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

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.

Section: Resultsmentioning

confidence: 85%

Circular Dichroism of the Photoreceptor Pigment Oxyblepharismin

Pieroni

Plaza

Mahet

et al. 2005

“…Hypericin, a natural compound identified in the plants of genus Hypericum (13), has been largely investigated as a photodynamic dye in the treatment of viral, bacterial infections and tumors (14–16). Binding of the photosensitizer to a biological substrate is important as it cannot only increase the residence time of the sensitizer but it may also change its photophysics ([17] and references therein). The photosensitizing effect of hypericin was described as singlet oxygen production when it is in organic solvent (18) or when it is bound to liposomic media (19) whereas when aggregated, e.g.…”

Section: Introductionmentioning

confidence: 99%

Fluorescence Study on the Interaction Between Hypericin and Lens Protein “α‐Crystallin”

Youssef¹

2009

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.

“…2), they do share the common feature of extended absorption from the visible to the UV. There is growing evidence from NMR (77,78), calculations (79), and spectroscopic studies of the separated hypericin enantiomers (80) that the complexity of the hypericin absorption spectrum does not arise from inhomogeneity of the ground state. The calculations performed here for the dihydroxy and the dimethoxy perylene quinone spectra can be explained in terms of transitions to a manifold of excited singlet states.…”

Section: Discussionmentioning

confidence: 99%

Synthesis of Hydroxy and Methoxy Perylene Quinones, Their Spectroscopic and Computational Characterization, and Their Antiviral Activity^¶

Krishnamoorthy¹,

Webb²,

Nguyen³

et al. 2005

Hydroxy and methoxy perylene quinones are synthesized in an attempt to isolate the essential spectroscopic and biological features of light‐induced antiviral agents such as hypericin and hypocrellin. Unlike their naturally occurring counterparts, these synthetic quinones bear the carbonyl, hydroxyl, and methoxy groups in the “bay region.” The hydroxy and methoxy compounds have rich absorption spectra with broad features in the visible (∼450–800 nm) and relatively more intense and narrow features at wavelengths ≤350 nm. High‐level ab initio quantum mechanical calculations assign the features in the absorption spectra to electronic transitions from So to S2 and to higher‐lying electronic states. The calculations indicate that in the ground state the trans dihydroxy isomer is 12.5 kcal/mol lower in energy than the cis dihydroxy isomer and is thus the only species present. The lowest‐energy trans methoxy ground state isomer and the lowest‐energy cis methoxy ground state isomer are found to be degenerate. An additional cis methoxy isomer 6.3 kcal/mol higher in energy than the global minimum is assumed to contribute to the spectrum and is also considered. Finally, the synthetic compounds exhibit similar light‐induced antiviral activity to each other, but significantly less than that of hypericin.