One of the factors required for the antiviral activity of the synthetic nucleoside, ribavirin (1-#->-ribofuranosyl-1,2,4-triazole-3-carboxamide), is the ability of the molecule to adopt the substrate conformation specified by the enzyme for which it is a competitive inhibitor, inosine 5'-phosphate dehydrogenase (IMP:NAD+ oxidoreductase, EC 1.2.1.14). The calculated glycosidic minimum for ribavirin is the high syn conformation, which is in agreement with experimental determinations of the molecule's solution conformation. The similarity in solution between the conformation of the active ribavirin molecule and the conformation of its inactive 5-methyl and 5-chloro derivatives indicate that some other substrate conformation is specified by the enzyme. The high anti conformation, found by these calculations to be close in energy to the high syn minimum, is postulated to be the active conformation required by the enzyme. The inactivity of the 5-methyl and 5-chloro derivatives is attributed to the much greater stability of these derivatives in the inactive high syn conformation.Ribavirin is an important new antiviral agent (1, 2). Its solution properties have been investigated by nuclear magnetic resonance (3) and circular dichroism*. In the solid state it has been found to crystallize in two different forms (4). The ability of ribavirin to act as a strong competitive inhibitor of inosine-5'-phosphate (IMP) dehydrogenase (IMP:NAD+ oxidoreductase, EC 1.2.1.14), because of its resemblance to IMP or the feedback inhibitor guanosine-5'-phosphate (GMP), has been established (5). The broad spectrum antiviral properties (2) of this molecule and the availability of many derivatives (6-11) increase the interest in a theoretical study of its conformational properties.We have calculated the variation in the total energy of ribavirin as a function of rotation about the glycosidic bond. The calculated high syn minimum corresponds with the solution conformation (3, *). The inactivity of 5-methyl and 5-chloro derivatives of ribavirin (7) as inhibitors of IMP dehydrogenase, whose solution conformations are identical* with the unsubstituted ribavirin's solution conformation, indicate that another conformation is specified at the active site of the enzyme. Our calculations show that the high anti conformation (4) is close in energy to the high syn conformation found in solution.Conformational energy calculations on the 5-substituted "inactive" derivatives of ribavirin show that the high anti conformation, postulated here as being the conformation required by the enzyme, is ruled out for these derivatives. According to this hypothesis, any effective inhibitory analogs designed to inhibit IMP dehydrogenase should not preclude stability in the high anti region. Insight into normal and aberrant purine metabolism (12, 13) may be obtained by an investigation of one of the key metabolic conversions involving IMP. IMP occupies an important position in purine metabolism since this nucleotide is partitioned between metabolic pathways lea...

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A method is presented for localizing molecular orbitals, based on diagonalizing subunits of the density matrix. First, nonbonding orbitals are found by diagonalizing the monatomic subunits; then, diatomic a or -r bonding and antibonding orbitals are obtained from the diatomic subunits for all bonded pairs of atoms; finally, the delocalized ir-orbitals for particular chromophores are found by projecting the first set out of the self-consistent field (SCF) Hamiltonian. The results show good general agreement with other localization methods, with advantages in the ability to display group orbitals in complex molecules which most closely resemble the SCF orbitals for simple prototypes. The impetus for obtaining localized orbitals is well recognized and much has been written about the applications and uses of localized orbitals (1-3).A set of localized orbitals may be obtained by a direct calculation of self-consistent field (SCF) localized molecular orbitals (4) or, given an arbitrary set of delocalized orbitals, one may construct a unitary matrix that will transform the delocalized orbitals into localized orbitals. The two most commonly used procedures for doing the latter are those of Boys (5) and Edmiston and Ruedenberg (6). Closely related methods (7, 8), improvements in efficiency (9), and general discussions of the various methods have been presented elsewhere (10-14).In this work a method is presented that is based on the density matrix formalism and can be used with either ab initio or semi-empirical programs. In the latter case the required input consists of the overlap matrix, the molecular Hamiltonian, and the first-order density matrix. METHODThe localization process is based on the diagonalization of appropriate subunits of the density matrix (15). First, the nonbonding and vacant atomic orbitals are obtained by diagonalizing the individual monatomic components, which are individually delineated by the basis orbitals for each atom. Ideally, a nonbonding orbital would be associated with the eigenvalue 2 and a vacant orbital would have the value 0. In practice it has been found convenient to take all orbitals with ni > 1.6 as nonbonding and those for which nj < 0.4 as vacant. The remaining eigenvectors are hybrids or linear combinations thereof associated with bonding to nearest neighbors.Although it is tempting to use the remaining eigenvectors as hybrids for localized bonding and antibonding orbitals, this process is defeated by the near degeneracy (ideally, 1) for such orbitals, and extensive mixing occurs. The problem is avoided by the second step.The diatomic portions for all pairs of atoms are isolated and diagonalized. Again

Purine nucleoside analogs modified by replacement of the nitrogen atom at the 3 position by a CH group give a characteristic circular dichroism curve that is not substantially modified by chemical substitution at the 8 position.Since it is rather well established that 8-substituted purine nucleosides are predominantly in the syn conformation in aqueous solution, it follows that the 3-deazapurine nucleosides, whether substituted at position 8 or not, also favor the syn conformation. These data are in sharp contrast to the circular dichroism data obtained on 8-halogenated and 8-alkylated derivatives of adenosine and guanosine, which give circular dichroism profiles substantially different from those obtained on the parent compounds. Certain purine-nucleoside-utilizing enzymes fail to interact effectively with either the unsubstituted 3-deaza analogs or the 8-substituted derivatives of adenosine and guanosine. The hypothesis recently given that the inactivity of the 8-substituted derivatives springs from their syn-conformational preference is tentatively accepted to explain the inactivity of the 3-deaza analogs. Included in the search for purine analogs active against bacteria, tumor cells, and viruses have been nucleosides and nucleotides modified by replacement of the nitrogen atom at the 3 position by a CH group (1-9). This modification has, however, rarely led to recognized biological activity, giving rise to speculation over the biological significance of N3 (2,3,(8)(9)(10)(11). It has been suggested several times (12)(13)(14), and as early as 1962, that one role of N3 is to stabilize an optimal glycosyl conformation at the enzyme surface by internally hydrogen bonding with the ribose ring. Direct interaction between N3 and the enzymes has also been proposed (2).Unfortunately the "active" conformation of a substrate molecule and its energy are at present beyond the scrutiny of any experimental technique. In aqueous solution intermolecular hydrogen bonding prevails; nevertheless the discovery of the preferred solution conformation is relevant to either the internal or the external hydrogen bonding role of N3, for the energy of the "active" conformation relative to the stable solution conformation must influence the overall kinetics and energetics of complex formation. A comparative study of the solution conformations of purine nucleosides and their 3-deaza analogs has not been made despite the obvious importance of understanding the conformational consequences of replacing nitrogen by carbon. This paper presents a preliminary report on a comparative study of the solution conformation of purine nucleosides and their 3-deaza analogs. An analysis of the circular dichroism data obtained so far shows rather conclusively that 1,3-dideazapurine nucleoside and 3-deaza purine nucleoside have strong if not exclusive preferences for the syn conformation in aqueous solution, in sharp contrast to the substantial anti population of the parent compounds. Moreover, gas phase conformations from molecular orbital calcula...

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