Spectroscopic characterization of hypericin and related compounds

Abstract

<p>Absorbance and fluorescence spectra of hypericin were studied in a variety of solvents. Hypericin was found to be monomeric in most polar organic solvents, aggregated in non-polar solvents and water, and protonated in acidic solvents. The solvatochromatic shifts for monomeric hypericin were also investigated and found to be related to the solvent's proton donor ability for protic solvents and to the solvent's proton acceptor ability and dipolarity/polarizability for aprotic solvents. Surface enhanced resonance Raman spectra (SERRS) for neutral hypericin, protonated hypericin, and deprotonated hypericin were compared in order to corroborate the previously assigned electronic transition dipoles of hypericin. The SERRS data supports the assignments of the redmost transition (S[subscript]1) of hypericin as polarized through the short axis of the molecule, and the S[subscript]2 transition as polarized through the carbonyl groups;The vibrational spectra of the chromophore for three Stentor protein complexes (stentorin I, stentorin II, and stentorin IIB) were compared using SERRS. The chromophore for each protein complex produces the same spectrum, suggesting that the structure of the chromophore in each complex is very closely related or identical;Monolayer films of hypericin were prepared at the air water interface and transferred to solid substrates. These films were investigated using absorption, fluorescence and SERRS spectroscopies. Both the spectroscopic data and the [pi]-A isotherms suggest that hypericin forms [pi]-[pi] aggregates that are oriented vertically to the subphase. However there is also evidence for some multilayer formation. The effect of subphase on the [pi]-A isotherm was also investigated.</p>