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 350C and 3.8 M MgCl2 at room temperature. The analysis of the A → Z transition in RNA by circular dichroism (CD), 1H and 31P 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 NaClO4 and exhibits a unique CD signature. Z/sub D/-RNA is found in concentrated MgCl2 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-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-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-1 provide evidence of differences in backbone geometry between Z/sub D-/ and Z/sub R/-RNA