Urocanic acid is a bioorganic molecule found in human skin. It acts as a  UV-filter, presumedly protecting DNA from radiation-induced damage.  Upon UV-irradiation, it is able to undergo E/Z-isomerization.Urocanic acid exhibits a noteworthy wavelength-dependent E/Z-isomerization  quantum yield. Upon absorption in the short-wavelength region of the  absorption spectrum the quantum yield for isomerization amounts to only  5%, whereas absorption of longer wavelengths has the quantum yield rise  up to 50%. We propose that this behavior originates from the topography  of excited state potential energy surfaces.The detailed mechanism of photochemical isomerization has not been  elucidated up to this day. We will show CASSCF potential energy surfaces  (corrected by CASPT2 or XMCQDPT2) along the  isomerization coordinate as well as other internal coordinates. Possible  mechanisms for photophysical deactivation of urocanic acid associated  with retention of the (E)- (or (Z)-) configuration and for photochemical isomerization will be outlined.

Photochemistry and photophysics of urocanic acid: A quantum chemical study

243rd National Meeting (2012)

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