Spectroscopic Studies on Vapour Phase Tautomerism of Natural Bases Found in Nucleic Acids

Infrared absorption spectra, in the vapour phase, have been recorded in the regions of NH, OH and carbonyl stretching frequencies, for a series of 1-substituted uracils and 9-substituted adenine and hypoxanthine, formal analogues of the natural nucleosides of uracil, adenine and hypoxanthine found in DNA and/or RNA. A number of related analogues was also examined, including the N-methyl derivatives of the known mutagen and chemotherapeutic agent, 5-fluorouracil. The infrared absorption spectra provide unequivocal evidence for the existence of 1 -substituted uracils as the 2,4-diketo tautomer, of 9-substituted hypoxanthine as the 6 -keto tautomer, and of 9-substituted adenine as the 6 -amino tautomer. These are the known predominant tautomeric forms in solution, so that the gas phase results are in sharp contrast with those for other nitrogen heterocycles, where the tautomeric equilibrium constants may differ by several orders of magnitude in going from solution to the vapour phase. The significance of these results is evaluated in relation to the types of heterocyclic bases found in natural nucleic acids, and to concepts of spontaneous and induced m utations in terms of mis-pairing. The ultraviolet absorption spectra of the various compounds have also been examined in the gas phase, but proved of relatively limited use in studies on tautomeric equilibria under these conditions. Heats of vaporization have been determined for most of the compounds examined. In particular the heat of vaporization for 1,3-dimethyluracil, which is incapable of association by hydrogen bonding in the condensed phase, is much lower than for uracil and 1 (3) -methyluracils which can associate by hydrogen bonding.