SINGLET OXYGEN GENERATION FROM LIPOSOMES - A COMPARISON OF TIME-RESOLVED 1270 NM EMISSION WITH SINGLET-OXYGEN KINETICS CALCULATED FROM A ONE-DIMENSIONAL MODEL OF SINGLET-OXYGEN DIFFUSION AND QUENCHING

Time-resolved measurements of 1270 nm singlet-oxygen emission following pulsed-laser excitation were made from unilamellar dimyristoyl 1-alpha-phosphatidylcholine liposomes labeled with zinc phthalocyanine. The effect of the hydrophobic quenchers, beta-carotene and ethyl beta-apo-8' trans carotenoate, and the hydrophilic quenchers, histidine and methionine, upon the kinetics of the 1270 nm singlet oxygen emission was studied. Hydrophobic quenchers principally lowered the intensity of the 1270 nm emission and caused only modest changes in the lifetime of the 1270 nm emission. The decrease in 1270 nm emission caused by hydrophobic quenchers was related to the size of the liposomes. The larger the radius of the liposome, the greater the decrease in 1270 nm emission caused by a given concentration of hydrophobic quencher. In contrast, hydrophilic quenchers principally decreased the lifetime of the 1270 nm emission. The effect of hydrophilic quenchers was independent of the size of the liposomes. There was good agreement between the experimental results and the kinetics of the singlet-oxygen emission calculated using a one dimensional model of singlet-oxygen quenching and diffusion. The kinetics of singlet-oxygen emission from liposomes without added hydrophobic quenchers closely approximated the theoretical kinetics of singlet oxygen in a homogeneous aqueous solution.