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The interaction of Actinomycin C(1) (AM) with purine nucleosides was investigated by circular dichroism. While dG and dGMP show identical interactions, rG, rGMP and 8-aza-GMP show lower tendency for complex formation. ara-G shows no complex formation. 6-Thio-GMP shows the strongest affinity for AM in accordance with previous results. Substitution on N-7 (methylation, protonation) or C-8 (8-Br-rG, 8-SH-rG) eliminate completely the complexing capacity. Another type of weaker complexes is formed by dA, dAMP and rIMP, indicated by the appearance of a positive band at 440 nm. It is concluded that in these complexes both the electron donor capacity of the purine base as well as the steric flexibility of the sugar around the glycosidic linkage determine their complexing capacity.