Photooxidation of Xanthobilirubic Acid in Aqueous Solution: Product and Mechanism Studies

Xanthobilirubic acid, an oxodipyrrylmethene with a chromophore very similar to that of bilirubin, was aerobically irradiated as its sodium salt in borate buffer at pH 8. Two photooxidation products, methylethylmaleimide and 5‐formyl‐2,4‐dimethyl‐1H‐pyrrole‐3‐propanoic acid, were isolated. These products can be rationalized on the basis of either a Type I or a Type II (singlet oxygen) photooxygenation mechanism. The reaction is inhibited by azide, a singlet oxygen quencher, and sensitized by methylene blue. However, both the self‐sensitized and the methylene blue‐sensitized reactions are enhanced, rather than inhibited, by high concentrations of 1,4‐diazabicyclo[2.2.2]octane, another known singlet oxygen quencher. It is therefore proposed that the diazabicyclooctane can also act as an electron donor. The self‐sensitized photooxidation of xanthobilirubic acid is an autocatalytic reaction. This is supported by the fact that addition of a previously irradiated solution to a freshly‐prepared solution significantly increases the rate of photodegradation. A similar catalyst is formed, but much more slowly, from xanthobilirubic acid in the dark in the presence of oxygen.