The evaluation of the amount of cis- and trans-urocanic acid in the stratum corneum by Raman spectroscopy.

Urocanic acid (UCA) is a major component in the epidermis which absorbs ultraviolet light (UV) and is isomerized from the trans to the cis form upon UV exposure. This study aimed to evaluate the amounts of cis- and trans-UCA in the stratum corneum (SC) in vivo using Raman spectroscopy. UV damage has previously been investigated by calculating the amount of trans-UCA by fitting Raman spectra. cis-UCA at pH 5.0 was also included in the fitting algorithm in this study. For validation, the SC was stripped from the subjects and the levels of cis- and trans-UCA were measured using high performance liquid chromatography (HPLC). Comparison between these two methods was performed at UV exposure and non-UV exposure sites on the volar forearm of 10 healthy Japanese subjects. Finally, the protective capacity of sunscreen was evaluated by Raman spectroscopy on the volar forearm of 18 healthy subjects. The amount of cis-UCA increased and the amount of trans-UCA decreased after UV exposure when measured by Raman spectroscopy. This was confirmed by HPLC analysis of the SC stripped from the same area of skin used in the Raman measurement. The protection against the increase in the cis-UCA and decrease in the trans-UCA amount after UV exposure by the application of sunscreen products was observed using Raman spectroscopy. The results show this method can be used to measure cis- and trans-UCA levels in vivo.