Protonation of hypoxanthine, guanine, xanthine, and caffeine

Abstract

The protonation equilibria of three hydroxypurines, hypoxanthine, guanine, and xanthine, and of related caffeine have been studied by 1 H and 13 C nmr and uv spectroscopies in aqueous sulfuric or perchloric acids. The results have been interpreted on the basis of the excess acidity method. The [Formula: see text] values and the protonation sites are discussed and comparisons are made with results of recent theoretical calculations.