Sunscreen agents are utilized to absorb UVB (and sometimes UVA) radiation for skin protection from these harmful solar UV rays. However, when sunscreen chemical agents absorb UV light they often undergo photodissociation, which may result in a reduced UV absorption capacity or even the formation of carcinogenic adducts. Thus, understanding the photochemistry of these sunscreen chemical agents is important. Para-aminobenzoic acid (PABA) is one of the first sunscreen agents used. In this study, the photolysis of PABA was studied at three different UV ranges (UVA: 355 nm, UVB: &gt;280 nm, and UVC: 266 nm and 213 nm) using parahydrogen (pH2) matrix isolation Fourier-Transform infrared (FTIR) spectroscopy. PABA was stable under UVA (355 nm) irradiation. However, PABA dissociated to form 4-aminylbenzoic acid (the PABA radical) through amino hydrogen atom loss under UVB and UVC irradiation. The production of the PABA radical supports a previously proposed mechanism of the formation of the carcinogenic PABA-thymine adduct. This work shows that pH2 matrix isolation spectroscopy is effective for sunscreen agent photochemical studies.

UV photolysis study of para-aminobenzoic acid using parahydrogen matrix isolation spectroscopy

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ACS Fall 2022

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