BIOPHYSICAL THEORY AND MODELING |
Stability and phase separation in mixed monopolar lipid:bolalipid layers
Gabriel S. Longo 1, David H. Thompson 1 and Igal Szleifer 1*
1 Purdue University
* To whom correspondence should be addressed. E-mail: igal{at}purdue.edu.
Submitted on December 7, 2006
Revised on January 10, 2007
Accepted on 4 June 2007
 |
Abstract |
|---|
The phase stability of a fluid lipid layer that is a mixture of conventional monopolar lipids and C20 bipolar bolalipids was studied using a mean field theory that explicitly includes molecular details and configurational properties of the lipid molecules. The effect of changing the fraction of bolalipids, as well as the length of the hydrocarbon chain of the monopolar lipids was probed. A phase separation between two liquid lipid phases was found when a mismatch in the optimal hydrophobic thicknesses of the pure bolalipid and monopolar lipid layers exists. The lipid mixture phase separates into a thin bolalipid-rich layer and a thicker monopolar-rich layer. The thin membrane phase is mainly composed of transmembrane bolalipid molecules whose polar heads are positioned at opposite membrane-water interfaces. In the monopolar lipid-rich phase, bolalipids are the minor component and most of them assume a looping configuration where both head groups are present at the same membrane-water interface. For mixed layers that form a single lipid phase across all bolalipid concentrations, the hairpin-transmembrane ratio strongly depends on the hydrocarbon chain length of the monopolar lipid and the bolalipid concentration. The C-D bond order parameters of the different species have been calculated. Our findings suggest that the concentration-dependant phase transition should be experimentally observable by measuring of the order parameters through quadrupolar splitting experiments. The driving force for the phase separation in the monopolar lipid:bolalipid mixture is the packing mismatch between hydrophobic regions of the monopolar lipid hydrocarbon chains and the membrane-spanning bolalipid chains. The results from the molecular theory may be useful in the design of stable lipid layers for integral membrane protein sensing.
Key Words:
CD order parameters, bolalipids, hydrophobic mismatch, mixed lipid layers, phase transition
