Molecular mechanism of decoupling between the fluorescence anisotropy thermograms of diphenylhexatriene and the thermotropic phase transitions of biomimetic ion pair amphiphile bilayers

Abstract

The fluorescence anisotropy (FA) thermogram is a common technique to characterize the thermotropic phase properties and to determine the phase transition temperature of a lipid bilayer. For several biomimetic bilayer systems composed of ion pair amphiphiles (IPAs), however, the FA thermograms obtained with the 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probe fail to capture their phase transition behaviors. Here, we employed molecular dynamics (MD) simulation to examine the dynamics of common fluorescent probes, including DPH and 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH), and evaluated the corresponding FA signals from the probe's rotational autocorrelation functions. The resulting FA thermogram of DPH in a DPPC bilayer indeed exhibits a sigmoidal feature that correlates with the phase behavior for the membrane. In contrast, the DPH FA thermogram fails to reflect the phase transition of IPA membranes with asymmetric or short alkyl tails. Such decoupling phenomenon is attributed to (1) the inherently disordered chain packing within the IPA bilayers of short alkyl tails, or (2) the flat orientation of DPH within the IPA bilayers featuring asymmetric alkyl chains. The FA thermograms further reweighted by the ratio of DPH flat orientation in IPA bilayers show excellent agreements with experimental data, demonstrating the importance of DPH orientation. Conversely, the amphiphilic TMA-DPH can capture the structural changes near the bilayer surface during the phase transition, applicable for a variety of IPA systems. The combined results provide important insights into the phase transition behaviors of different bilayer systems and offer guidance for probe selection in FA studies.