Tauto Merism and Ionization of Xanthosine

Abstract

Tautomerism and ionization of xanthosine have been studied by infrared spectroscopy. N-methyl and O-methyl model compounds which are isoelectronic with possible keto and enol tautomers have been synthesized. Comparisons of their spectra with neutral and with ionized xanthosine demonstrate that unionized xanthosine has the diketo form and that on acid dissociation (pK 5.7), the first proton is lost from N3 (rather than N1) to give the 6-keto-2-enolate anion. Specific labelling at the 2- and 6-positions with ¹⁸O confirms these conclusions and supports assignment of the highest frequency bands in the double bond region to carbonyl stretching vibrations. These conclusions are relevant to two different proposed structures for the ordered form of polyxanthylic acid (pH 7), one based upon keto and one upon enol tautomeric forms.