Xanthine

Xanthines are a chemical class of organic compounds that are purine-derived alkaloids, built on a fused double-ring purine skeleton bearing carbonyl groups. Members differ by the methyl groups attached to the ring nitrogens. They act primarily as antagonists at adenosine receptors, producing mild stimulant effects, and include the most widely consumed stimulant on earth. The methylated purine skeleton is the defining structural feature of the class.