Solvatochromic effects in the electronic absorption and nuclear magnetic resonance spectra of hypericin in organic solvents and in lipid bilayers

The natural product hypericin was tested in recent years as a biological ph otosensitizer with a potential for viral and cellular photodamage. We thus studied extensively its spectroscopy and membrane partitioning. Absorption, fluorescence excitation and emission spectra of the sodium salt (HyNa) wer e measured in 36 protic and aprotic, polar and apolar, solvents. Electronic transition bands as well as vibrational progressions were identified. Aggr egation in some nonpolar solvents and protonation in organic acids were dem onstrated. Modeling solvatochromism was done by Lippert equation, by the E- T(30) parameter and by the Tan multiparameter approach. In all cases, separ ation into protic and aprotic solvents gave much better fits to the models. C-13 chemical shift data could also be correlated with solvent polarity. T hey correlated best with Lippert's Deltaf polarity measure, but tended to f all into two distinct solvent groups-each along different lines-correspondi ng to protic and aprotic media, respectively. This interesting phenomenon s uggests that in the case of the charged and slightly water soluble HyNa, tw o mechanisms of solvation are involved, each resulting in its own line equa tion. In aprotic media, dipole-dipole interaction is the predominant solvat ion mechanism. In protic solvents, the most effective means of solvation is likely to be hydrogen bonding. When intercalated into the liposomal phosph olipid bilayer, HyNa is oriented at an angle to the interface, thus experie ncing a gradient of solvent polarities: a highly polar environment (similar to methanol) for C-2/5, suggesting that they lie not far from the interfac e; a moderately polar environment (similar to that of n-propanol) for C-6a/ 14a, which are somewhat deeper within the bilayer; and a more lipophilic en vironment (akin to n-hexanol) for C-10/11. The fluorescence excitation peak in liposomes also correlates with an aprotic medium of relatively high pol arity, as might be excepted from a molecule in a shallow position in the bi layer.