The Quadruplex Form of Polyriboinosinic Acid as Studied by UV Resonance Raman Spectroscopy

Four-stranded forms of DNA have recently become of great interest due to the discovery of four stranded segments of DNA in such significant structures as telomers[l]. Polyriboinosinic acid (Poly[rI]) forms a self-associated quadruplex in high concentrations of salt, but also exists as a single stranded form at low ionic strength[2], thus forms a useful model for studying such four-stranded forms of polynucleotides, and many studies have been made of this system using a wide range of experimental techniques such as CD, ORD, UV absorption, IR and Raman spectroscopies, and X-ray crystallography. We have studied this system using CD and Resonance Raman Spectroscopy with excitation wavelengths of 257 and 280 nm, thereby allowing us to obtain vibrational spectra with low concentrations of polynucleotide (0.4mg/ml) such as those used for UV and CD spectroscopy. Thus we have been able to follow certain processes which are not readily observed at the higher concentrations required by IR and classical Raman spectroscopies. We also performed a trial experiment with Terbium Tb3+ ions, a fluorescent marker normally used as a structural probe for guanine residues.