Homo-association of hypericin in water and consequences on its photodynamic properties

The hypericin absorption spectrum in water is clearly different from that observed in organic solvents where hypericin exists as monomer. The influence of the percentage of EtOH in ethanol-water mixtures. the total hypericin concentration and the temperature show that from 5.10(-7) to 5.10(-5) M hypericin is in a dimeric form in water. The aggregation seems to be mainly due to Van der Waals interactions between the hydrophobic cores of monomers and we suggest from the exciton theory that the dipole moments in dimers are oriented obliquely (alpha = 95 degrees). The hypericin association in water lends to the quenching of luminescence and inhibition of phodynamic properties: singlet oxygen was observed neither by direct detection of I.R. luminescence nor by indirect E.S.R. measurements. Superoxide could not been detected by cytochrome C reduction and by E.S.R. by spin trapping with DMPO. The excited state radiationless deactivation could be in part due to a charge-transfer. The transient existence of charge separation in dimers might then induce O-2-radical-anion generation. Consequently, the hypericin photodynamic properties in liposomic or micellar aqueous dispersions are caused by monomers which are localized in the lipidic phase.