Proton magnetic resonance studies of actinomycin D complexes with mixtures of nucleotides as models for the binding of the drug to DNA.

The proton magnetic resonance spectra of actinomycin solutions with mixtures of deoxynucleotides have been investigated to determine the relative preference for the binding of guanine and adenine nucleotides to the two nucleotide binding sites of actinomycin D. An analysis of the chemical shifts of the actinomycin D resonances shows that adenine and guanine nucleotides competitively bind to the benzenoid portion of the phenoxazone ring of actinomycin D while guanine nucleotides bind stronger than adenine nucleotides to the quinoid portion of the phenoxazone ring. The chemical shift data for the titrations of actinomycin D with pdG-dG, pdC-dC, and an equimolar mixture of these complementary deoxydinucleotides show that: (1) pdG-dG forms a stacked complex much like dGMP; (2) pdC-dC does not bind to actinomycin D under the conditions used in these experiments; (3) in the titration of actinomycin D with the equimolar mixture of pdG-dG + pdC-dC, a miniature intercalated complex is formed.