Guanine residue: A normal‐coordinate analysis of the vibrational spectra

The force field established for guanine is applied here to guanine‐containing biopolymers by considering the model compound 9‐methylguanine, in which the methyl group is taken as a dynamic unit whose mass is concentrated on the carbon. In‐plane normal‐mode frequencies for this model compound and its N‐deuterated analog are calculated. Band frequencies observed for guanine residue in Raman biopolymer spectra, such as those for DNA, RNA, or poly(G), are associated with calculated modes having similar wavelengths. They are discussed by taking into account observed and calculated D, 15N, and 18O isotopic shifts. The atomic displacements for the normal modes corresponding to the principal bands are illustrated and a number of assignments proposed.