Nucleic Acids in the PO- Stretching Region

Vibrational circular dichroism (VCD) spectra were measured for H20 solutions of several natural and model DNAs (single and double strands, oligomers and polymers) in the B-form, poly(dG-dC).poly(dG-dC) in the Z-form, and various duplex RNAs in an A-form over the POstretching region. Only the symmetric POstretch at -1075 cm-' yields a significant intensity VCD signal. Differences of the POstretching VCD spectra found for these conformational types are consistent with the spectral changes seen in the base region, but no sequence dependence was seen in contrast to VCD for base modes. The B to Z transition is accompanied by an inversion of the PO VCD spectra, which is characteristic of the change in the helical sense of the nucleic acid backbone. A-RNAs give rise to the same sense of coupletVCD as do B-DNAs but have a somewhat different shape because of overlapping ribose modes. These POVCD spectral characteristics have been successfully modeled using simple dipole coupling calculations. The invariability of the symmetric PO stretching mode VCD spectra to the base sequence as opposed to that found for the C=O stretching and base deformation modes is evidence that this mode will provide a stable indication of the DNA helical sense.