Photochemical formation of singlet molecular oxygen (1O2) in illuminated aqueous solutions of p-aminobenzoic acid (PABA)

Evidence is presented for the photochemical formation of singlet molecular oxygen (1O2) in air-saturated buffered aqueous solutions of p-aminobenzoic acid (PABA) using sunlight-range illumination. This is significant because PABA is widely used as an active ingredient in sunscreen preparations that are applied to the surface of the skin and 1O2 is known to cause oxidative damage to cells via the formation and subsequent reactions of lipid peroxides. Furfuryl alcohol (FFA), a well known chemical trap for 1O2, was added to aqueous PABA solutions prior to illumination. The FFA was consumed when the solution was illuminated, but no loss of FFA occurred in the dark and loss by direct photolysis was negligibly slow. Further evidence for the formation of 1O2 in illuminated aqueous PABA solutions is provided by the results of experiments in which individual solutions containing PABA and FFA that were diluted with D2O exhibited an increased rate of FFA consumption due to the increased lifetime and concentration of 1O2 in this solvent.