Xanthosine is a nucleoside derived from xanthine and ribose. It is the biosynthetic precursor to 7-methylxanthosine by the action of 7-methylxanthosine synthase. 7-Methylxanthosine in turn in the precursor to theobromine (active alkaloid in chocolate), which in turn is the precursor to caffeine, the alkaloid in coffee and tea. The 6-oxopurine xanthine (Xan, neutral form 2,6-diketopurine) differs from the corresponding 6-oxopurines guanine (Gua) and hypoxanthine (Hyp) in that, at physiological pH, it consists of a approximately 1:1 equilibrium mixture of the neutral and monoanionic forms, the latter due to ionization of N(3)-H, in striking contrast to dissociation of the N(1)-H in both Gua and Hyp at higher pH. In xanthosine (Xao) and its nucleotides the xanthine ring is predominantly, or exclusively, a similar monoanion at physiological pH.
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