Permeability and morphology of low temperature phases in bilayers of single and of mixtures of phosphatidylcholines.

The properties of subtransitions were studied in aqueous dispersions of saturated phosphatidylcholines (PC) by means of permeability measurements, freeze-fracture electron microscopy, and differential scanning calorimetry (DSC). For dispersions of C16PC, a C16PC analog (2,3-dipalmitoyl-cyclopentano-1-phosphocholine with four methylene residues between the nitrogen and the phosphorus atoms) and C17PC, there was good agreement between phase properties (including subtransitions) as observed by DSC and temperature-related permeability. C16PC and C17PC dispersions also displayed a 'crinkled' surface morphology in the subgel state. The phase diagram for mixtures of C14PC and C16PC was consistent with ideal mixing of these two components in the subgel state and also illustrated the relative independence of the subtransition on acyl chain length as compared to the pre- and main transitions. Together, these results indicate that (i) permeability, DSC and freeze-fracture electron microscopy measurements do correlate reasonably well with the existence of a subgel state, (ii) mixtures of lipids with similar acyl chain lengths can be used to investigate subtransitions, (iii) the development of a subtransition appears to be mainly a function of the non-acyl chain moiety of the phospholipid.