Fluorescence quenching with lindane in small unilamellar L,alpha-dimyristoylphosphatidylcholine vesicles

The lateral mobility and lipid-water partition of the pesticide lindane was studied by fluorescence quenching of N-isopropylcarbazole (NIPC) and L,alpha-palmitoyl-beta-(N-carbazolyl) undecanoylphosphatidylcholine (PCUPC) in liposomes of dimyristoylphosphatidylcholine at 50 degrees C. In isotropic solvents the quenching reaction was highly inefficient. A scheme for dynamic quenching, in which the monomolecular quenching rate constant is small, was valid. In lipid bilayers the same scheme was applied to describe the quenching results but the rate constant of the back-reaction of the excited complex to quencher and excited probe was of comparable magnitude to the monomolecular quenching rate constant. This phenomenon results in biexponential decays of the fluorescent probe in the presence of quencher. All the rate constants of the scheme could be determined. Stern-Volmer plots at different membrane concentrations were obtained from fluorescence intensity and decay time measurements. From these plots the true bimolecular quenching rate constant, Kq, and the rate constant for lateral diffusion, kd, were determined: kq[NIPC] = 3.2 +/- 0.5 x 10(8) M-1 s-1, kq[PCUPC] = 1.9 +/- 0.4 x 10(8) M-1 s-1, kd[NIPC] = 6.6 +/- 0.8 x 10(8) M-1 s-1. The smaller value of kq compared to kd for the quenching reaction of NIPC with lindane indicates that this quenching reaction is not diffusion controlled. The lateral diffusion coefficient D of lindane was found to be 1.7 +/- 0.2 x 10(-6) cm2/s in dimyristoylphosphatidylcholine vesicles at 50 degrees C. The partition coefficient of lindane in these lipid bilayers is very high (greater than 2000).