A new photoisomerization of bilirubin.

Photoirradiation of solutions of natural (4Z,15Z)-bilirubin-IX alpha in chloroform with approximately 366 nm UV light leads rapidly to a new photoisomer that has been characterized by NMR spectroscopy and by its (ground-state reaction) adducts with methanol and other protic nucleophiles. Unlike the previously described Z-->E geometric isomerization important in phototherapy of neonatal jaundice, the new photoisomerization involves regiospecific photoautomerization to afford 2-ethenyl-18-ethylidene- 1,10,19,22,24-pentahydro-2,7,13,17-tetramethyl-1,19- dioxo-21H-biline-8,12-dipropanoic acid.