Vibrational Raman Optical Activity of DNA and RNA

The study of solution structure and dynamics of biopolymers remains at the forefront of biomolecular science. A promising technique for such studies is Raman optical activity (ROA), which measures vibrational optical activity by means of a small difference in the intensity of Raman-scattered light from chiral molecules in right- and left-circularly-polarized incident light. Thanks to new instrumentation developed over the past few years, ROA can now be measured routinely on a wide range of biological molecules in aqueous solution. Following ROA studies of pyrimidine nucleosides and synthetic polyribonucleotides, we report here the first observations on naturally occurring DNA and RNA samples which demonstrate that, on account of its ability to probe directly the central chiral elements of biomolecular structure, ROA may enhance the already considerable value of conventional Raman spectroscopy in the study of nucleic acids. Figure 1 shows the backscattered Raman (I super(R) + I super(L)) and ROA (I super(R) - I super(L)) spectra of calf thymus DNA (top pair), phenylalanine specific transfer RNA (tRNA super(Phe)) from brewers yeast (middle pair), and the same tRNA super(Phe) but with the Mg super(2+) ions removed (bottom pair). The instrument used was that described earlier. These spectra are generally more complex than the synthetic polyribonucleotide ROA spectra published previously: one reason is that there are now four different bases present compared with one in the single- and two in the double-stranded synthetic polyribonucleotides; another is the possible presence of more conformational substates. We shall confine the discussion to a few general remarks since to date we have only obtained definitive ROA spectra of model polynucleotides in A-type conformations. A more detailed analysis of the data will follow from further studies on oligo- and polynucleotides in canonical B- and Z-conformations as well as less regular structures.