Phase transition properties of 1,2- and 1,3-diacylphosphatidylethanolamines with modified head groups

The phase transition properties of dilute aqueous suspensions of "nonhydrated" (i.e., lipid suspensions which had not been heated above room temperature or above the main phase transition temperature of the fully hydrated lipid, whichever was lower) and hydrated 1,2(alpha)- and 1,3(beta)-dipalmitoylphosphatidylethanolamines with modified head groups have been determined by high-sensitivity differential scanning calorimetry at a scan rate of 0.1 K min-1. In both the 1,2 and 1,3 series, the head-group modifications of the phosphoethanolamine moiety included N-methyl, N,N-dimethyl, and N,N,N-trimethyl (phosphocholine). In the 1,2 series, additional modifications were dinitrophenyl, trinitrophenyl, N-(dinitrophenyl)aminocaproyl, N-(trinitrophenyl)aminocaproyl, and N-4-nitro-2,1,3-benzoxadiazole. Also included in this study were 1,2-dihexadecylphosphatidylethanolamine and the corresponding N-methyl-substituted lipid. In general, increasing bulkiness of the head-group substituent caused increasing lowering of the transition temperature, the most extreme cases among the hydrated lipids being the 45 degrees C lowering produced by the N-(dinitrophenyl)aminocaproyl substitution and its trinitrophenyl analogue in the 1,2 series. No simple trend is evident in the changes produced in the calorimetric enthalpy of transitions.