Abstract
Lipid components in membranes are known to mix non-ideally, but the thermodynamics of this mixing remains poorly understood. Deviations from ideality may be characterized in part by the heat that is absorbed or released when components mix. This study aims to directly measure the heat of mixing of two phospholipid species in a bilayer membrane, using isothermal titration microcalorimetry. Unilamellar vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phsophocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) were mixed in a calorimeter cell in the presence of methylated beta-cyclodextrin, which served as a lipid transfer catalyst. We expected on the basis of reduced head-group charge repulsions that mixing of the two lipids would be energetically favorable. The measured heat of mixing for a 1:1 mixture of POPC and POPG is -0.15 kJ/mol.
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