Spectroscopic study of visible-light effects on hypericin-lens proteins systems

Molecular interactions between hypericin and alpha-, beta- and gamma -cryst allin proteins have been studied by means of absorption and steady-state fl uorescence spectroscopy, aiming to clarify if and how the pigment binds to the proteins and to investigate the effects of visible-light irradiation on these molecular systems. Such a study is a prerequisite for assessing the possibility of using hypericin as a mild antidepressant and/or as a photody namic agent for the treatment of eye tumors and eye viral and bacterial dis eases without side injuries to the lens. We have shown that in dark-kept sa mples, with increasing alpha -crystallin concentration, both the fluorescen ce emission intensity and the ratio of the absorption maxima around 590 and 550 nm of hypericin increase. These effects have been attributed to the mo nomerization of nonfluorescent hypericin aggregates caused by the binding o f the pigment to alpha -crystallin. The binding constant of hypericin has b een evaluated to be of the order of 3.0 (mg/mL)(-1), corresponding to a dis sociation constant of the order of 0.3 mg/mL. Following irradiation with li ght of wavelengths over 400 nm, at an irradiance of 20 mW/cm(2), both trypt ophan and hypericin fluorescence emission intensities decrease. These effec ts are suggested to be the consequence of a spatial rearrangement of the pr otein framework which takes place following the alpha -crystallin photopoly merization sensitized by hypericin itself described in the literature. For the sake of comparison hypericin has been studied also in the presence of b eta (H)-, beta (L)- and gamma -crystallins at the same concentration.