KINETICS AND YIELD OF SINGLET OXYGEN PHOTOSENSITIZED BY HYPERICIN IN ORGANIC AND BIOLOGICAL MEDIA

The spectroscopy and photophysics of the photosensitizer hypericin whe n in homogeneous solutions and when bound to liposomes were studied. H ypericin was found to partition efficiently into DMPC liposomes, with a binding constant of 58 (mg lipid/mL)(-1). In these liposomes the sin glet oxygen production quantum yield was 0.43 +/- 0.09. To determine t he deactivation constant of singlet oxygen in lipid bilayers for the f irst time, we calculated extrapolated values from its quenching by DMP C and lecithin in homogeneous solutions and obtained decay times of 36 .4 and 12.2 mu s, respectively. We also measured the quenching of sing let oxygen, sensitized by hypericin in DMPC liposomes, by NaN3, diphen yl isobenzofuran and H2O:D2O mixtures and explained the results on the basis of singlet oxygen diffusing rapidly out of the lipid bilayer in to the aqueous medium. The observed temperature effect on the lifetime of singlet oxygen of about 50% over a 15 degrees C range in liposome suspension contrasts with a 3% change in a homogeneous solution in 1-n onanol and is explained by the temperature effect on the diffusion out of the liposome. A strong pH effect was observed, indicating that the deprotonated species formed above about pH 10 is a much weaker photos ensitizer of singlet oxygen than the native, protonated species.