QUENCHING OF MEROCYANINE-540 TRIPLET-STATE BY NITROXYL RADICALS IN LIPOSOMAL SYSTEMS - A LASER FLASH-PHOTOLYSIS STUDY

Laser flash photolysis experiments were undertaken to investigate the interaction between stearic acid nitroxide spin probes and photoexcite d merocyanine 540 (MC540) in dimyristoyl-L-alpha-phosphatidylcholine l iposomes (membrane model). The measurements of the paramagnetic signal decay kinetics of four different spin-labelled stearic acids (n-DSA) show that the direct interaction between the dye and the probe is affe cted by the position of the nitroxyl group along the carbon chain. Las er flash photolysis results reveal a significant decrease in the MC540 triplet lifetime in the presence of n-DSA, the effect depending on th e depth at which the nitroxyl moiety is localized in the bilayer. Prev ious results on the rate of disappearance of the electron spin resonan ce (ESR) nitroxide signal on continuous photolysis of the same systems do not show the same dependence on the localization of the nitroxyl m oiety in the liposome. Although the MC540 triplet state seems to be im plicated in the reaction process, the results suggest that ESR and las er flash experiments demonstrate two different kinds of mechanism.