Kinetics and thermodynamics of flip-flop in binary phospholipid membranes measured by sum-frequency vibrational spectroscopy.

In order to better characterize the dependence of lipid flip-flop rate and thermodynamics on the nature of the lipid headgroup, we have studied the kinetics of flip-flop for single-lipid and mixed-lipid bilayers consisting of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) as a function of both pressure and temperature. The kinetics of flipping were studied by sum-frequency vibrational spectroscopy (SFVS), which does not require exogenous chemical labeling of the lipid species of interest. Additionally, SFVS may be employed to track only a single species (DSPE or DSPC) within a binary mixture by selective deuteration of the matrix lipid to make it spectroscopically inactive. Using this approach, we have found the flip-flop of pure DSPE to be slower than the flip-flop of pure DSPC by nearly 2 orders of magnitude. The thermodynamics of the pure systems were analyzed in order to better understand the physical factors underlying their transmembrane dynamics. Headgroup hydrophobicity and associated solvent effects, as well as lipid packing constraints, appear to play a key role in determining the rate of flip-flop for these two species. For mixtures of DSPE + DSPC, both components exhibited similar rates of flip-flop at a given mole fraction of DSPE. The kinetics and thermodynamics of flip-flop in the mixtures did not vary uniformly with changing composition but were well correlated to changes in the molecular packing as a function of DSPE content in the bilayer.