Steady-state and time-resolved spectroscopic studies on low-density lipoprotein-bound Zn(II)-phthalocyanine

Abstract: Steady-state and time-resolved spectroscopic studies on Zn(II)-phthalocyanine (ZnPc)-low-density lipoprotein (LDL) complexes obtained through the transfer of ZnPc molecules from dipalmitoyl-phosphatidylcholine (DPPC) or palmitoyl-oleoyl-phosphatidylcholine (POPC)-dioleoyl-phosphatidylserine (OOPS) liposomes have been carried out. The number of ZnPc molecules bound to the LDL particles varies with the liposome type and the ZnPc/LDL concentration ratio in the incubation medium; the ZnPc transfer to LDL is more efficient with POPC-OOPS liposomes. Steady-state fluorescence measurements show a reduction of the ZnPc fluorescence yield as the number of ZnPc molecules bound to LDL increases (about 40% reduction is observed with a ZnPc/LDL molar ratio of 60 as compared to 1). The fluorescence decay of ZnPc is best fitted by a monoexponential decay with a lifetime of 3.8 ns at a ZnPc/LDL ratio equal to 1, while at higher ratios a shorter-living fluorescence component is detected whose amplitude increases as the ZnPc/LDL ratio is increased. This behaviour suggests the presence of ZnPc aggregates in the LDL particle.