The enthalpy of acyl chain packing and the apparent water-accessible apolar surface area of phospholipids

The energetics of phospholipid aggregation depend on the apparent water-acc essible apolar surface area (ASA(ap)), ordering effects of the chains, and headgroup interactions. We quantify the enthalpy and entropy of these inter actions separately. For that purpose, the thermodynamics of micelle formati on of lysophosphatidylcholines (LPCs, chains C-10, C-12, C-14, and C-16) an d diacylphosphatidylcholines (DAPCs, chains C-5, C-6, and C-7) are studied using isothermal titration calorimetry. The critical micelle concentration (CMC) values are 90, 15, and 1.9 mM (C-5-C-7-DAPC) and 6.8, 0.71, 0.045, an d 0.005 mM (LPCs), The group contributions per methylene of Delta DeltaG(0) = -3.1 kJ/mol and Delta DeltaC(p) = -57 J/(mol.K) for LPCs agree with lite rature data on hydrocarbons and amphiphiles. An apparent deviation of DAPCs (-2.5 kJ/mol, 45 J/(mol.K)) is due to an intramolecular interaction betwee n the two chains, burying 20% of the surface. The chain/chain interaction e nthalpies in a micelle core are by similar to -2 kJ/(mol) per methylene gro up more favorable than in bulk hydrocarbons. We conclude that the impact of the chain conformation and packing on the interaction enthalpy is very pro nounced. It serves to explain a variety of effects reported on membrane bin ding. Interactions within the water-accessible region show considerable Del taH, but almost no DeltaG(0). The heat capacity changes suggest about three methylene groups (ASA(ap) approximate to 100 Angstrom (2)) per LPC remain exposed to water in a micelle (DAPC: 2 CH2/70 Angstrom (2)).