DIFFERENTIAL SCANNING CALORIMETRY OF CHAIN-MELTING PHASE-TRANSITIONS OF N-ACYLPHOSPHATIDYLETHANOLAMINES

Phosphatidylethanolamines in which the polar headgroup is N-acylated b y a long-chain fatty acid (N-acyl PEs) are present in many plasma memb ranes under normal conditions, and their content increases dramaticall y in response to membrane stress in a variety of organisms. The thermo tropic phase behavior of a homologous series of saturated N-acyl PEs, in which the length of the N-acyl chain is equal to that of the O-acyl chains attached at the glycerol backbone, has been investigated by di fferential scanning calorimetry (DSC). All fully hydrated N-acyl PEs w ith even chain lengths from C-12 to C-18 exhibit sharp endothermic cha in-melting phase transitions in the absence of salt and in 1 M NaCl. C ooperative chain-melting is demonstrated directly by the temperature d ependence of the electron spin resonance spectra from probe phospholip ids bearing a spin label group in the acyl chain, The calorimetric tra nsition enthalpy and the transition entropy obtained from DSC depend a pproximately linearly on the chain length with incremental values per CH, group that exceed those of normal diacyl phosphatidylethanolamines , but to an extent that underrepresents the additional N-acyl chain, A thermodynamic model is constructed for the chain length dependences a nd end effects of the calorimetric quantities, which includes a defici t proportional to the difference in O-acyl and N-acyl chain lengths fo r nonmatched chains, as is found and justified structurally for mixed- chain diacyl phospholipids. From data on the chain-length dependence o f N-acyl diC(16)PEs, it is then deduced that the N-acyl chains are les s well packed than the O-acyl chains and, from the data on the matched -chain N-acyl PEs, that the O-acyl chain packing is similar to that in normal diacyl PEs, The gel-to-fluid phase transition temperatures of the N-acyl PEs in the absence of salt are practically the same as thos e of the normal diacyl PEs of the corresponding chain lengths, althoug h the transition enthalpies and entropies are appreciably greater, ind icating entropy-enthalpy compensation, In 1 M NaCl, the transition tem peratures are 3-4.5 degrees higher than in the absence of salt, repres enting the contribution of the electrostatic surface potential of the N-acyl PEs.