A Kinetic Study of the Interaction Between Bilirubin and Thermally Produced Singlet Oxygen

Thermal decomposition of 3‐(4‐methyl‐1‐naphthyl)‐propionic acid‐1,4‐endoperoxide at 34,7°C has been used to produce singlet oxygen in non‐photochemical conditions. Thus the reaction of bilirubin with singlet oxygen has been studied. A kinetic scheme is proposed taking into account physical and chemical quenching of singlet oxygen by bilirubin and quenching by the endoperoxide. From the lifetime, τ = 100 μs, of 102 in methylene chloride the following rate constants have been determined: for bilirubin: kphys quenching = 5.0 × 109 M−1s−1,kchem quenching = 5.0 × 108 M−1s−1 and for the endoperoxide: k = 8.5 × 107 M−1s−1. These same constants in the same order as above but in water (τ = 3μs) are: 4.9 × 109 M−1s−1, 1.0 × 109 M−1 s−1 and 9.0 × 108 M−1's−1 Moreover it has been shown that bilirubin is scarcely protected by albumin from singlet oxygenation. The protection in photochemical systems seems to be due to the lower ability of bilirubin to behave as a photosensitizer.