A Relatively Slow Membrane Remodeling after the Salt Induced Collapse of a Giant Unilamellar Vesicle Revealed by the Multimodal Imaging Method

To probe the detailed structural evolution of molecules in lipid membranes provides critical information for understanding the biophysics in cells. This study employed second harmonic generation (SHG) plus two-photon fluorescence (TPF) and SHG plus bright field imaging techniques to monitor the structural evolution of a giant unilamellar vesicle (GUV) composed of cardiolipin under hyperosmotic stress induced by an electrolyte (NaCl). After the contraction of the lipid vesicle upon the influence of the electrolyte, rapid local collapses and relatively slow remodeling processes lasting over 100 s were alternatively observed multiple times before the complete collapse of the vesicle. With the aid of probe molecule (E)-4-(2-(pyren-1-yl)vinyl)-1-(3-(trimethylammonio)propyl)pyridin-1-dibromide (Pyr-Py⁺-N⁺, PPN), periodic reorientation and recovery of the interfacial order of lipid molecules in the membrane were revealed. This work provides new insight for analyzing the orientational structures and kinetics of molecules in membranes.