CONFORMATIONAL TRANSITIONS OF POLY(DI-DC) IN AQUEOUS-SOLUTION AS STUDIED BY ULTRAVIOLET RESONANCE RAMAN-SPECTROSCOPY

Poly(dI-dC) in aqueous solution can undergo different equilibrium geom etries, which strongly depend on salt nature and concentrations. These equilibrium structures have been monitored by resonance Raman spectro scopy (RRS) measurements in the ultraviolet region, i.e. by using 257 and 281 nm laser excitation wavelengths which favor the resonance enha ncement of the Raman contributions from inosine and cytosine residues of poly(dI-dC), respectively. Spectral changes depending on the NaCl c oncentration and on the presence of Ni2+ ions have been observed and i nterpreted in comparison with RRS results previously obtained for othe r alternating purine-pyrimidine polydeoxyribonucleotides, i.e. poly(dG -dC), poly(dA-dT) and poly(dA-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(dI-dC) as in the low-salt fo rm (0.1 M NaCl) of the polymer, ii) however, the high-salt structure y ields important differences from a B-helix (obtained in low-salt solut ion) as regards the nucleoside conformations (sugar puckering and base -sugar orientation), and : iii) the addition of 9 mM NiCl2 in the high -salt (5 M NaCl) solution of poly(dI-dC) induces the Z-conformation of the polymer.