CALORIMETRIC DETECTION OF CURVATURE STRAIN IN PHOSPHOLIPID-BILAYERS

Phospholipids in biological membranes are arranged as bilayers. When c onstrained to pack into planar bilayers, certain phospholipids will fo rm unstable structures as a consequence of their molecular shape and n oncovalent bonding. This produces curvature strain which may provide e nergy for certain membrane processes. We demonstrate that an exothermi c process associated with the relief of curvature strain can be detect ed calorimetrically. The enthalpy for the incorporation of a few perce nt lysophosphatidylcholine into large unilamellar vesicles of monometh yldioleoylphosphatidylethanolamine at pH 7.4 is exothermic but it is e ndothermic for stable bilayers such as this same lipid at pH 9 or diol eoylphosphatidylcholine at pH 7.4 or 9. The addition of lysophosphatid ylcholine to monomethyldioleoylphosphatidylethanolamine at pH 7.4 is e xothermic only for the addition of the first few percent of lysophosph atidylcholine and then it becomes endothermic. The size of the exother mic heat change is sensitive to changes in temperature, while the endo thermic processes are relatively temperature-insensitive. The exotherm ic heat is also larger when 1 or 2 mol % of diolein is incorporated in to vesicles of monomethyldioleoylphosphatidylethanolamine. These resul ts are all consistent with the exothermic process corresponding to the relief of curvature strain in bilayers having a tendency to convert t o the hexagonal phase. It provides a demonstration that considerable e nergy may be released upon the incorporation of certain molecules into membranes which have a low radius of spontaneous curvature.