Vibrational Raman optical activity as a probe of polyribonucleotide solution stereochemistry

The backscattered vibrational Raman optical activity (ROA) spectra of three single-stranded polyribonucleotides, poly(rA), poly(rC), and poly(rU), together with two double-stranded polyribonucleotides, poly(rA)· poly(rU) and poly(rG)·poly(rC), which both adopt A-type double helices, are reported in buffered H2O and D2O solutions between ∼650 and 1750 cm-1. The ROA spectra are subdivided into three distinct regions that contain information on different stereochemical elements of nucleic acid structure. Between ∼1550 and 1750 cm-1, ROA is generated through coupling of the vibrational coordinates on adjacent stacked bases that are chirally disposed. Large differences are observed between the spectra recorded in H2O and D2O in this region. Between ∼1200 and 1550 cm-1, vibrational coordinates in the sugar and base rings couple in the normal modes to produce ROA signals that reflect the mutual orientation of these two rings and also the conformation of the sugar ring itself. Between ∼950 and 1150 cm-1, a base-independent ROA triplet is observed that is characteristic of the particular sugar ring and sugar−phosphate backbone conformation present in A-type helices. These first results indicate that ROA is a powerful new probe of nucleic acid solution stereochemistry