Study of cholesterol phase effect on the dynamics of DOPC and DPPC small vesicle membranes using single-molecule fluorescence correlation spectroscopy

Abstract

The effect of gradual cholesterol increase on the dynamics of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membranes of small unilamellar vesicles (SUVs) sized below 50 nm was explored using single-molecule fluorescence correlation spectroscopy (SM-FCS). The dye 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine (DiD) was entrenched into the membrane as a fluorescent probe, and silica nanoparticles were used to obtain SUVs of similar size, termed Si-supported small unilamellar vesicles (Si-SUVs). Si-SUVs of three different compositions for each DOPC and DPPC with cholesterol were synthesized. Auto-correlation and cross-correlation methods were implemented to analyze the fluorescence trajectories of single DiD molecules to determine their diffusion frequencies and triplet state lifetimes. The rigidity of the DOPC membrane increased with increasing cholesterol concentration, which led to an increase of the triplet state lifetime and a decrease in the diffusion frequency of DiD molecules. Thus, pure DOPC Si-SUVs, homogeneous in nature, showed a single block in the 2D correlation plot between triplet state lifetime and diffusion frequency, whereas multiple blocks appeared in the case of cholesterol-added DOPC Si-SUVs due to the inhomogeneity of the rigid membrane. On the contrary, DPPC Si-SUVs showed a reverse effect, where the membrane rigidity decreased with increasing cholesterol concentration. Thus, three blocks in the 2D plot of pure DPPC Si-SUVs were merged into one block when the cholesterol concentration in the membrane reached 40% of the total molar concentration as the membrane became homogeneous.