Photodynamic Damage to Egg Lecithin Liposomes*

Egg lecithin liposomes were lysed by singlet oxygen ( 1 O* 2 ) generated by red light irradiation of oxygenated methylene blue (MB + ) and by superoxide (O − 2 ) generated by blue‐green irradiation of oxygenated riboflavin (RF) in the presence of ethylenediaminetetraacetic acid (EDTA) or reduced nicotinamide adenine dinucleotide (NADH). The MB + /O 2 sensitized process is more efficient in alkaline solutions, attributed to the stabilization of the triplet state di‐cation. The small increase of the lysis efficiency from pH4 to pH9 for the RF/EDTA/O 2 system suggests that HO − 2 may be more damaging than O − 2 . Estimates based on large‐target reaction kinetics indicate that 1 O* 2 is about 200 times more effective than superoxide for inducing liposome lysis. Similar experiments with saturated 1 ‐α‐phosphatidylcholine dipalmitoyl liposomes showed no photosensitized lysis for either system.