Relationships Between Bilayer Phase and Equilibration Rates of Patman and Laurdan

Abstract

Patman is a fluorescent membrane probe related to laurdan. The structural distinctions between the two probes are the lengths of the aliphatic tails (eleven carbons in laurdan and fifteen in patman) and the presence of a trimethylammonium forming a positively-charged head on patman. Studies exploring patman as a probe to detect membrane properties during apoptosis revealed that the two edges of the emission spectrum (435 and 500 nm) stabilize at different rates as the probe binds to the cell membrane. To test whether these differences represent dissimilarities in probe binding to ordered and disordered domains, experiments were conducted to monitor patman equilibration with bilayers composed of various mixtures of saturated and unsaturated phosphatidylcholines at temperatures above, at, and below the main thermotropic phase transition. In general, patman equilibrated more rapidly with bilayers in the liquid disordered phase than in the solid ordered phase. With solid phase membranes, the fluorescence stabilized faster at 435 nm than at 500 nm. Similar yet more subtle results occurred in the lipid disordered phase. In contrast, the situation was reversed at the phase transition temperature; equilibration was faster at 500 nm than at 435 nm. To determine whether these results reflected specific properties of patman, the experiments were repeated with laurdan, and several distinctions were observed. First, equilibration with solid phase lipids was faster than for patman and not different from equilibration with the fluid phase. Second, differences in rates between the two wavelengths were less than with patman for solid phase membranes but greater than with patman for melted bilayers. Third, at the phase transition temperature, equilibration rates favored 435 nm over 500 nm, the opposite of the result obtained with patman. Computer simulations were used to assist with interpretation of these results.