Heat Changes in Lipid Membranes under Sudden Osmotic Stress

Abstract
Closed lipid vesicles act as osmometers increasing or decreasing their volume under the influence of osmotic gradients. The enthalpy changes accompanying membrane compression or expansion have not been measured yet, and first results obtained with high-sensitivity titration calorimetry are reported here. Phospholipid vesicles suspended in and in equilibrium with an electrolyte or nonelectrolyte with a defined initial concentration of ci, were injected into a solution with a final concentration of cf, and the heat changes were monitored with a titration microcalorimeter. Osmotic compression (Δc = cfci > 0) produced an exothermic heat change with ΔH ≈ −500 ± 100 cal/mol and osmotic expansion (Δc < 0) an endothermic heat change with ΔH ≈ 1000 ± 200 cal/mol; both results normalized to a concentration gradient of |Δc| = 1 M NaCl. The heats of compression and expansion varied linearly with the lipid content and the size of the osmotic gradient but were independent of the vesicle size. The cubic thermal expansion coefficient αv which equals (1/V)(∂V/∂T)p could be derived and was found to be 1.25 × 10-3 and 2.5 × 10-3 K-1 for the compressed and expanded bilayer vesicles, respectively. The entropy changes associated with compression and expansion could be estimated. Compression of the membrane led to a negative entropy change and increased the hydrocarbon chain order. Expansion of the membrane was accompanied by a positive entropy change which can be explained, in part, by more disordered hydrocarbon chains. Vesicle expansion and compression thus appear to be asymmetric as far as the thermodynamic driving force is concerned.