The Separation of Hypericin's Enantiomers and Their Photophysics in Chiral Environments¶

On the diffusion of hypericin in dimethylsulfoxide/water mixtures-the effect of aggregation

Hypericin (Hyp) is a natural photosensitizing pigment with a possible application in the photodynamic therapy of cancer. Hyp is readily dissolved in dimethylsulfoxide (DMSO) but forms nonsoluble aggregates in an aqueous environment. Fluorescence spectroscopy and diffusion coefficient measurements are used to investigate the self-association of Hyp molecules in DMSO/water mixtures. Fluorescence measurements reveal that Hyp remains in its monomeric form in DMSO/water mixtures containing up to ∼20-30 wt % water. At higher water concentration, Hyp starts to form nonfluorescent aggregates. To determine the size of the aggregates, the diffusion coefficient of Hyp is determined for different DMSO/water mixtures both experimentally and theoretically. Our data indicate that the size of the aggregates increases as more water is added into DMSO. At 50 wt % water content, the effective diffusion coefficient is about 30% smaller than the calculated value for the stacked Hyp tetramer. The results indicate that in an aqueous environment, Hyp presumably produces large molecular weight stacked H-aggregates. We have also confirmed that in an aqueous environment at alkaline pH, molecules of Hyp remain in the monomeric state.

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.

Fluorescence study on the interaction between hypericin and lens protein "alpha-crystallin"

Hypericin has been reported as a potent photosensitizing agent exhibiting antiviral, antibacterial, antineoplastic activities. Although its photophysics and mode of action are strongly modulated by the binding protein, detailed information about its mechanism of interaction with possible cellular targets, including proteins, is still lacking. Previous in vitro studies demonstrated that hypericin can be uptaken by intact lens and is able to bind to the major lens protein "alpha-crystallin." In this study, the mechanism of interaction of this potent drug with alpha-crystallin was studied using the chemical denaturant guanidine hydrochloride (GdnHCl) and the hydrophobic surface probe, 8-anilino-1-naphthalenesulfonic acid (ANS). Fluorescence measurements showed that the increased exposure of tryptophan resulting from partial unfolding of alpha-crystallin incubated with 1.0 mol L(-1) of GdnHCl corresponds to the maximum accessibility of hydrophobic sites to ANS at the same GdnHCl concentration. Interestingly at this additional hydrophobicity of the protein, hypericin exhibited its maximum fluorescence intensity. This in vitro study implied that hydrophobic sites of alpha-crystallin play a significant role in its interaction with hypericin. The binding between alpha-crystallin and hypericin was found to be enhanced by partial perturbation of the protein.

Circular Dichroism of the Photoreceptor Pigment Oxyblepharismin

Circular dichroism (CD) was used to study the structure of oxyblepharismin (OxyBP), the photoreceptor chromophore for the photophobic response of the blue form of Blepharisma japonicum. Both the chromophore associated to its native protein and the free chromophore in ethanol solution were investigated. CD spectra in the far-UV range indicate that OxyBP induces a slight increase in the alpha-helix content of the protein matrix. CD spectra in the near-UV and visible region of the spectrum show that OxyBP adopts a chiral conformation with a preferential geometry not only when associated to its protein matrix, but also when isolated and dissolved in ethanol. This experimental result is related to the existence of a high-energy interconversion barrier between two enantiomeric structures of the molecule and discussed on the basis of an asymmetric biosynthesis of its precursor, blepharismin.