MODULATION OF LIPOSOME MATRIX BY CHOLESTEROL FOR EFFICIENT LOCALIZATION OF PHOTOSENSITIZER - A FLUORESCENCE QUENCHING STUDY ON CHLOROPHYLL EXTRACT FROM SPINACH LEAVES

To develop potential biomimetic solar energy conversion devices, egg p hosphatidylcholine (EPC) liposomes were prepared with extracts from sp inach leaves as the photosensitizer. Fluorescence energy transfer stud ies using these liposomes established that the extract consists of an organized assembly of components such as chlorophyll a (Chl a), chloro phyll b (Chl b), pheophytin, etc. For homogeneous solutions in CHCl3, the spectroscopic properties of the extract are as follows: Chl a, lam bda(ab) = 430 nm (Soret) and 668 nm (Q), lambda(fl) = 670 nm; Chl b, l ambda(ab) = 460 nm (Soret), lambda(fl) = 650 nm; in liposomes: Chl a, lambda(ab) = 420 nm, lambda(fl) = 678 nm, lambda(exc) = 420 nm; Chl b, lambda(ab) = 460 nm lambda(fl) = 678 nm, lambda(exc) = 470 nm (monito red at 678 nm). A fluorescence quenching technique using anthraquinone -1,5-disulphonate (1,5-AQDS2-) as the electron acceptor (A) and ethyle nediaminetetraacetic acid (EDTA) as the sacrificial electron donor (D) , was employed to study the localization of the photosensitizer in the liposome matrix on modulation with cholesterol. The Stern-Volmer quen ching constants K(SV) for 1,5-AQDS2- are nearly diffusion controlled i n homogeneous solutions: K(SV) = 1100 M-1 in CH3COCH3-H2O (60:40) and K(SV) = 1420 M-1 in CH3CN-H2O (60:40). In unmodified liposomes with A in the outer pool and D in the inner pool, the value decreases to 540 M-1; this value is reduced further on addition of 10% cholesterol, rem ains constant up to 30% cholesterol and decreases sharply on addition of 50% cholesterol. A reverse situation is observed when A is located in the inner pool and D is added gradually to the outer pool: a low va lue is obtained for 0% cholesterol, which increases slightly at 10% ch olesterol, remains constant up to 30% cholesterol and increases sharpl y at 50% cholesterol. These changes indicate the movement of Chl a int o the interior of the liposome and follow the changes in structure and size of EPC liposomes on addition of cholesterol as discussed by Huan g and Mason. The distribution of Chl a within these modified unilamell ar liposomes is a function of the physicochemical properties of the ma trix and the dimensions of the liposome. Our study indicates that for constant performance, EPC liposomes containing 15% cholesterol may be prepared to improve the structural fluidity of the liposome, to obtain appropriate localization of the photosensitizer and to optimize the e fficiency of quenching and stability of the system.