Raman spectroscopic study of left-handed Z-RNA

The solvent conditions that induce the formation of a left-handed Z form of poly(r(G-C)) have been extended to include 6.5 M NaBr at 35/sup 0/C and 3.8 M MgCl/sub 2/ at room temperature. The analysis of the A ..-->.. Z transition in RNA by circular dichroism (CD), /sup 1/H and /sup 31/P NMR, and Raman spectroscopy shows that two distinct forms of left-handed RNA exist. The Z/sub R/-RNA structure forms in high concentrations of NaBr and NaClO/sub 4/ and exhibits a unique CD signature. Z/sub D/-RNA is found in concentrated MgCl/sub 2/ and has a CD signature similar to the Z form of poly(d(G-C)). Significant differences in the glycosyl angle and sugar pucker between Z-DNA and Z-RNA are suggested by the 16-cm/sup -1/ difference in the position of this band. The Raman evidence for structural difference between Z/sub D/- and Z/sub R/-RNA comes from two groups of bands: First, Raman intensities between 1180 and 1600 cm/sup -1/ of Z/sub D/-RNA differ from those for Z/sub R/-RNA, corroborating the CD evidence for differences in base-stacking geometry. Second, the phosphodiester stretching bands near 815 cm/sup -1/ provide evidence of differences in backbone geometry between Z/sub D-/ and Z/sub R/-RNA.