Structural Effects of Cytosine Methylation on DNA Sugar Pucker Studied by FTIR

This FTIR investigation concerns structural consequences of 5-methylation of cytosine in a DNA decamer in solution. Methylation of DNA is an important functional signal in transcription, but its effect on DNA structure is variable and not fully understood. Here, single and multiple 5-methylcytosine substitutions are introduced into the self-complementary sequence d(CCGGCGCCGG)2. No major structural effect of methylation on the DNA duplex in solution is seen in the IR spectra: The overall B-form character of the backbone and S-type of sugar puckering are maintained in all the studied sequences, in agreement with previous literature. However, certain significant effects are detected in the IR regions sensitive to sugar pucker and glycosidic torsional angle. A single or multiple 5-methylcytosine substitution in d(CCGGCGCCGG)2 leads to a doublet splitting of the S-type 840–820 cm−1 sugar conformational band. The results suggest the coexistence of two different major sugar puckers within the S-conformational family, with an increased relative contribution of the C2′-endo type of sugar in the methylated sequences. In addition, a partial or full downshift of the guanosine/anti marker band at 1375 cm−1 in the methylated sequences reflects a change in the value of the dihedral angle χ of guanosine upon methylation. The IR spectra are interpreted in terms of localized transitions between the BI and BII subconformational states of the B-DNA backbone caused by the methylation. An increased amount of the BII subconformer in the methylated sequences should give rise to a structurally more rigid conformation, in agreement with earlier observations on DNA backbone dynamics and bending flexibility in methylated DNA.