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Lateral Pressure Dependence of the Phospholipid Transmembrane Diffusion Rate in Planar-Supported Lipid Bilayers

Department of Chemistry, University of Utah, Salt Lake City, Utah

Correspondence: Address reprint requests to John C. Conboy, Tel.: 801-585-7957; E-mail: conboy{at}chem.utah.edu.

The dependence of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) flip-flop kinetics on the lateral membrane pressure in a phospholipid bilayer was investigated by sum-frequency vibrational spectroscopy. Planar-supported lipid bilayers were prepared on fused silica supports using the Langmuir-Blodgett/Langmuir-Schaeffer technique, which allows precise control over the lateral surface pressure and packing density of the membrane. The lipid bilayer deposition pressure was varied from 28 to 42 mN/m. The kinetics of lipid flip-flop in these membranes was measured by sum-frequency vibrational spectroscopy at 37°C. An order-of-magnitude difference in the rate constant for lipid translocation (10.9 x 10^–4 s^–1 to 1.03 x 10^–4 s^–1) was measured for membranes prepared at 28 mN/m and 42 mN/m, respectively. This change in rate results from only a 7.4% change in the packing density of the lipids in the bilayer. From the observed kinetics, the area of activation for native phospholipid flip-flop in a protein-free DPPC planar-supported lipid bilayer was determined to be 73 ± 12 Ų/molecule at 37°C. Significance of the observed activation area and potential future applications of the technique to the study of phospholipid flip-flop are discussed.