The tautomeric equilibria of thio analogues of nucleic acid bases. Part 1. 2-Thiouracil: background, preparation of model compounds, and gas-phase proton affinities

Abstract

The preparation is reported of all four of the monoalkyl derivatives of 2-thiouracil and of four of the six possible dialkyl derivatives required as models for a study of the tautomeric equilibria by physical methods. Gas-phase proton affinities are determined using ion cyclotron resonance mass spectrometry, and are used to provide quantitative estimates of individual tautomer stabilities in the vapour state. These quantitative results agree well with qualitative deductions of predominant structures for the monoalkyl derivatives from i.r. spectroscopy.