Protonation of hypoxanthine, guanine, xanthine, and caffeine

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