Lateral pressures in biomembranes estimated from the dynamics of fluorescent probes.

A theoretical model is proposed which states that the time-independent fluorescence anisotropy of the rod-shaped molecule diphenylhexatriene incorporated into lipid bilayers is a direct result of forces constraining the diphenyl-hexatriene molecule. These forces are postulated as equating with the lateral pressure operating within the bilayer independently of the probe molecule. Insertion into the model of experimental observations (recorded in the literature) on anisotropy of diphenylhexatriene in lipid bilayers as a function of temperature yielded values of lateral pressure, which decreased with temperature, and sharply at the temperature defining the transition from gel phase to fluid phase. The values so predicted for the mid-point of the transition and for the entirely fluid phase, respectively, compared favourably with estimates of the lateral pressures in these physical states, that have been reported elsewhere and arrived at either from theories describing lipid chain behaviour or from lipid monolayer compression experiments. Previously documented effects on anisotropy induced by incorporation of cholesterol into fluid lipid bilayers have been interpreted as reflections of rises in intramembranal lateral pressure.