Use of the trans-cis isomerization of the urocanic acid as in vitro measurement of protective effectiveness of sunscreens

The urocanic acid (UCA) undergoes trans-cis isomerization after exposure to UV light. In this study we estimated in vitro the sunscreen's ability to absorb UV-light. The sunscreens were put in touch with the UCA-standard-solutions into an apparatus built for this specific problem. The apparatus was irradiated using a UVB lightsource Philips TL12 (6 tubes 40 W). Afterwords we tested the amount of cis-UCA formed from trans-UCA. The results were compared with those obtained without sunscreen's application. In this experience we demonstrated a strict inverse correlation between sunscreens's Protection Factor and amount of cis-UCA. This method is useful for an initial screening of the sunscreen's ability to shade the UVB-light. But tests 'in vivo' are necessary to estimate the real photoprotective sunscreen's ability to protect the skin from UVB.