Excited-state properties of hypericin: electronic spectra and fluorescence decay kinetics

Abstract

Hypericin (Hyp), the parent chromophore of the photoreceptor protein (stentorin) participates in photophobic movements in Stentor coeruleus. The photoreactivity in the lowest singlet excited state (SI) of hypericin has been studied with respect to the solvent effects on absorption and fluorescence spectra and fluorescence decay kinetics. The primary photoreaction of hypericin was probed through (1) hydrogen-bonding effect, (2) deuterium solvent isotope effect, (3) pH dependence and (4) fluorescence quenching by p-benzoquinone. From the pH dependence of the spectra, the following equilibrium reactions are established: ( H y p H y p ) + 20H-+ 2Hyp-+ 2Hz0, in aqueous solution; Hyp + OH-+ Hyp-+ HzO, in water/Triton X solution, with pKa = 11.3 in SO and pKa* = 12.2 in SI. The neutral species, Hyp, emits fluorescence (Amax = 600 nm) strongly, whereas the anionic species, Hyp-, emits fluorescence (Amax = 660-680 nm) very weakly. The dimer (Hyp-Hyp) shows virtually no emission. Fluorescence quenching occurs effectively in the presence of p-benzoquinone, which functions as an electron acceptor. Possible photoreaction pathways of hypericin (SI), Le., deprotonation and electron transfer, are discussed in relation to the primary photoreaction in stentorin.

Effect of protonation on the singlet–singlet excited-state absorption of meso-tetrakis(p-sulphonatophenyl) porphyrin

“…The light excitation induces an absorption increase in the 500-620 nm range, similar to the nonprotonated TPPS 4 porphyrine sample, and a decrease between 620 and 700 nm. Changes in the red-ox characteristics, followed by pK shifts in organic molecules after excitation are well documented [24][25][26][27]. Based on that, we attribute the difference in the r 01 (k) and r 1n (k) profiles to the partial TPPS 4 deprotonation that appears due to a negative pK shift at excitation.…”

Hypericin, Hypocrellin, and Model Compounds: Primary Photoprocesses of Light-Induced Antiviral Agents

“…To our knowledge, the fluorescence decay of hypericin and its analogues does not depend on excitation wavelength, visible and ultraviolet excitation yielding similar results. 8,25 This result should not be confused with the excitation wavelength dependence of the picosecond transients in hypericin. 15 These latter transients report on primary processes that are not resolvable in the measurement of the longer lived fluorescence decay.…”

“…Hypericin is unlike the other three compounds investigated Normalized absorption (-) and emission (---) spectra in DMSO of (a) hypericin (λex ) 550 nm), (b) hexamethoxyhypericin (λex ) 410 nm), (c) hypocrellin (λex ) 400 nm), and (d) tetramethoxyhypericin (λex ) 412 nm). The extinction coefficients for hypericin 8,25 and hypocrellin 27 are taken or estimated from the literature. The arrows indicate the wavelengths at which pump-probe transient absorption experiments were performed.…”

Sodium cholate, a solubilizing agent for the necrosis avid radioiodinated hypericin in rabbits with acute myocardial infarction

“…However, its poor biocompatible solubility in water-based formulations is a major limiting factor for its successful launch in market despite its pharmacological potentials (Van De Putte et al, 2006). In organic solvents, at the pH interval between 4 and 11, it forms monobasic salts (Weiner & Mazur, 1992;Yamazaki et al, 1993), which undergo homoassociation in the presence of water. The resulting aggregates lack necrosis targetability and exhibit unwanted retention in organs of the mononuclear phagocyte system (MPS) (Van De Putte et al, 2006).…”