Studies on the structure of chemically methylated DNA

CD and melting temperature measurements on the nature of DNA with chemically methylated guanine-rich sites indicate that the stable secondary structure of DNA depicted by Ramstein et al- involves considerable distortions resulting from decreased base-base stacking interaction. Besides that quantum chemical data gained from PPP calculations are in favor of a weaker hydrogen bonding interaction in the methylated guanine-cytosine base pair. CD measurements demonstrate that methylated DNA-regions differ from the nonmethylated helical structure, since formation of a condensed conformation as occurs in the transition from B to the C-uke structure is prevented by positively charged methylated guanine residues. An increase in helix winding angle, however, can not be excluded. Binding ability of the dyes acridine orange, phenosafranine, and the antibiotic actinomycin C is lowered for methylated DNA, while binding of proflavine is, in accordance with the results of Ramstein and Leng, slightly enhanced. The reason for the opposite behavior of proflavine is at present not fully understood. In particular changes in the binding ability with dyes could not be correlated with base specificity of complex formation. It is discussed that structural changes in DNA towards a loose conformation decrease the binding tendency for acridine orange, phenosafranine, and actinomycin C.