No abstract

The effects of certain substituents and solvents on the circular dichroism (CD) spectra are reported for a number of guanine nucleoside derivatives from 320 to 200 nm. Both theoretical and empirical analyses of the data suggest that the anti conformation predominates in aqueous solution, but that the syn conformation is preferred in alcoholic solvents, at low pH in water, and when the heterocycle carries a large substituent on carbon 8 of the imidazole ring. Theoretical optical calculations based on the bond-bond coupled oscillator theory are included to check the validity of the theory with experimental data. The interaction of guanine nucleoside derivatives with actinomycin is also reported and the data suggest that the anti conformation is necessary for complex formation.A fundamental aspect of the conformational analyses of nucleic acids and their fragments concerns the allowed conformational states of the individual nucleotide or nucleoside monomers.In particular the relative position of the sugar and the base about the glycosidic bond, as described by the torsion angle, <£CN, has received a great deal of attention.1-4 Steric considerations lead to two extreme conformations as demonstrated by 1 and 5 in Figure 1, which are designated anti and syn, respectively. Crystallographic studies have pointed to the almost exclusive presence of the anti conformation.5 A number of exceptions, however, are known: deoxyguanosine, which is syn in a mixed crystal with 5-bromodeoxycytidine;6 3',5' cyclic AMP, which can exist simultaneously in the anti and syn conformations in the same crystal structure;7 8bromoguanosine;8 8-bromoadenosine;8 and 3'-0-acetyladenosine.9 Several independent calculations using (1) A.