Assignments of guanosine UV resonance Raman bands on the basis of13C,15N and18O substitution effects

Abstract

UV resonance Raman spectra of guanosine and its seven isotope-substituted analogs (2-13 C, 2-15 N, 6-18 O, 7-15 N, 8-13 C, 9-15 N and 1 -13 C) were measured with 257 nm excitation in H 2 O and D 2 O solutions. In-plane vibrations of the guanine ring were selectively enhanced in the UV resonance Raman spectra, and most Raman bands showed significant wavenumber shifts upon isotopic substitution. The observed isotope shifts were used to assign the Raman bands to vibrations of the peripheral sites (N1-H, C2-NH 2 and C6 O), the pyrimidine ring and/or the imidazole ring. Previous assignments for some Raman bands were shown to be inconsistent with the isotopic data and they were revised. Relationships between the vibrational modes and the sensitivities to hydrogen bonding or conformation are discussed for known Raman marker bands. Each hydrogen bond marker arises from a vibration that involves, at least partly, the proton donor or acceptor atom. All the marker bands of glycosidic bond orientation and ribose ring puckering actually involve atomic displacements around the N9-C1 moiety connecting the guanine ring to ribose, permitting vibrational coupling between them. The isotopic wavenumber shifts reported here may be useful in improving the force field for the 9-substituted guanine ring and in interpreting the vibrational spectra of guanine nucleoside and nucleotides.

Computational prediction of vibrational spectra of normal and modified DNA nucleobases

“…The corresponding mode in dGMP was observed at 1681 cm −1 by us [60] and at 1679 cm −1 by Fodor et al [51] The other important high wavenumber band, assigned to NH 2 scissoring, occurs at 1602 cm −1 for guanine, and is reproduced by DFT at 1667 cm −1 in Giese and McNaughton's work. [52] It shifts to 1643 cm −1 in guanosine [58] but, interestingly, does not shift in GMP. [51,60] As pointed out by Toyama et al, [58] this band is susceptible to shift in wavenumber upon hydrogen bonding and is a good hydrogen-bond marker.…”

“…[60] The Raman, [52,56] SERS, [53] and FTIR [56] spectra of guanine in the solid state and in strong acidic solutions, UVRR spectrum of guanosine, [58,59] as well as GMP [51,60] and theoretical studies [52,54,55,57,61] on its geometry, spectroscopic properties, tautomerism, and non-planar nature, have all been reported. The important carbonyl stretching mode C 6 O occurs as a weak band at 1675 cm −1 for guanine, with a considerable degree of coupling with C 5 -C 6 stretching and N 1 -H bending as reported by Giese and McNaughton.…”

“…In Mathlouthi et al's work, [56] however, two bands were recorded at 1700 and 1676 cm −1 (IR), which have been assigned to carbonyl stretching coupled to NH 2 group scissoring. Toyama et al [58] have carried out a detailed UVRR study of guanosine, which absorbs strongly at 257 nm excitation. They have substituted various carbon and nitrogen atoms in the pyrimidine and imidazole rings as well as the single oxygen atom with their heavier isotopes and compared the spectra recorded with various substitutions.…”

Raman, polarized Raman and ultraviolet resonance Raman spectroscopy of nucleic acids and their complexes

“…the introduction of a spectroscopically informative isotope label at a unique site of the macromolecule. 24,44 …”

“…11(D)], which appears at much lower wavenumber (¾1596 cm 1 ), has been assigned to a guanine ring vibration involving in-plane bending of the C2-N1-H linkages. 24 The observed difference has been ascribed to the presence of Hoogsteen hydrogen bonding (N1-HÐ Ð ÐO6) in telomeric sequences in lieu of solvent hydrogen bonding (N1-HÐ Ð ÐOH 2 ) in non-telomeric sequences. No comparable band can be resolved in conventional Raman spectra, owing to an apparently low Raman scattering cross-section and possible obscuration by intense neighboring bands of the guanine (1575 cm 1 ) and thymine rings (1660 cm 1 ) [ Fig.…”