Incorporation and distribution of benzoquinone into liposomes containing amphilic zinc(II) porphyrin as studied by the quenching of the excited state

Abstract

The amphiphilic porphinato zinc(II), i.e.[5, 10, 15,20-tetra{α,α,α,α,-o-[2′,2′-dimethyl-20′-((2″-trimethyl-ammonio) ethyl) phosphonatoxy octaneamido]phenyl}porphinato]zinc(II) complex (abbreviated to ZnP), was embedded in the liposomes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The electron-transfer reaction from the excited state of the liposome-embedded ZnP to benzoquinone (BQ) added to the outer aqueous phase has been studied. Electron transfer occurred from both the triplet and the singlet excited states. The study of the singlet lifetime shows that the fluorescence is quenched statically, indicating the incorporation of BQ into the liposomes. Stepwise incorporation of BQ into the quenching sphere around ZnP was proposed to explain the quenching behaviour and, by calculation, the equilibrium constant of the incorporation was correlated with the relative emission yield. The equilibrium constant for the incorporation of BQ into the quenching sphere around ZnP was obtained from the quenching experiments and the distribution of BQ in the quenching sphere is presented.