Salt-induced conformational transition of poly(d2NH2A-dT) studied by ultraviolet resonance Raman spectroscopy.

The conformational changes of poly(d2NH2A-dT) in aqueous solution, induced by increasing the NaCl concentration from 0.1M to 4M, have been monitored by ultraviolet resonance Raman spectroscopy, in using the 222-, 257- and 281 nm excitation wavelengths. These changes have been interpreted in comparing the polymer spectra to those of the mononucleotide compounds on one hand, and to those of other alternating purine-pyrimidine polymers on the other hand, i.e. poly(dG-dC) and poly(dA-dT) which showed a B to Z transition in going from low- to high salt concentrations. The high salt poly(d2NH2A-dT) spectra do not show any Raman marker line of the Z conformation. The spectroscopic results indicate that most of the ribose puckering goes from C2'-endo/anti to C3'-endo/anti in increasing the salt concentration. In addition the base stacking interactions, to which the resonance Raman effect is very sensitive, are not drastically changed upon salt variations. Thus the high salt structure of poly(d2NH2A-dT) remains a right-handed helix, likely under a dominant A conformation.