Fluorescence-quenching study of percolation and compartmentalization in two-phase lipid bilayers

Fluorescence quenching of a lipid-labeled fluorophore by a lipid spin-labeled quencher has been studied experimentally in two-component, two-phase phosphatidylcholine bilayers to examine the effect of phase connection and disconnection on quenching. Both fluorophore and quencher prefer the fluid phase. At the percolation threshold, the point at which the fluid phase becomes subdivided into may small disconnected domains, the quenching drops abruptly. This decrease in quenching is a function of the fluid-phase fraction and is due to the heterogeneous distribution of fluorophores and quenchers over the fluid-phase domains. Computer simulations of the system were carried out with a triangular lattice divided into closed compartments of variable size and reactant occupancy. The simulations demonstrate that the degree of quenching is reduced in the disconnected systems and that the reduction is correlated with the size of the disconnected domains. The combination of experimental data with simulations leads to the conclusion that at constant temperature the size of fluid-phase domains, nfluid, in the region of the coexistence of the fluid and gel phases is proportional to the fluid fraction, Xfluid. This is in a qualitative agreement with a previous electron spin resonance study of interlipid spin-spin interactions in the same two-component, two-phase bilayer system.