The partition and distribution of porphyrins in liposomal membranes. A spectroscopic study

Spectroscopic techniques were employed to establish the first comparative evaluation of the partition of the photosensitizers hematoporphyrin derivative (HPD) and photofrin II (PF-II) into phosphatidylcholine (PC) and PC/cholesterol liposomes. The fluorescence of the dyes was monitored while they were titrated with liposomes, yielding the dyes' effective binding constants to the membranes. The binding constants of HPD and PF-II to PC liposomes are 12.2 ± 0.3 (mg/ml)^-1 and 9.2 ± 0.8 (mg/ml)^-1 and to PC/cholesterol (50% w/w) liposomes they are 6.7 ± 0.9 (mg/ml)^-1 and 8.0 ± 0.8 (mg/ml)^-1, respectively. The vertical distribution of the dyes in the bilayer was determined by quenching their fluorescence with spin-labeled stearic acids. PF-II was found to reside deeper in the membrane than HPD. Cholesterol was found to modulate the distribution of the two dyes to a greater extent then DPPC and DMPC. The modulation mechanism is discussed.