FROM NUCLEIC BASE TO POLYNUCLEOTIDE - A VIBRATIONAL ANALYSIS OF THE REPETITIVE SUBUNITS INVOLVED IN POLY(RI) STRANDS

Ultraviolet resonance Raman spectra of inosine and its deuterated spec ies (C8-d, N1-d and C8-, N1-deuterated derivatives) in aqueous solutio n have been collected (lambda(exc) = 257 and 281 nm) and reported in t he spectral region between 400 and 1800 cm-1. The observed vibrational modes have been assigned using the Wilson GF method with an empirical harmonic valence force field and a non-redundant set of internal co-o rdinates. This normal mode analysis is based on previous work on the v ibrational assignments of the hypoxanthine nucleic base and the furano se ring with various geometries. Here the analysis has been carried ou t by using three different nucleoside conformations as determined from x-ray studies of molecular crystals. The most striking features of th e present work are: (i) the assignments of inosine vibrational modes a re very different from those of the hypoxanthine base; (ii) the best f it between measured and calculated vibrational wavenumbers and their i sotopic shifts upon selective deuteration has been obtained from inosi ne structures containing an N-type sugar (C3'-endo) associated with a low-anti base. In addition, we have tested the reliability of the forc e field in the case of inosine residues involved in a canonical A-RNA helical structure by comparing the calculated wavenumbers of the most characteristic vibrational modes with those observed in off- and on-re sonance Raman spectra of poly(rI).