Quantitative infrared spectra in D20 of some nucleosides, nucleotides, and polynucleotides; a new measure of polynucleotide interaction

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.

Interbase vibrational coupling in G:C polynucleotide helices

The carbonyl stretching vibrations of G and C residues undergo frequency changes when a helical complex is formed from nonordered G and C components. We have prepared a series of two-stranded G:C helices having chemical and isotopic ((18)O) substitutions which lead to variation of the observed vibrational frequencies. Infrared spectra of these helices clearly demonstrate that vibrational coupling occurs between the guanine and cytosine residues in the helix. As a result of this coupling through the hydrogen bonds, the two carbonyl bands undergo frequency shifts of equal magnitude and opposite sign, with mixing of G and C character in the observed interacted vibrations.

Tautomerism and Protonation of Guanosine

Guanosine has been demonstrated, by infrared and nuclear magnetic resonance spectroscopy, to have a keto-amino structure in neutral aqueous solution and to undergo protonation at N(7) in acid solution.