FLUORESCENCE QUENCHING AND FLASH-PHOTOLYSIS STUDIES OF PHOTOINDUCED ELECTRON-TRANSFER ACROSS ZN-TETRAPHENYL PORPHYRIN-ENCAPSULATED LIPOSOMES - EVIDENCES FOR REDUCTIVE QUENCHING BY EDTA

The photosensitizer, zinc-tetraphenyl porphyrin (ZnTPP), encapsulated liposomes incorporating reductive quencher (EDTA) (0.5 M) in the inner pool have been used to initiate photo-induced electron transfer acros s liposome membrane using (i) BQ, (ii) MV2+ and (iii) 1,5-AQDS2- as ox idative quenchers in the outer pool. Two types of lipids have been use d: egg-phosphatidyl choline, EPC (T(c) approximately 15-degrees-C) and dipalmitoyl-phosphatidyl choline, DPPC (T(c) approximately 38-degrees -C). The EPC liposome matrix has been further modulated by cholesterol and the localisation of the photosensitizer studied by fluorescence q uenching technique. From modified Stern-Volmer equation it is observed that about 60% of ZnTPP is not available for quenching by AQDS2- adde d to the outer pool. Availability is 100% for cholesterol modified lip osomes, K(SV) = 2.7 x 10(3)M-1. For DPPC, the liposomes contained two different concentrations of EDTA in the inner pool, Lip A 0.5 M and Li p B 0.25 M. On addition of AQDS2- as the quencher in the outer pool, K (SV) is found to be a function of [EDTA] in the inner pool suggesting definite rate determining role of the reductive quencher EDTA in trans membrane electron transfer process. These observations are confirmed b y flash photolysis studies on the DPPC liposomes, Lip A and Lip B, of composition as above and the Lip C obtained by adding AQDS2- to the ou ter 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 red uctive quenching of photoexcited ZnTPP by EDTA in the inner pool, foll owed by electron transfer to AQDS2- in the outer pool, promoting ET fr om inside to outside.