Reaction of Sapienic Acid with Hydroxyl Radical in Water: A Kinetic and Product Study

Sapienic acid (SA; <i>(Z)-</i>hexadec-6-enoic acid) is the most abundant unsaturated fatty acid on the human skin surface. SA possesses a carbon-carbon double bond that allows it to react with reactive oxygen species, such as the hydroxyl radical (OH), originating from the gas phase or formed directly at the skin surface. Here, time-resolved electrospray ionization mass spectrometry analysis of the oxidation of SA by OH, generated by the irradiation of hydrogen peroxide with ultraviolet light, revealed the formation of several oxygen-incorporated products in the aqueous phase, rather than smaller products resulting from C-C bond scission. Further experiments using D₂O and H₂¹⁸O as solvents instead of H₂O indicated the number of -OH/-OOH/-C═O moieties in the OH-oxidation products. One of the products possessed three -OOH groups and was categorized as a highly oxygenated organic molecule (HOM)-like species. The fact that mass spectral signals assigned to products possessing multiple -OOH groups retained their intensity for 19 h in the dark implied that these species may persist on the skin surface for at least a day under dark conditions. A relative rate analysis afforded an aqueous-phase rate constant for the SA + OH reaction of (6.5 ± 1.8) × 10⁹ M^-1 s^-1 at 298 ± 3 K.