Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high‐performance liquid chromatography on immobilized polysaccharide‐type chiral stationary phases and off‐column racemization experiments

Direct enantiomer separation of hypericin, pseudohypericin, and protohypericin was accomplished by high-performance liquid chromatography (HPLC) using immobilized polysaccharide-type chiral stationary phases (CSPs). Enantioselectivities up to 1.30 were obtained in the polar-organic elution mode whereby for hypericin and pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5-dichlorophenylcarbamate)] and for protohypericin Chiralpak IA (chiral selector being the 3,5-dimethylphenylcarbamate of amylose) gave favorable results. Enantiomers were distinguished by on-line electronic circular dichroism detection. Optimized enantioselective chromatographic conditions were the basis for determining stereodynamic parameters of the enantiomer interconversion process of hypericin and pseudohypericin. Rate constants delivered by computational simulation of dynamic HPLC elution profiles (stochastic model, consideration of peak tailing) were used to calculate averaged enantiomerization barriers (DeltaG(enant)(#)) of 97.6-99.6 kJ/mol for both compounds (investigated temperature range 25-45 degrees C). Complementary variable temperature off-column (i.e., in solution) racemization experiments delivered DeltaG(enant)(#) = 97.1-98.0 kJ/mol (27-45 degrees C) for hypericin and DeltaG(enant)(#) = 98.9-101.4 kJ/mol (25-55 degrees C) for pseudohypericin. An activation enthalpy of DeltaH(#) = 86.0 kJ/mol and an activation entropy of DeltaS(#) = -37.7 J/(K mol) were calculated from hypericin racemization kinetics in solution, whereas for pseudohypericin these figures amounted to 74.1 kJ/mol and -82.6 J/(K mol), respectively. Although the natural phenanthroperylene quinone pigments hypericin and pseudohypericin as well as their biological precursor protohypericin are chiral and can be separated by enantioselective HPLC low enantiomerization barriers seem to prevent the occurrence of an excess of one enantiomer under typical physiological conditions--at least as long as stereoselective intermolecular interactions with other chiral entities are absent.