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J. Soc. Cosmet. Chem., 48, 151-157 (May/June 1997) Photochemistry of urocanic acid: Evidence that urocanic acid should be used with caution in cosmetic formulations KERRY M. HANSON and JOHN D. SIMON*, Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, LaJolla, CA 92093-0341. Accepted for publication July 26, 1997. Synopsis Data from a previously reported spectroscopic investigation of urocanic acid are summarized. The data support the conclusion that urocanic acid should be used with caution as an ingredient in sunscreen and conditioning cosmetic formulations because reactive intermediates can be produced following absorption of the sun's lower UV-B wavelengths. INTRODUCTION Trans-urocanic acid (t-UA) (Scheme 1) is a naturally occurring ultraviolet (UV) chro- mophore formed from the action of histidase upon histidine in the stratum corneum. t-UA was originally identified as a component of sweat, and determined to have a broad, structureless absorption spectrum in the ultraviolet region of the solar spectrum (1). As a result, the chromophore was proposed to act potentially as a natural sunscreen, ab- sorbing solar UV in the stratum corneum. The effective result of t-UA in the stratum comeurn was assumed to be the attenuation of UV radiation before penetration into deeper epidermal layers where damage to other UV-absorbing chromophores, like DNA, H 0 H \ •• AbsorptioUVof H trans-urocanic acid cis-urocanic acid Scheme 1. * Current address: Department of Chemistry, Duke University, Durham, NC 27708 151