Correlation between vibrational frequencies and hydrogen bonding states of the guanine ring studied by UV resonance Raman spectroscopy of 2′-deoxy-3′,5′-bis(triisopropylsilyl)guanosine dissolved in various solvents

Ultraviolet resonance Raman spectra of 2′-deoxy-3′,5′-bis(triisopropylsilyl)guanosine (TPS-dGuo) were recorded in non-hydrogen bonding, proton acceptor, and proton donor/acceptor solvents. Raman spectral changes observed on going from inert to proton acceptor solvents were ascribed to the hydrogen bonding at the proton donor sites of the guanine ring (N1H and C2NH 2 ), and the spectral changes associated with the solvent change from proton acceptor to donor/acceptor were ascribed to the hydrogen bonding at the proton acceptor sites (N3, C6O, and N7). A Raman band appearing at 1624 cm −1 in inert solvents is assigned mainly to the NH 2 scissors mode and its frequency changes to ≈ 1640 cm −1 in acceptor solvents, reflecting the hydrogen bonding at C2NH 2. Another band at 1581 cm −1, arising largely from the N1H bend, shows an upshift of ≈ 10 cm −1 upon hydrogen bonding at either N1H or acceptor sites. Hydrogen bonding at the acceptor sites also produces frequency shifts of other Raman bands (at 1710, 1565, 1528, 1481, and 1154 cm −1 in 1,2-dichloroethane solution). Among the Raman bands listed above, the 1710 cm −1 band due to the C6O stretch decreases in frequency, whereas the others increase. The downshift of the C6O stretching frequency is correlated with the strength of hydrogen bonding at C6O. The frequency of the 1481 cm −1 band increases with a decrease of the C6O stretching frequency, indicating that the 1481 cm −1 band is also a marker of hydrogen bonding at C6O. This finding is in sharp contrast to the previously proposed correlation with the hydrogen bonding at N7. The 1565 cm −1 band is assigned to a vibration mainly involving the N1C2N3 linkage, and its frequency increases with increasing strength of the hydrogen bond at N3. Three bands around 1315, 1180, and 1030 cm −1, which are known to be sensitive to the ribose ring puckering and glycosidic bond orientation, also show small frequency changes upon hydrogen bonding. The Raman marker bands for the hydrogen bonds at C2NH 2, N1H, C6O, and N3 are expected to be useful in studying the structures of nucleic acids and nucleic acid-protein complexes containing guanine residues.