Establishment of a kinetic model for urocanic acid photoisomerization

The photoisomerization reactions of the E and Z isomers of urocanic acid (UCA), which are natural constituents of the mammalian epidermis, were studied by time-resolved hash photolysis and quantum yield determination in the pH range 3.0-10.0. A kinetic model for the processes is presented on the basis of the following observations: (1) the sum of the quantum yields for E --> Z and Z --> E reactions can be more than unity; (2) a kinetic component with almost equal rate parameters for both reactions is observed in flash photolysis; (3) fluorescence and (4) loss of material were not detected; (5) the quantum yield of formation of the isomerization product is represented by a short-lived decay component in the E --> Z and by a long-lived component in the Z --> E reaction. Correlations between the experimental data and possible molecular arrangements are examined.