Enzymatic Synthesis of 18O(6)-Guanosine and Spectroscopic Characterization of Hoogsteen and Reverse Hoogsteen Hydrogen-Bonded Guanosine Structures

Adenylic acid deaminase has been successfully used for the first time to convert Cl(6)-guanosine to 18 O(6)-guanosine in the presence of H 2 18 O. Observations of infrared and Raman spectra of both guanosine and 18 O(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 νC 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 νC 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.