Infrared spectroscopy of polynucleotides in the carbonyl region in H2O solution: A.U systems.

Strong absorbance of water at 1645 cm-1 has previously prevented dependable observation of infrared spectra of H2O solutions in the carbonyl region. We have obtained reliable spectra in pure water and in alcohol-water mixtures in this region and used them to study A.U polynucleotide interaction. Spectra in pure water were obtained by computer subtraction of solvent from that of solution measured in short path length cells. Mixed solvents, by diluting the water, permit use of longer cells and standard double-beam compensation methods. Poly(adenylic acid) in H20 solution has a strong NH2 deformation band at 1659 cm-1 coupled to an A ring vibration at 1605 cm-1. Poly(uridylic acid) has a broad band at approximately 1695 cm-1, predominantly C2 carbonyl stretch, an unresolved CR carbonyl band (probably near 1660 cm-1), and a ring vibration at approximately 1636 cm-1. The large changes these spectra undergo when helical structures are formed are discussed and compared with changes observed in D2O.