Temperature-jump studies of merocyanine 540 relaxation kinetics in lipid bilayer membranes

Abstract

The temperature-jump technique was used to study the rapid kinetics of merocyanine 540 (M-540) interactions with single-walled phosphatidylcholine (PC) vesicles. The absorption spectrum of M-540 in PC vesicles has an isosbestic point at 560 nm at low [PC]/[M-540], whereas solution M-540 and membrane-bound M-540 dimers are present, and an isosbestic point at 548 nm at high [PC]/[M-540], where membrane-bound M-540 monomers and dimers are present. In response to a 15-kV discharge across a solution containing M-540 and PC vesicles (2.5.degree. C temperature increment), there was a rapid increase in absorbance at 575 nm (< 5 .mu.s) followed by a slower (.apprx. 1 ms), monoexponential relaxation process of opposite sign and approximately equal amplitude to the initial rise. The amplitude of the slower process was wavelength-dependent and reversed sign at .apprx. 540 nm. The slower relaxation time constant decreased as [PC] was increased at constant [M-540]. A proposed model for the potential sensitivity of M-540 involves intramembrane reorientation of dye molecules and dimerization. The results obtained here suggest that reorientation of dye molecules is the rate-limiting step, with a rate constant for reorientation from parallel to perpendicular to the plane of the membrane of 1340 .+-. 200 s-1 at 23.degree. C.