The Collaborative International Dictionary of English v.0.48:

Murexan \Mu*rex"an\, n. [From Murexide.] (Chem.)
   A complex nitrogenous substance obtained from murexide,
   alloxantin, and other ureids, as a white, or yellowish,
   crystalline which turns red on exposure to the air; -- called
   also uramil, dialuramide, and formerly purpuric acid.
   [1913 Webster]