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Molecular orbital calculations on anti-viral agents
Journal of Molecular Structure (Theochem), 207 (1990) 15-22 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MOLECULAR AGENTS
ORBITAL CALCULATIONS
15
ON ANTI-VIRAL
L.S. FLORA and P.C. YATES* Pharmaceutical Sciences Institute, Department of Pharmaceutical Sciences, Aston University, Aston Triangle, Birmingham B4 7ET (Gt. Britain) (Received
17 July 1989)
ABSTRACT Ab initio molecular-orbital calculations have been performed on isomers of the anti-viral agents pyrazofurin, bredinin and ribavirin, AICA riboside which is an intermediate in purine biosynthesis, and the puke base guanine. Ribavirin is predicted to have the highest energy of these compounds and guanine the lowest, the energies of different isomers being very close for each compound. Charge distributions differ considerably between these isomers, and in the case of the anti-viral agents agree well with those in both AICA riboside and in guanine. Differences in geometry between isomers are most pronounced in the carboxamide region, which tends to twist out of the plane of the rest of the molecule. The orientation of the hydroxyl groups in both pyrazofurin and bredinin leads to significant changes in bond length which in the latter compound also affect the geometry of the ring. Bond lengths in the optimised conformations of ribavirin and guanine differ considerably from those in the corresponding crystal structures.
INTRODUCTION
Despite advances in medicine and the availability of new drugs, the development of anti-viral agents has been relatively slow. Viral diseases such as measles and respiratory illness continue to be prevalent in developing countries, while elsewhere herpes and AIDS are just two which have become widespread in recent years. There is thus a considerable need for the development of new drugs in this area [ 11, which may be assisted by a better understanding of the properties and action of those agents with proven anti-viral activity. A central nucleotide intermediate in the biosynthesis of purines is 5-aminol- (P-D-ribofuranosyl) imidazole-4-carboxamide 5’ -phosphate (AICAR). The corresponding nucleoside, 5-amino-l- (P-D-ribofuranosyl) imidazole-4-carboxamide (AICA riboside, I) bears a close structural resemblance to a number of existing anti-viral agents. *Present address: Department Gt. Britain.
0166-1280/90/$03.50
of Chemistry,
University
of Keele, Keele, Staffordshire
0 1990Elsevier Science Publishers
B.V.
ST5 5BG,
16
H
9
IW):
HC
N,N H
v:
HC
Fig. 1. Structural isomers of the compounds studied in this work: I, AICA riboside; II, pyrazofurin; III, bredinin; IV, ribavirin; V, guanine. The atomic numbering scheme for each molecule is similar to that shown for V.
17
Pyrazofurin (II) is a naturally occurring purine C-nucleoside which shows a broad spectrum of activity against both DNA and RNA viruses [ 21, but has not been developed as an anti-viral agent because of its high toxicity. The corresponding nucleotide is an inhibitor of both orotidylic acid decarboxylase [ 3 ] and AICAR formyltransferase [ 41. These are involved in the biosynthesis of pyrimidine and purine nucleotides, respectively. Bredinin (III) and l-(P-D-ribofuranosyl)-1,2,4-triazole-3-carboxamide (ribavirin, IV) are both synthetic purine nucleoside analogues. The corresponding nucleotide of bredinin inhibits guanine biosynthesis [ 51, while that of ribavirin [6] interferes with purine biosynthesis by the inhibition of IMP dehydrogenase, AICA ribotide transformylase and adenylosuccinase. In the present paper we describe molecular-orbital calculations on various isomers of these four compounds and also on the purine base guanine (V ) and compare their optimised geometries and charge distributions. The crystal structures of AICA riboside [ 71, ribavirin [ 81 and guanine [ 91 are also corn. pared with the corresponding optimised structures. EXPERIMENTAL
Differences in the structure and charge distribution of the five compounds are likely to be small. Therefore we chose to use an ab initio molecular-orbital treatment. However, due to considerations of computer memory and speed, our calculations only treat the base in each molecule; the ribose moiety was replaced’by a single hydrogen atom in each case. A number of structural isomers of these relatively rigid fragments exist for each molecule; those we have considered are shown in Fig. 1. TABLE 1 Summary of the results of the molecular-orbital calculations Compound IA IB IIA IIB IIC IID IIIA IIIB IIIC IVA IVB V
Et
E,
E*
HOMO
p
UW
U%J
(Ed
WiJ
(D)
-441.87 -441.88 -461.35 -461.35 - 461.37 - 461.36 -461.47 -461.41 -461.34 - 403.30 - 403.30 - 532.47
- 887.59 - 878.40 - 898.98 - 898.68 - 898.86 - 898.38 - 899.57 - 902.63 -898.11 - 753.29 - 752.76 - 1119.26
435.72 436.52 437.63 437.33 437.49 437.02 438.10 441.22 436.77 349.99. 349.46 586.79
1.424 1.430 1.094 1.111 1.095 1.098 1.319 1.329 1.402 1.150 1.066 1.187
4.44 4.02 3.99 3.21 2.61 2.67 3.52 3.22 6.54 4.37 3.79 4.75
n
t (s)
68 61 1 1 1 1 57 53 28 1 1 33
76907
67222 760 757 773 768 59814 55080 29700 1518 503 65139
-0.435 -0.277 0.101 0.023 0.313 -0.233 -0.121 -0.033 -0.290 0.193 0.210 0.230 0.228 0.093
-0.410 -0.390 0.200 -0.016 -0.266 0.125 -0.318 -0.293 0.178 0.197 0.211 0.175 0.088 0.232
Nl N3 c4 c5 N7 C8 N9 010 Hll H12 H31 H32 H8 H9
Nl 03 c4 C5 C6 N7 N8 C9 010 HlOl H102 H3 H7 H9
Atom IIA
IB
Atom IA
-0.416 -0.396 0.195 -0.013 -0.250 0.121 -0.320 -0.243 0.171 0.169 0.214 0.173 0.087 0.229
Pyrazofurin, II
AICA riboside, I
Atomic charge distributions
TABLE 2
-0.262 0.189 0.207 0.221 0.229 0.087
-0.019
-0.439 -0.297 0.092 0.052 0.311 -0.234 -0.137
IIB -0.441 -0.308 0.075 0.057 0.311 -0.226 -0.138 -0.013 -0.278 0.212 0.202 0.218 0.248 0.081
IIC -0.440 -0.269 0.083 0.037 0.310 -0.245 -0.126 -0.011 -0.270 0.181 0.208 0.198 0.257 0.087
IID Nl 03 C4 c5 C6 N7 C8 N9 010 Hll H12 H3 H8 H9 H7
-0.426 -0.301 0.228 -0.032 0.279 -0.248 0.120 -0.323 -0.235 0.172 0.177 0.265 0.087 0.235
Atom IIIA
Bredinin, III
-0.408 -0.302 0.235 -0.042 0.304 -0.258 0.123 -0.323 -0.314 0.190 0.202 0.269 0.088 0.237
IIIB
0.121 0.231 0.266
-0.418 -0.347 0.276 -0.056 0.280 -0.263 0.194 -0.327 -0.418 0.193 0.169
IIIC Nl N4 C5 C6 N7 C8 N9 010 Hll H12 H8 H9
-0.408 -0.145 0.123 0.296 -0.253 0.137 -0.236 -0.238 0.188 0.182 0.247 0.106
Atom IVA
Ribavirin, IV
-0.429 -0.127 0.124 0.310 -0.270 0.137 -0.235 -0.259 0.195 0.206 0.248 0.104
IVB
Nl C2 N3 c4 C5 C6 N7 C8 N9 010 Nil Hlll H112 Hl H8 H9
- 0.360 0.350 - 0.320 0.183 - 0.009 0.310 - 0.248 0.129 -0.315 - 0.263 - 0.388 0.203 0.185 0.217 0.089 0.237
Atom V
Guanine, V
19
Initial atomic coordinates for all molecules except IV were obtained by building the appropriate structure using standard geometries with the ChemX modelling system [lo]. These coordinates were then subjected to full geometry optimisation using the GAUSSIAN 80 program [ll] accessed from within Chem-X. Coordinates for ribavirin were taken from a previous study using a modified version of the GAUSSIAN 70 program [ 121. As these coordinates had already been optimised, a fixed-point calculation was performed on each isomer using GAUSSIAN 80. The STO-3G basis set was used in each calculation. RESULTS
A summary of the results of the molecular-orbital calculations is given in Table 1, showing total energy, E,, electronic energy, E,, nuclear energy, E,, the energy of the highest occupied molecular orbital (HOMO), dipole moment (p) , number of cycles (n) required for convergence, and total CPU time t for each molecule. Table 2 shows the charge distribution for each isomer considered. (Full details of each geometry are given in the supplementary publication, available from B.L.L.D. as SUP 26384 (16 pages). ) DISCUSSION
Of the compounds studied, Table 1 shows that isolated base guanine (V) is the most stable, while ribavirin (IV) is the least stable. A more detailed examination shows that ribavirin has the most stable nuclear structure and guanine the least; the opposite, however, is true for the electronic structures. As might be expected, the only charged species (IIIC ) has the highest dipole moment. AICA riboside (I) has the highest HOMO and thus would be expected to form an ion most readily out of this group of compounds. The isomers of each compound do show some significant differences in charge distribution. The carbonyl oxygen (01) is rather more negative in IB than it is in IA, and this is compensated by a higher positive charge on HlOl and H102. The remaining atoms have very similar partial charges. More variation is apparent on the atoms in pyrazofurin (II), the largest variations (of up to 0.039e) occurring on 03, C5, H3 and HlOl. The charged species of bredinin (IIIC ) shows a large variation in charge distribution from the uncharged molecules IIIA and IIIB. These are very similar except for the carbonyl oxygen 01 which has charges of -0.235 and -0.314, respectively. The ribavirin molecules IVA and IVB are very different, with N4, N7, NlO, HlOl and H102 showing significant variations. A comparison of the charge distribution of each drug molecule with the appropriate isomer of AICA riboside and with guanine may provide information on its mode of action. In the case of pyrazofurin, isomers IIB and IID are very
20
similar in the carboxamide region to IA, while charges on the opposite side of the heterocyclic ring are quite different. The same is true for isomers IIA and IIC with IB. The charges on N8 (pyrazofurin) and C8 (AICA riboside) are equal in magnitude but of opposite sign. The same general observations are also made when comparing IIB and IID with guanine (V), but in the case of IID we also see disagreement at 03. The bredinin isomers IIIA and IIIB show excellent agreement with the charges on IA and IB, respectively; while this would be expected over most of the molecule because of the identical ring substitution patterns, it is interesting that the hydroxyl group in III is able to mimic the amine in I. The extent of agreement between the charged molecule IIIC and IA is less, but nevertheless all charges do agree within O.le. Good agreement is also obtained when IIIA and IIIC are compared with V. The carboxamide moiety of ribavirin isomers IVA and IVB have close charge distributions to those of IA and IB, respectively. However, the ring substitution patterns are quite different, and this leads to significant differences in charge on N4, C5, H8 and H9. Similar differences are apparent between IVA and V. The most pronounced difference in the geometry of the two conformations of AICA riboside occurs in the carboxamide region; the Nl-C6 bond length in IA is 1.469 A while in IB it is 1.432 A. In the corresponding crystal structure the value is even shorter at 1.333 (6) A. Torsion angles around the C&C6 bond are, by definition, different in the two isomers, but IA shows the greatest deviation from planarity with C4-C5-C6-Nl being 20”, C4-C5-C6-010 157” and N7-C5-C6-010 20’. The corresponding values in IB are - 167”, 10” and -170”, while in the crystal they are 175.7(4)“, 175.7(4)’ and 176.3(4)“. It thus appears that packing forces play a considerable part in determining the crystal structure of AICA riboside, with a slight deviation from planarity being preferred in the free molecules. From the torsion-angle values obtained, it is clear that the crystal structure corresponds to isomer IB, although as the data in Table 1 show there is very little difference in energy between the two conformations. There is little variation in the magnitude of the structural parameters of the isomers of pyrazofurin (II), except where these arise as a result of the initial definition of geometry. However, one notable exception is in the 03-C4 bond length which is 1.386, 1.355, 1.355 and 1.388 A in isomers IIA, IIB, IIC and IID, respectively. It thus appears that the orientation of the hydroxyl group, which is the same in IIA and IID and also in IIB and IIC may play an important part in determining the overall structure. This variation in the C-O (hydroxyl) bond length is also pronounced in bredinin (III). Values for 03-C4 are 1.370,1.366 and 1.230 A in isomers IIIA, IIIB and IIIC, respectively. However, there is now also a corresponding variation in C4-N9 which takes values of 1.382, 1.380 and 1.477 A in the three
21
isomers. In general, bond lengths in the charged species IIIC are rather different from those in IIIA and IIIB, which is not too surprising. One notable difference between IIIA and IIIB in the carboxamide region is the Nl-C6 length which has values of 1.487 and 1.419 A, respectively. The largest difference between angles in this section of the two isomers is for C5-C6-010 which is 128.5” in IIIA and 118.8” in IIIB. Isomer IIIA shows the greatest deviation from planarity, with torsion angles of - 18” for C4-C5-0%Nl, 157” for C4C5--C6-010 and - 18” for N7-C5-C6-010. In IIIB and IIIC all torsion angles are within 10” of 0” or 180”. Bond lengths in the two isomers of ribavirin (IV) are very similar, but greater variations are seen between the two conformations determined from the crystal structure. The two sets of values also differ, indicating that a considerable relaxation of the molecules has occurred during the quantum-mechanical optimisation. The greatest difference in bond angles occurs at the carboxamide carbon, C5-C6-010 being 122.1’ for IVA and 117.8” for IVB. There is little variation in bond angles between crystal conformations, and generally there is good agreement between the crystal and optimised angles. Torsion angle values show that both crystal conformations are generally similar to that of IVA, although they are slightly different as evidenced by values of 1.5” and 14.7”, respectively, for N4-C5-C6-Nl, the corresponding values of N7-C5-C6-010 being 1.3” and 14.2”, respectively. There is thus a considerable twist of the carboxamide group from planarity in the latter conformation. The optimised structure of guanine (V) has considerably different bond lengths from those observed in the crystal structure. The greatest deviation is for C2-Nll which is 1.334 A in the crystal but optimises to 1.431 A. The lengths of Nl-C6, N3-C4 and C5-C6 also show differences of greater than 0.05 A. Bond angles and torsion angles show good agreement between optimised and crystal structures. It is thus apparent that a number of ~fferen~es in geometry may result between isomers of a particular compound. These may well be important in determining the mode of action of the anti-viral agents considered here. From a comparison of charge distributions, we conclude that pyrazofurin, bredinin and ribavirin may all be able to mimic the electrostatic properties of both AICA riboside and guanine. ACKNOWLEDGEMENTS
We would like to thank Dr. C.H. Schwalbe for helpful discussions during this work.
22 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
R.J. Whitley, in M.R. Harnden (Ed.), Approaches to Antiviral Agents, Macmillan, Basingstake, 1985, p. 3. M.R. Harnden and D.N. Planterose, in M.R. Harnden (Ed.), Approaches to Antiviral Agents, Macmillan, Basingstoke, 1985, p. 153. G.E. Gutowski, M.J. Sweeney, D.C. DeLong, R.L. Hamill, K. Gerzon and R.W. Dyke, Ann. N.Y. Acad. Sci., 255 (1975) 544. J.F. Wonalla and M.J. Sweeney, Cancer Res., 40 (1980) 1482. P.C. Srivastava, D.G. Streeter, T.R. Matthews, L.B. Allen, R.W. Sidwell and R.K. Robins, J. Med. Chem., 19 (1976) 1020. R.K. Smith and W. Kirkpatrick (Eds.), Ribavirin - A Broad Spectrum Antiviral Agent, Academic Press, New York, 1980. D.A. Adamiak and W. Saenger, Acta Crystallogr., Sect. B, 35 (1979) 924. P. Prusiner and M. Sundralingham, Acta Crystallogr., Sect. B, 32 (1976) 419. U. Thewalt, C.E. Bugg and R.E. Marsh, Acta Crystallogr., Sect. B, 27 (1971) 2358. Chem-X, developed and distributed by Chemical Design Ltd., Oxford, Gt. Britain. U. Chandra Singh and P. Kollman, GAUSSIAN 80, QCPE Bull., 2 (1982) 117. A.P. Burgess and C.H. Schwalbe, unpublished work.
MOLECULAR AGENTS
ORBITAL CALCULATIONS
15
ON ANTI-VIRAL
L.S. FLORA and P.C. YATES* Pharmaceutical Sciences Institute, Department of Pharmaceutical Sciences, Aston University, Aston Triangle, Birmingham B4 7ET (Gt. Britain) (Received
17 July 1989)
ABSTRACT Ab initio molecular-orbital calculations have been performed on isomers of the anti-viral agents pyrazofurin, bredinin and ribavirin, AICA riboside which is an intermediate in purine biosynthesis, and the puke base guanine. Ribavirin is predicted to have the highest energy of these compounds and guanine the lowest, the energies of different isomers being very close for each compound. Charge distributions differ considerably between these isomers, and in the case of the anti-viral agents agree well with those in both AICA riboside and in guanine. Differences in geometry between isomers are most pronounced in the carboxamide region, which tends to twist out of the plane of the rest of the molecule. The orientation of the hydroxyl groups in both pyrazofurin and bredinin leads to significant changes in bond length which in the latter compound also affect the geometry of the ring. Bond lengths in the optimised conformations of ribavirin and guanine differ considerably from those in the corresponding crystal structures.
INTRODUCTION
Despite advances in medicine and the availability of new drugs, the development of anti-viral agents has been relatively slow. Viral diseases such as measles and respiratory illness continue to be prevalent in developing countries, while elsewhere herpes and AIDS are just two which have become widespread in recent years. There is thus a considerable need for the development of new drugs in this area [ 11, which may be assisted by a better understanding of the properties and action of those agents with proven anti-viral activity. A central nucleotide intermediate in the biosynthesis of purines is 5-aminol- (P-D-ribofuranosyl) imidazole-4-carboxamide 5’ -phosphate (AICAR). The corresponding nucleoside, 5-amino-l- (P-D-ribofuranosyl) imidazole-4-carboxamide (AICA riboside, I) bears a close structural resemblance to a number of existing anti-viral agents. *Present address: Department Gt. Britain.
0166-1280/90/$03.50
of Chemistry,
University
of Keele, Keele, Staffordshire
0 1990Elsevier Science Publishers
B.V.
ST5 5BG,
16
H
9
IW):
HC
N,N H
v:
HC
Fig. 1. Structural isomers of the compounds studied in this work: I, AICA riboside; II, pyrazofurin; III, bredinin; IV, ribavirin; V, guanine. The atomic numbering scheme for each molecule is similar to that shown for V.
17
Pyrazofurin (II) is a naturally occurring purine C-nucleoside which shows a broad spectrum of activity against both DNA and RNA viruses [ 21, but has not been developed as an anti-viral agent because of its high toxicity. The corresponding nucleotide is an inhibitor of both orotidylic acid decarboxylase [ 3 ] and AICAR formyltransferase [ 41. These are involved in the biosynthesis of pyrimidine and purine nucleotides, respectively. Bredinin (III) and l-(P-D-ribofuranosyl)-1,2,4-triazole-3-carboxamide (ribavirin, IV) are both synthetic purine nucleoside analogues. The corresponding nucleotide of bredinin inhibits guanine biosynthesis [ 51, while that of ribavirin [6] interferes with purine biosynthesis by the inhibition of IMP dehydrogenase, AICA ribotide transformylase and adenylosuccinase. In the present paper we describe molecular-orbital calculations on various isomers of these four compounds and also on the purine base guanine (V ) and compare their optimised geometries and charge distributions. The crystal structures of AICA riboside [ 71, ribavirin [ 81 and guanine [ 91 are also corn. pared with the corresponding optimised structures. EXPERIMENTAL
Differences in the structure and charge distribution of the five compounds are likely to be small. Therefore we chose to use an ab initio molecular-orbital treatment. However, due to considerations of computer memory and speed, our calculations only treat the base in each molecule; the ribose moiety was replaced’by a single hydrogen atom in each case. A number of structural isomers of these relatively rigid fragments exist for each molecule; those we have considered are shown in Fig. 1. TABLE 1 Summary of the results of the molecular-orbital calculations Compound IA IB IIA IIB IIC IID IIIA IIIB IIIC IVA IVB V
Et
E,
E*
HOMO
p
UW
U%J
(Ed
WiJ
(D)
-441.87 -441.88 -461.35 -461.35 - 461.37 - 461.36 -461.47 -461.41 -461.34 - 403.30 - 403.30 - 532.47
- 887.59 - 878.40 - 898.98 - 898.68 - 898.86 - 898.38 - 899.57 - 902.63 -898.11 - 753.29 - 752.76 - 1119.26
435.72 436.52 437.63 437.33 437.49 437.02 438.10 441.22 436.77 349.99. 349.46 586.79
1.424 1.430 1.094 1.111 1.095 1.098 1.319 1.329 1.402 1.150 1.066 1.187
4.44 4.02 3.99 3.21 2.61 2.67 3.52 3.22 6.54 4.37 3.79 4.75
n
t (s)
68 61 1 1 1 1 57 53 28 1 1 33
76907
67222 760 757 773 768 59814 55080 29700 1518 503 65139
-0.435 -0.277 0.101 0.023 0.313 -0.233 -0.121 -0.033 -0.290 0.193 0.210 0.230 0.228 0.093
-0.410 -0.390 0.200 -0.016 -0.266 0.125 -0.318 -0.293 0.178 0.197 0.211 0.175 0.088 0.232
Nl N3 c4 c5 N7 C8 N9 010 Hll H12 H31 H32 H8 H9
Nl 03 c4 C5 C6 N7 N8 C9 010 HlOl H102 H3 H7 H9
Atom IIA
IB
Atom IA
-0.416 -0.396 0.195 -0.013 -0.250 0.121 -0.320 -0.243 0.171 0.169 0.214 0.173 0.087 0.229
Pyrazofurin, II
AICA riboside, I
Atomic charge distributions
TABLE 2
-0.262 0.189 0.207 0.221 0.229 0.087
-0.019
-0.439 -0.297 0.092 0.052 0.311 -0.234 -0.137
IIB -0.441 -0.308 0.075 0.057 0.311 -0.226 -0.138 -0.013 -0.278 0.212 0.202 0.218 0.248 0.081
IIC -0.440 -0.269 0.083 0.037 0.310 -0.245 -0.126 -0.011 -0.270 0.181 0.208 0.198 0.257 0.087
IID Nl 03 C4 c5 C6 N7 C8 N9 010 Hll H12 H3 H8 H9 H7
-0.426 -0.301 0.228 -0.032 0.279 -0.248 0.120 -0.323 -0.235 0.172 0.177 0.265 0.087 0.235
Atom IIIA
Bredinin, III
-0.408 -0.302 0.235 -0.042 0.304 -0.258 0.123 -0.323 -0.314 0.190 0.202 0.269 0.088 0.237
IIIB
0.121 0.231 0.266
-0.418 -0.347 0.276 -0.056 0.280 -0.263 0.194 -0.327 -0.418 0.193 0.169
IIIC Nl N4 C5 C6 N7 C8 N9 010 Hll H12 H8 H9
-0.408 -0.145 0.123 0.296 -0.253 0.137 -0.236 -0.238 0.188 0.182 0.247 0.106
Atom IVA
Ribavirin, IV
-0.429 -0.127 0.124 0.310 -0.270 0.137 -0.235 -0.259 0.195 0.206 0.248 0.104
IVB
Nl C2 N3 c4 C5 C6 N7 C8 N9 010 Nil Hlll H112 Hl H8 H9
- 0.360 0.350 - 0.320 0.183 - 0.009 0.310 - 0.248 0.129 -0.315 - 0.263 - 0.388 0.203 0.185 0.217 0.089 0.237
Atom V
Guanine, V
19
Initial atomic coordinates for all molecules except IV were obtained by building the appropriate structure using standard geometries with the ChemX modelling system [lo]. These coordinates were then subjected to full geometry optimisation using the GAUSSIAN 80 program [ll] accessed from within Chem-X. Coordinates for ribavirin were taken from a previous study using a modified version of the GAUSSIAN 70 program [ 121. As these coordinates had already been optimised, a fixed-point calculation was performed on each isomer using GAUSSIAN 80. The STO-3G basis set was used in each calculation. RESULTS
A summary of the results of the molecular-orbital calculations is given in Table 1, showing total energy, E,, electronic energy, E,, nuclear energy, E,, the energy of the highest occupied molecular orbital (HOMO), dipole moment (p) , number of cycles (n) required for convergence, and total CPU time t for each molecule. Table 2 shows the charge distribution for each isomer considered. (Full details of each geometry are given in the supplementary publication, available from B.L.L.D. as SUP 26384 (16 pages). ) DISCUSSION
Of the compounds studied, Table 1 shows that isolated base guanine (V) is the most stable, while ribavirin (IV) is the least stable. A more detailed examination shows that ribavirin has the most stable nuclear structure and guanine the least; the opposite, however, is true for the electronic structures. As might be expected, the only charged species (IIIC ) has the highest dipole moment. AICA riboside (I) has the highest HOMO and thus would be expected to form an ion most readily out of this group of compounds. The isomers of each compound do show some significant differences in charge distribution. The carbonyl oxygen (01) is rather more negative in IB than it is in IA, and this is compensated by a higher positive charge on HlOl and H102. The remaining atoms have very similar partial charges. More variation is apparent on the atoms in pyrazofurin (II), the largest variations (of up to 0.039e) occurring on 03, C5, H3 and HlOl. The charged species of bredinin (IIIC ) shows a large variation in charge distribution from the uncharged molecules IIIA and IIIB. These are very similar except for the carbonyl oxygen 01 which has charges of -0.235 and -0.314, respectively. The ribavirin molecules IVA and IVB are very different, with N4, N7, NlO, HlOl and H102 showing significant variations. A comparison of the charge distribution of each drug molecule with the appropriate isomer of AICA riboside and with guanine may provide information on its mode of action. In the case of pyrazofurin, isomers IIB and IID are very
20
similar in the carboxamide region to IA, while charges on the opposite side of the heterocyclic ring are quite different. The same is true for isomers IIA and IIC with IB. The charges on N8 (pyrazofurin) and C8 (AICA riboside) are equal in magnitude but of opposite sign. The same general observations are also made when comparing IIB and IID with guanine (V), but in the case of IID we also see disagreement at 03. The bredinin isomers IIIA and IIIB show excellent agreement with the charges on IA and IB, respectively; while this would be expected over most of the molecule because of the identical ring substitution patterns, it is interesting that the hydroxyl group in III is able to mimic the amine in I. The extent of agreement between the charged molecule IIIC and IA is less, but nevertheless all charges do agree within O.le. Good agreement is also obtained when IIIA and IIIC are compared with V. The carboxamide moiety of ribavirin isomers IVA and IVB have close charge distributions to those of IA and IB, respectively. However, the ring substitution patterns are quite different, and this leads to significant differences in charge on N4, C5, H8 and H9. Similar differences are apparent between IVA and V. The most pronounced difference in the geometry of the two conformations of AICA riboside occurs in the carboxamide region; the Nl-C6 bond length in IA is 1.469 A while in IB it is 1.432 A. In the corresponding crystal structure the value is even shorter at 1.333 (6) A. Torsion angles around the C&C6 bond are, by definition, different in the two isomers, but IA shows the greatest deviation from planarity with C4-C5-C6-Nl being 20”, C4-C5-C6-010 157” and N7-C5-C6-010 20’. The corresponding values in IB are - 167”, 10” and -170”, while in the crystal they are 175.7(4)“, 175.7(4)’ and 176.3(4)“. It thus appears that packing forces play a considerable part in determining the crystal structure of AICA riboside, with a slight deviation from planarity being preferred in the free molecules. From the torsion-angle values obtained, it is clear that the crystal structure corresponds to isomer IB, although as the data in Table 1 show there is very little difference in energy between the two conformations. There is little variation in the magnitude of the structural parameters of the isomers of pyrazofurin (II), except where these arise as a result of the initial definition of geometry. However, one notable exception is in the 03-C4 bond length which is 1.386, 1.355, 1.355 and 1.388 A in isomers IIA, IIB, IIC and IID, respectively. It thus appears that the orientation of the hydroxyl group, which is the same in IIA and IID and also in IIB and IIC may play an important part in determining the overall structure. This variation in the C-O (hydroxyl) bond length is also pronounced in bredinin (III). Values for 03-C4 are 1.370,1.366 and 1.230 A in isomers IIIA, IIIB and IIIC, respectively. However, there is now also a corresponding variation in C4-N9 which takes values of 1.382, 1.380 and 1.477 A in the three
21
isomers. In general, bond lengths in the charged species IIIC are rather different from those in IIIA and IIIB, which is not too surprising. One notable difference between IIIA and IIIB in the carboxamide region is the Nl-C6 length which has values of 1.487 and 1.419 A, respectively. The largest difference between angles in this section of the two isomers is for C5-C6-010 which is 128.5” in IIIA and 118.8” in IIIB. Isomer IIIA shows the greatest deviation from planarity, with torsion angles of - 18” for C4-C5-0%Nl, 157” for C4C5--C6-010 and - 18” for N7-C5-C6-010. In IIIB and IIIC all torsion angles are within 10” of 0” or 180”. Bond lengths in the two isomers of ribavirin (IV) are very similar, but greater variations are seen between the two conformations determined from the crystal structure. The two sets of values also differ, indicating that a considerable relaxation of the molecules has occurred during the quantum-mechanical optimisation. The greatest difference in bond angles occurs at the carboxamide carbon, C5-C6-010 being 122.1’ for IVA and 117.8” for IVB. There is little variation in bond angles between crystal conformations, and generally there is good agreement between the crystal and optimised angles. Torsion angle values show that both crystal conformations are generally similar to that of IVA, although they are slightly different as evidenced by values of 1.5” and 14.7”, respectively, for N4-C5-C6-Nl, the corresponding values of N7-C5-C6-010 being 1.3” and 14.2”, respectively. There is thus a considerable twist of the carboxamide group from planarity in the latter conformation. The optimised structure of guanine (V) has considerably different bond lengths from those observed in the crystal structure. The greatest deviation is for C2-Nll which is 1.334 A in the crystal but optimises to 1.431 A. The lengths of Nl-C6, N3-C4 and C5-C6 also show differences of greater than 0.05 A. Bond angles and torsion angles show good agreement between optimised and crystal structures. It is thus apparent that a number of ~fferen~es in geometry may result between isomers of a particular compound. These may well be important in determining the mode of action of the anti-viral agents considered here. From a comparison of charge distributions, we conclude that pyrazofurin, bredinin and ribavirin may all be able to mimic the electrostatic properties of both AICA riboside and guanine. ACKNOWLEDGEMENTS
We would like to thank Dr. C.H. Schwalbe for helpful discussions during this work.
22 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
R.J. Whitley, in M.R. Harnden (Ed.), Approaches to Antiviral Agents, Macmillan, Basingstake, 1985, p. 3. M.R. Harnden and D.N. Planterose, in M.R. Harnden (Ed.), Approaches to Antiviral Agents, Macmillan, Basingstoke, 1985, p. 153. G.E. Gutowski, M.J. Sweeney, D.C. DeLong, R.L. Hamill, K. Gerzon and R.W. Dyke, Ann. N.Y. Acad. Sci., 255 (1975) 544. J.F. Wonalla and M.J. Sweeney, Cancer Res., 40 (1980) 1482. P.C. Srivastava, D.G. Streeter, T.R. Matthews, L.B. Allen, R.W. Sidwell and R.K. Robins, J. Med. Chem., 19 (1976) 1020. R.K. Smith and W. Kirkpatrick (Eds.), Ribavirin - A Broad Spectrum Antiviral Agent, Academic Press, New York, 1980. D.A. Adamiak and W. Saenger, Acta Crystallogr., Sect. B, 35 (1979) 924. P. Prusiner and M. Sundralingham, Acta Crystallogr., Sect. B, 32 (1976) 419. U. Thewalt, C.E. Bugg and R.E. Marsh, Acta Crystallogr., Sect. B, 27 (1971) 2358. Chem-X, developed and distributed by Chemical Design Ltd., Oxford, Gt. Britain. U. Chandra Singh and P. Kollman, GAUSSIAN 80, QCPE Bull., 2 (1982) 117. A.P. Burgess and C.H. Schwalbe, unpublished work.