VIBRATIONAL-MODE ASSIGNMENTS OF THE SUBUNITS INVOLVED IN POLY-RIBOINOSINIC ACID HELICAL STRUCTURES - FROM HYPOXANTHINE BASE TO RIBOINOSINICRESIDUE

Ultraviolet resonance Raman spectra of hypoxanthine base as well as in osine, along with their deuterated species, have been collected in neu tral aqueous solution (lambda(exc) = 257 and 281 nm) in the spectral r egion between 400 and 1800 cm(-1). The observed vibrational modes have been assigned by a normal mode analysis which rests on previous work on the vibrational assignments of hypoxanthine nucleic base and furano se ring with various geometries. Normal mode analysis has been carried out on inosine by using three different nucleoside conformations, as determined from X-ray studies of molecular crystals. The most striking features of the present work are : i) the assignments of inosine vibr ational modes are very different from those of the hypoxanthine base, and ii) : the best fit between measured and calculated vibrational wav enumbers and their isotopic shifts upon selective deuterations has bee n obtained for inosine structures containing an N-type sugar (C3'-endo ) associated with a low-anti base. In addition, we have tested the rel iability of this force field in the case of inosine residues involved in a canonical A-RNA helical structure, by comparing the calculated wa venumbers of the most characteristic vibrational modes with those obse rved in off- and on-resonance Raman spectra of poly(rI).