Adenylic acid deaminase has been successfully used for the first time to convert Cl(6)-guanosine to O-18(6)guanosine in the presence of (H2O)-O-18. Observations of infrared and Raman spectra of both guanosine and O-18(6)-guanosine permit assignment of the carbonyl vibrations in guanosine tetramer. In-plane normal mode wavenumbers have been calculated using a valence force field that incorporates an interaction force constant of vC=O transition dipole coupling. This coupling alone between carbonyl groups of adjacent guanine residues provides the physical basis for understanding the observed C=O splittings in guanosine tetramer and in hydrogen-bonded guanine bases adopting the Hoogsteen structure. This coupling is, however, absent in the dimer of 2＇-deoxy-3＇,5＇-bis(triisopropylsilyl),guanosine which is known to adopt the reverse Hoogsteen structure. The vC=O spectral profiles, then, permit the distinction between Hoogsteen and reverse Hoogsteen hydrogen-bonded guanine nucleobases in G*G.C triplets that may be present in nucleic acid triple helices.