Infrared spectra and tautomeric structure of polyinosinic acids in D2O solution

IR study of base stacking interactions

For D2O solutions of 1,3-dimethyluracil, cytidine, caffeine, inosine and 2'-deoxyadenosine the concentration dependence of IR spectra (1800-1400 cm-1) have been found which reflects stacking association of these compounds. A method is proposed to use this data to obtain thermodynamic parameters of association and the molecular spectra in the monomer and associated forms. The homoassociation constant for 1,3-dimethyluracil was estimated as k=0.65. Stacking is shown to change radically the spectra, inducing a high-frequency shift of carbonyl vibrations and a decrease in the intensity of skeletal stretching vibration bands. This allows one to distinguish between the stacking interactions and hydrogen bonding. A conclusion is made about the considerable contribution of stacking interactions into the change of IR spectra of DNA and other polynucleotides following conformational transitions.

Tautomerism and Site of Protonation of 1-Methylcytosine: Proof by Nuclear Magnetic Resonance Spin-Spin Coupling

New nuclear magnetic resonance data lead to a completely unambiguous proof that the predominant tautomeric form of 1-methylcytosine is the amino form and that in acid solution the molecule protonates at 3-N. The close similarity of the nuclear magnetic resonance spectra of this compound to the spectra of the cytosine nucleosides indicates that these structures also exist in the nucleosides. These conclusions are reached on the basis of the spectra of analogs labeled with N15 and of the proton spin decoupling measurements made at low temperature.

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.