Examining the effects of cholesterol on model membranes at high temperatures: Laurdan and Patman see it differently

At high temperature, the presence of cholesterol in phospholipid membranes alters the influence of membrane dipoles, including water molecules, on naphthalene-based fluorescent probes such as Laurdan and Patman (solvatochromism). Although both of these probes report identical changes to their emission spectra as a function of temperature in pure phosphatidylcholine bilayers, they differ in their response to cholesterol. Computer simulations of the spectra based on a simple model of solvatochromism indicated that the presence of cholesterol reduces the probability of bilayer dipole relaxation and also blunts the tendency of heat to enhance that probability. While the overall effect of cholesterol on membrane dipoles was detected identically by the two probes, Laurdan was influenced much more by the additional effect on temperature sensitivity than was Patman. A comparison of the fluorescence data with simulations using a coarse-grained bilayer model (de Meyer et al., 2010) suggested that these probes may be differentially sensitive to two closely related properties distinguishable in the presence of cholesterol. Specifically, Patman fluorescence correlated best with the average phospholipid acyl chain order. On the other hand, Laurdan fluorescence tracked more closely with the area per lipid molecule which, although affected generally by chain order, is also impacted by additional membrane-condensing effects of cholesterol. We postulate that this difference between Laurdan and Patman may be attributed to the bulkier charged headgroup of Patman which may cause the probe to preferentially locate in juxtaposition to the diminutive headgroup of cholesterol as the membrane condenses.