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The carbonyl stretching vibrations of G and C residues undergo frequency changes when a helical complex is formed from nonordered G and C components. We have prepared a series of two-stranded G:C helices having chemical and isotopic ((18)O) substitutions which lead to variation of the observed vibrational frequencies. Infrared spectra of these helices clearly demonstrate that vibrational coupling occurs between the guanine and cytosine residues in the helix. As a result of this coupling through the hydrogen bonds, the two carbonyl bands undergo frequency shifts of equal magnitude and opposite sign, with mixing of G and C character in the observed interacted vibrations.
Howard et al. (Wed,) studied this question.