 contribution of Contribution the atomic diamagnetic sus0.35 ceptibility.
0.48
0.32
0.12
0.87
0.86
0.23
-0.23
0.30
0.14
-0.06
0.27
0.08
Polarization contribution
-0.76
0.14
0.15
-1.12
0.19
0.22
-0.30
113
Nucleic Acids Research not be greater than 0.6
p.p.m..
tions predict for this
proton
For the an
same
helical structure
upfield shift of 0.89
ciably larger value in spite of the fact that the three bases coplanar with the (Table I) which
one
was
studied induce
a
our
p.p.m.,
an
calculaappre-
which
downfield shift of -0.12
are
p.p.m
neglected by Pinnavaia et al.. This result clearly
shows the possible importance of contributions other than the sole
ring
current in the chemical shift variation observed for helix coil transition.
CONCLUSION
The
present work has
tes the feasability of
a
double meaning. On the
hand, it indica-
one
theoretical evaluation of the proton chemical
a
shifts associated with hydrogen bonding between purine and pyrimidine bases,
whether complementary
sents the results of such
or
an
not in the Watson-Crick
evaluation for
a
sense,
and it
number of essential such
pairs. On the other hand, it indicates also the possibility of evaluation of
pre-
a
direct
the proton chemical shifts associated with the formation of
helical structures from random coils without the necessity of decomposing
their value into "intrinsic" (descriptive of isolated base pairs) and due stacking components.
to the
It must, of course, be realized that the computations presented here
pertain to the interactions between free bases, in they produce values of
vacuum.
The fact that
chemical shifts,in particular for the imino protons,
of the order of magnitude of those observed in solution is that the solvent has possibly only
a
moderate effect
on
an
indication
these shifts. The
fact remains to be verified, however, and work is in progress presently in our
of
laboratory in this our
direction. The exceedingly successful utilization
previous computations
complex
as
on
the "ring current" effect in problems
as
the determination of the solution conformation of tRNA's, in
spite of the fact that these computations have also been performed for molecules in due to
a
vacuum,
points to their practical significance. This may be
large extent to their correct estimation of the relative trends
or in solution. The present computation complefor the interpretation of NMR spectra involof data necessary te the set
in A6's, whether in vacuum
ving purine-pyrimidine interactions in
a
homogenous way.
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