N (1)—C (6) N CH C (2)—C (5)—N (7) HC C NH C (8) CH N (3)—C (4)—N (9) N C—N Purin nucleus. Purin. To describe the individual _derivatives of purin_ we have to indicate to which particular atom of the _purin nucleus_ the combining groups are attached. Thus, for example, _adenine_ in structure is classed as a 6-amino-purin, and accordingly has the following formula: N==C—NH₂ | | HC C—NH ‖ ‖ \ ‖ ‖ CH ‖ ‖ / N—C—N Adenine (6-amino-purin). Other important bodies built up round the purin nucleus C₅N₄, variously designated as xanthine, alloxuric and nuclein bodies:— HN—C==O HN—C==O | | | | H₂NC C—NH O==C C—NH ‖ ‖ \ | ‖ \ ‖ ‖ CH | ‖ CH ‖ ‖ // | ‖ // N—C—N HN—C—N Guanine Xanthine (2-amino-6-oxypurin). (2-6-dioxypurin). HN—C==O HN—C==O | | | | HC C—NH O==C C—NH ‖ ‖ \ | ‖ \ ‖ ‖ CH | ‖ C==O ‖ ‖ // | ‖ / N—C—N HN—C—NH Hypoxanthine Uric acid (6-oxypurin). (2-6-8-trioxypurin). H₃C—N—C==O HN==C==O | | | | | | CH₃ | | CH₃ | | / | | / O==C C—N O==C C—N | ‖ \ | ‖ \ | ‖ CH | ‖ CH | ‖ // | ‖ / H₃C—N—C—N H₃C—N—C—N Caffeine Theobromine (1-3-7-trimethyl, (3-7-dimethyl, 1-2-6-dioxypurin). 2-6-dioxypurin). It will be seen that the _purin bases_ stand in very close chemical relationship to _uric acid_ in that the latter also is marked by the possession of a group called the _purin nucleus_; indeed, the relationship of uric acid to the purin bases is more intimate than to _urea_ (CON₂H₄), close though this latter be as may be seen from the study of its constitutional formula. (For uric acid may be regarded as composed of two urea radicles, linked by a tricarbon chain. By oxidation and hydrolysis, two molecules of urea may be obtained from one of uric acid, and conversely uric acid is produced by the condensation of urea with hydroxy acids).