Deactivation of singlet molecular oxygen by thiols and related compounds, possible protectors against skin photosensitivity.

Abstract— From time-resolved measurements of the decay of singlet molecular oxygen phosphorescence at 1270 nm in D2O, direct estimates have been gained for the rate constants of the singlet oxygen reactions with a group of sulphur compounds in the pD range 5 to 13. In the case of most of the thiols, the results are consistent with singlet oxygen reacting exclusively with the thiolate anions. At the normal physiological pH 7, the apparent rate constants (in units of M-1 s-1) were 8.9 times 106 (cysteine), 2.5 times 106 (N-acetyl cysteine), 2.9 times 106 (glutathione), 3.0 times 105 (2-mercaptoethanol), 2.3 times 107 (ergothioneine) and 2.7 times 106 (2-mercaptopropionyl glycine). For methionine the rate constant, 1.4 times 107, was independent of pD in the range studied. These sulphur compounds, in particular N-acetyl cysteine and ergothioneine, or related compounds, might be considered as possible candidates for protection against skin photosensitivity side effects associated with the photodynamic therapy of solid tumours and as observed in the disease erythropoietic protoporphyria.