EXCITED STATES OF BILIRUBIN

Abstract— The excited states of bilirubin (BR) in a variety of environments have been studied by 347 nm laser flash photolysis. Quantum yields of formation of triplet BR have been shown to be less than 0.005 in solution in water ( p H 9–11), methanolic ammonia, 10% aqueous mulgofen and in cetyl trimethyl-ammonium bromide. In benzene the quantum yield was 0.01 although this diminished to less than 0.005 on addition of triethylamine. Permanent products are formed with benzene and with 1% methanolic ammonia. With BR in HSA a transient decaying with k = 3.5 × 10⁵ s^-1 is formed by a monophotonic process together with a permanent product. Neither species is affected by oxygen or by iodide ion. Both originate from BR molecules in the strongest binding site in the HSA. The yields of both species are unaffected by salt but are temperature dependent. The decay of the transient is strongly temperature dependent corresponding to an activation energy of about 50–60 kj mol^-1. If this transient is a triplet it is formed with a quantum yield of 0.13 ± 0.01. The relevance of these results to an understanding of the photo therapeutic process is discussed.