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Ultraviolet resonance Raman spectra of inosine and its deuterated species (C&d, N1-d and C&, Nl-deuterated derivatives) in aqueous solution have been collected (k,,, = 257 and 281 nm) and reported in the spectral region between 400 and 1800 cm-'. 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-ordinates. This normal mode analysis is based on previous work on the vibrational assignments of the hypoxanthine nucleic base and the furanose ring with various geometries. Here the analysis has been carried out 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 vibrational modes are very different from those of the hypoxanthine base; (ii) the best fit between measured and calculated vibrational wavenumbers and their isotopic shifts upon selective deuteration has been obtained from inosine structures containing an N-type sugar (C3'-endo) associated with a low-mti base. In addition, we have tested the reliability of the force 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-resonance Raman spectra of poly(r1).

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Poly( dl-dC) in aqueous solution can undergo different equilibrium geometries, which strongly depend on salt nature and concentrations. These equilibrium structures have been monitored by resonance Raman spectroscopy (RRS) measurements in the ultraviolet region, i. e. by using 257 and 281 nm laser excitation wavelengths which favor the resonance enhancement of the Raman contributions from inosine and cytosine residues of poly( dl-dC), respectively. Spectral changes depending on the NaCI concentration and on the presence orNe+ ions have been observed and interpreted in comparison with RRS results previously obtained for other alternating purine-pyrimidine polydeoxyribonucleotides, i.e. poly(dG-dC), poly(d.Adn and poly(d.A-dC).poly(dG-dT), which also showed B to Z conformational transitions in varying the salt concentrations. It is shown here that : i) the base stacking geometries are nearly the same in the high-salt form (5 M NaCl) of poly( dl-dC) as in the low-salt form (0.1 M NaCI) ofthe polymer, ii) however, the high-salt structure yields important differences from a B-helix (obtained in low-salt solution) as regards the nucleoside conformations (sugar puckering and base-sugar orientation), and: iii) the addition of9 mM NiC1 2 in the high-salt(5 M NaCI) solution ofpoly(dl-dC) induces the Z-conformation of the polymer.