Hypericin, an intriguing internally heterogenous molecule, forms a covalent intramolecular hydrogen bond

On the origin of photodynamic activity of hypericin and its iodine-containing derivatives

The main photophysical properties, useful for establishing whether hypericin in anionic form and some of its derivatives containing heavy atoms such as iodine, can be proposed for their use in photodynamic therapy, were determined using density functional based computations. The results showed that in the anionic form and in the iodinated derivatives, the absorption wavelength undergoes a bathochromic shift, the singlet-triplet energy gap assumes values ​that allow to excite the oxygen molecule from its ground to the excited singlet state, and that the spin-orbit couplings between singlet and triplet states significantly increase.

Aromaticity and Electron Density of Hypericin

The influence of the substituents on the geometry of the central ring system of hypericin has been analyzed. Substitution that causes flattening of the hypericin central rings is connected with introducing the aromatic character of the empty rings. All the hypericin rings have an aromatic character illustrated by the Harmonic Oscillator Measure of Aromaticity (HOMA), Nucleus Independent Chemical Shift (NICS), Fluctuation Index (FLU), and Ellipticity Index (EL) indices. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analyses performed on 7,14-dihydrophenanthro[1,10,9,8-opqra]perylene, its substituted analogues, and hypericin show an influence of this substitution on electron density of the central rings.

Photodynamic effect of hypericin on the microorganisms and primary human fibroblasts

BACKGROUND

Hypericin (HYP) is a natural photosensitizer considered for the new generation of photodynamic therapy (PDT) drugs. The aim of this study was to evaluate the in vitro bactericidal effect of HYP-PDT on four bacterial species, assessing its photocytotoxicity to primary human fibroblasts to determine possible side effects.

METHODS

Effect of photosensitizer concentration (0.1, 0.3, 0.6, and 1 μg/ml) and light irradiation time (3, 5, 10 min) on photodynamic inactivation of microorganisms and primary human fibroblasts was investigated.

RESULTS

A 6.3 log killing was obtained for Staphylococcus aureus (ATCC 25923) treated with 1 μg/ml at 48 J/cm². For this set of PDT parameters, Enterococcus faecalis (ATCC 11700) showed 6.5 log killing, Escherichia coli (ATCC 25922) 6.2 log killing, and Pseudomonas aeruginosa (ATCC 27853) 0.7 log killing. Fibroblasts can be preserved by keeping the HYP concentration below 0.6 μg/ml and the light dose below 48 J/cm².

CONCLUSION

S. aureus, E. faecalis, and E. coli appear to be suitable for treatment with HYP-PDT without significant damage to fibroblasts.

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.

Photodynamic effect of hypericin on the conformation and catalytic activity of hemoglobin

Hypericin, extracted from H. perforatum, can induce the generation of reactive oxygen species by visible light irradiation, which may consequently induce the conformational change of hemoglobin. We have not only employed UV-vis spectroscopy to observe the changes of UV-vis spectra of the protein, which reveals the conformational changes of the protein, but also employed electrochemical method to obtain its enhanced peroxidase activity. The photodynamic effect of hypericin on the conformation and catalytic activity of the protein has also been proven to be strongly dependent on the irradiation time, the hypericin concentration and the presence of oxygen. This work is beneficial not only to the fabrication of more sensitive hydrogen peroxide biosensor, but also to the guidance of the usage of this medicinal herb molecule, since the conformational change of the protein and the enhanced peroxidase can be easily obtained only by visible light irradiation on hypericin, the process of which is so common to happen.

Characterization of the interaction of hypericin with protein kinase C in U-87 MG human glioma cells

A fluorescence imaging technique was used to monitor intracellular localization of protein kinase C (PKC) in U-87 MG human glioma cells in the presence of hypericin (Hyp) and phorbol 12-myristate-13-acetate (PMA). It is shown that PKC localization, which reflects its activity, is influenced by Hyp and this influence is different from that observed for PMA which acts as PKC activator. Fluorescence binding experiments were used to determine the binding constants of Hyp to several isoforms of PKC. The obtained values of K(d)s (approximately 100 nM) suggest that Hyp binds with high affinity to PKC. Finally, molecular modeling was used to compare structural models of the interaction of C1B domain of PKC (PKC isoforms alpha, delta, gamma) with Hyp and our previously published model of the (C1B domain PKCgamma)/PMA complex. The influence of Hyp on PKC translocation in U-87 MG cells in comparison with PMA, colocalization fluorescence pattern of Hyp and PKC, the higher binding affinity of Hyp to PKC in comparison with known binding constants of phorbol esters, as well as the binding mode of Hyp and PMA to the C1B domain of PKC suggested by molecular modeling, support the idea that Hyp and PMA might competitively bind to the regulatory domain of PKC.

Hypericin derivatives: substituent effects on radical-anion formation

The electron-transfer properties of the hypericin derivatives, dibromo-, hexaacetyl-, hexamethyl- and desmethylhypericin, were studied. Cyclovoltammetric measurements revealed that dibromo- and desmethylhypericin have almost the same redox potentials as the parent hypericin. Substitution of the hydroxyl groups by acetoxy leads to less negative E1/2 values, whereas methoxy substitution induces more negative values. Electron paramagnetic resonance (EPR)/electron nuclear double resonance/general TRIPLE spectroscopy and quantum mechanical calculations were used to establish the structure of the one-electron reduced stages of hypericin derivatives. Proton loss in the bay region, already demonstrated for hypericin, was also found for dibromo- and desmethylhypericin. The spin and charge of the radical ions are predominately confined to the central biphenoquinone moiety of the hypericin skeleton. Generation of the radical ions by in situ electrolysis indicates that the redox potentials of hypericin, dibromo- and desmethylhypericin, containing hydroxyls at the 1, 3, 4, 6, 8 and 13 positions, largely depend on the solvent. With phosphate-buffered saline (pH 7.4)/dimethylsulfoxide (DMSO) as the solvent the EPR spectra of the corresponding radical ions appear at markedly lower potentials than in pure DMSO and N,N'-dimethylformamide. However, this effect is not observable for hexaacetyl- and hexamethyl-hypericin-lacking hydroxyl groups. In all cases the EPR data and calculations revealed the presence of 7,14 tautomers.