Differential scanning calorimetry of dipalmitoyl phosphatidylcholine analogues and of their interaction products with basic polypeptides.

The thermotropic behaviour of dipalmitoyl phosphatidylcholine analogues with a varying number (n) of CH2 groups between the phosphate and the quaternary ammonium has been investigated. The temperature (Tm) and the enthalpy (deltaH) of the phase transition are non-monotonous functions of the number of CH2 groups. Tm oscillates between 40 and 45 degrees C and deltaH between 7 and 13 kcal/mol for a variation of n between 2 and 11. It is concluded that the hydrocarbon chains in the head groups do not penetrate the hydrocarbon region and do not contribute directly to the melting of the acyl chains. It is suggested that their length may affect the critical balance between the attractive and the repulsive forces within the bidimensional lattice of the head groups. Copolypeptides of lysine with phenylalanine do not appreciably affect the Tm but have a pronounced effect on deltaH of the lipid phase transition, which depends strongly on the ratio of the two amino acids in the polypeptide. The effect of copolypeptide of any defined composition on deltaH is also a non-monotonous function of the number of CH2 groups in the phosphatidylcholine head group, but it does not parallel completely the oscilations in the Tm and deltaH of the pure lipids.