Interpretation of DNA vibration modes: I--The guanosine and cytidine residues involved in poly(dG-dC).poly(dG-dC) and d(CG)3.d(CG)3.

A normal coordinate analysis has been carried out on guanosine and cytidine residues appearing in oligo and polynucleotides by using a simplified valence force field that allows the vibrational spectra of 5'-dGMP and 2'-deoxycytidine molecules to be reproduced. The role of both C2'-endo and C3'-endo conformations on sugar pucker, as well as that of glycosidic torsion angle (X), on several characteristic vibration modes of these residues have been studied. The present calculations based on a non-redundant set of internal coordinates preserving the harmonic approximation of the potential field, allows us to explain quite satisfactorily the modifications of the vibrational spectra in the 1550-1250 cm-1 and 785-500 cm-1 regions, when the right----left-handed conformational transition occurs.