Mechanisms of the photodegradation of bilirubin.

Bilirubin (I) can be regarded as a dipyrrolylmethane (shown in bold in formula 1) with conjugating substituents at C-5 and C-5’ (dipyrrolylmethane numbering).* These substituents (i.e. the unsaturated lactam rings) are very similar to one another, but are not identical (cf. I1 and 111). They have the electronic dichotomy associated with the amide function. Although they formally have the capability of being electron donors (shown at rings A and B in I), it seems more likely in view of the nexcessive nature of the pyrrole system that they will generally behave as electron-withdrawing groups (shown at rings C and D). From a photochemical point of view these substituents are most important, since they complete the chromophore or, more correctly, the two chromophores, I1 and 111. These two chromophores give rise to a broad envelope (Amax. x450nm in chloroform) in the absorption spectrum. This absorption corresponds to an energy of approx. 265 kJ/mol for the first excited singlet state (S,) of bilirubin. This state has been observed in emission from polar glasses at 7 7 K [A,,.. % 525nm; Bonnett et al. (1975); Matheson et al. (1975)]. The energy of the first excited triplet state (T,) is at present unknown. The photochemistry of bilirubin (and other linear tetrapyrroles) is of considerable interest in its own right, but undoubtedly the marked development in this area in recent years is largely due to the bearing that this work has on the phototherapeutic treatment of neonatal jaundice (Cremer et al., 1958; McDonagh, 1976). The problems that arise