Evidence for the "cold fusion" of bilayer membranes

Polymerization of the lipid headgroups inhibits calcium-induced fusion of s mall unilamellar vesicles of the lipid di-n-dodecyloxypropyl beta-nitrostyr yl phosphate but does not influence vesicle aggregation. Addition of a copo lymer of lauryl methacrylate and acrylamide (LMPAM) provides the vesicles w ith a steric shield that prevents both fusion and aggregation. Accurate mic rocalorimetric determination of the enthalpies of vesicle aggregation and f usion was possible by comparison of titrations of vesicles into CaCl2 in th e absence and presence of LMPAM, both before and after polymerization of th e lipids in the vesicles. Whereas calcium-induced aggregation is associated with an enthalpy of +2.6 +/- 0.1 kJ/mol of Lipid, fusion occurs with a min imal endothermic heat effect. We contend that the driving force of membrane fusion is of entropic origin.