✍ Scribed by Sophie Franceschi; Juliette Sirieix; Nancy Lauth-de Viguerie; Monique Rivière; Armand Lattes
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Resume -I'erude de la photo-isornerisation de l'acide urocanique E (photoprotecteur naturel de la peau) et de certains de ses derives a permis de montrer l'importance du coefficient d'extinction molaire des deux isorneres a la longueur d'onde d'irradiation. Lisomerisarion thermique des memes composes a lieu difficilement et uniquement dans Ie sens Z ~E. Les isorneres E sont thermodynamiquement les plus stables. Dans les derives non N-alkyles, en milieu basique, Ie passage par l'ion amidure delocalise abaisse la barriere energetique sans changer le sens de l'isomerisation, © Academic des sciences/Elsevier, Paris isomerisation / photo-isomerisation / acide urocaniqne / acide 3-(lH-imidazol-4-yl)-propano'ique / imidazole Abstract -Isomerisation and stability of Z/E isomers of urocanic acid and derivatives. Thermal and photochemical isomerisation of urocanic acid and its derivatives (0and/or N-alkylated) have been studied. The data show the wavelength and the molar extinction coefficient-dependent photochemistry of E urocanic acid and its derivatives. Thermal isomerisation of these compounds is difficult and undergoes a "one-way" Z ~E reaction. Thus, E isomers are thermodynamically more stable than the Zones. Under basic conditions, derivatives having an unsubstirued imidazole ring (NH function free) exist in their anionic forms, which lowers the energy barrier without changing the isomerisation pathway. © Acadernie des sciences/Elsevier, Paris isomerisation / photoisomerisation / uroeanic acid / 3•(lH-imidazol-4-yl)-propanoic acid / imidazole Figure 1. Equilibre photostationnaire des deux stereoisorneres de I'acide urocanique. De nombreux travaux ont permis de rnontrer que l'acide urocanique Z (Z-AU) joue un role important dans les phenomenes de photoimmunosuppression induits par l'irradiation 1387-1609/99/000200299 © Acadc!mie des sciences/Elsevier, Puis
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