Concerning the Photodiastereomerization and Protic Equilibria of Urocanic Acid and its Complex with Human Serum Albumin

Summary.

Photodiastereomerization of urocanic acid and its human serum albumin complex (its binding constant was estimated to amount 4.1·10⁻⁴ dm³·mol⁻¹) was investigated. It was found that although the photodiastereomerization rates were similar, the photoequilibrium positions differed significantly ((E):(Z)=33:77 for free urocanic acid, and 50:50 for the complex). This is thought to be due to a different stabilization of the corresponding orthogonal excited states. The thermal barrier of diastereomerization was estimated to amount to more than 250 kJ·mol⁻¹ making it a very unlikely process under physiological and photodiastereomerization conditions. The various prototropic species of the two diastereomers at various pH values were analyzed by means of a mathematical model and from these results a novel photoinduced pH-jump methodology allowing for fast, persistent, diffusion controlled, and bidirectional jumps is proposed.