Facile Lipid Flip-Flop in a Phospholipid Bilayer Induced by Gramicidin A Measured by Sum-Frequency Vibrational Spectroscopy
Jin Liu 1, Timothy C Anglin 1 and John C Conboy 1*
1 University of Utah
* To whom correspondence should be addressed. E-mail: conboy{at}chem.utah.edu.
Submitted on August 25, 2006
Revised on September 12, 2006
Accepted on 10 October 2006
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Abstract |
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The first direct experimental evidence that gramicidin A (gA), a transmembrane peptide, facilitates the translocation of unlabeled lipids in a phospholipid bilayer was obtained with sum-frequency vibrational spectroscopy (SFVS). SFVS was used to investigate the effect of gA on lipid flip-flop in a planar 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) lipid bilayer. The kinetics of lipid translocation were determined by an analysis of the SFVS intensity versus time at different temperatures in the presence of 2 mol% gA. The rate constants of DSPC flip-flop increase from two to ten times relative to the pure DSPC system. The results indicate that facial lipid exchange can be induced by a hydrophobic transmembrane helix. The increase in lipid flip-flop rates is correlated to an increase in the gauche content of the lipid tails. The results suggest that membrane defects induced by the presence of integral membrane proteins may play a large role in modulating the rate of lipid flip-flop.
Key Words:
gramicidin A, lipid flip-flop, membrane, planar supported lipid bilayer, sum-frequency vibrational spectroscopy, translocation
