Fluorescence quenching and flash photolysis studies of photo-induced electron transfer across Zn-tetraphenyl porphyrin-encapsulated liposomes: Evidences for reductive quenching by EDTA

Abstract

The photosensitizer, zinc-tetraphenyl porphyrin (ZnTPP), encapsulated liposomes incorporating reductive quencher (EDTA) (O.5<i>M</i>) in the inner pool have been used to initiate photo-induced electron transfer across liposome membrane using (i) BQ, (ii) MV²⁺ and (ill) 1,5-AQDS^2- as oxidative quenchers in the outer pool. Two types of lipids have been used: egg-phosphatidyl choline, EPC (T_c.~ 15°C) and dipalmitoyl-phosphatidyl choline, DPPC (T_c.~ 38°C). The EPC liposome matrix has been further modulated by cholesterol and the localisation of the photosensitizer studied by fluorescence quenching technique. From modified Stern-Volmer equation it is observed that about 60% of ZnTPP is not available for quenching by AQDS^2- added to the outer pool. Availability is 100% for cholesterol modified liposomes, <i>K_sv </i>=2.7 X <i>10³M ^-</i>¹. For DPPC, the liposomes contained two different concentrations of EDTA in the inner pool, Lip A 0.5<i>M </i>and Lip B <i>0.25M. </i> On addition of AQDS^2- as the quencher in the outer pool, <i>K_sv </i>is found to be a function of [EDTA] in the inner pool suggesting definite rate determining role of the reductive quencher EDTA in transmembrane electron transfer process. These observations are confirmed by flash photolysis studies on the DPPC liposomes, Lip A and Lip B, of composition as above and the Lip C obtained by adding AQDS^2- to the outer pool of Lip B. Biexponential decay for Lip A and Lip B and single exponential decay for Lip C is observed. From both singlet and triplet quenching studies it has been inferred that the first step is the reductive quenching of photoexcited ZnTPP by EDTA in the inner pool, followed by electron transfer to AQDS^2- in the outer pool, promoting ET from inside to outside.