THERMOTROPIC PHASE-BEHAVIOR OF MIXTURES OF LONG-CHAIN FATTY-ACID SPECIES OF CEREBROSIDE SULFATE WITH DIFFERENT FATTY-ACID CHAIN-LENGTH SPECIES OF PHOSPHOLIPID

The thermotropic phase behavior of asymmetric, long fatty acid chain s pecies of cerebroside sulfate, C24-CBS and C26-CBS, with symmetric spe cies of phosphatidylcholine (PC) containing fatty acid chains of 14-18 carbons in length (diC14-PC, diC16-PC, diC18-PC) and dimyristoylphosp hatidylethanolamine (diC14-PE) in 0.1 M KCl was studied by differentia l scanning calorimetry. Novel cerebroside sulfate (CBS) spin labels co ntaining long chain C24 and C26 fatty acid spin labels with the nitrox ide group on the twenty-second carbon were used to study the lipid org anization of the gel phases of these mixtures. The phase diagrams of a ll the mixtures indicated the presence of two immiscible gel phases at low CBS concentrations. All except the C26-CBS/diC14-PC mixture had e utectic phase behavior at low CBS concentrations suggesting that the l ong fatty acid chain of the CBS species had a destabilizing effect on the gel phase of most of the phospholipids. The C26-CBS/diC14-PC mixtu re had peritectic phase behavior at low CBS concentrations indicating a stabilizing effect of the CBS C26 acyl chain on diC14-PC. These resu lts are consistent with the relative compatibility of the CBS acyl cha in length with the bilayer thickness of the PC; only in the case of th e C26-CBS/diC14-PC mixture is the acyl chain of CBS long enough to spa n the PC bilayer. At intermediate to high CBS concentrations, the CBS and phospholipid (PL) were miscible with the exception of the C24-CBS/ diC18-PC combination, which had eutectic phase behavior over a wide co ncentration range. Thus when the PL acyl chain length was similar to t he sphingosine chain length of CBS, CBS bilayers could accommodate sym metric phospholipid molecules better than phospholipid bilayers could accommodate asymmetric molecules of CBS. Use of the spin labels indica ted that, at low temperatures and at intermediate to high CBS concentr ations, all of the mixtures were in a triple chain mixed interdigitate d gel phase which immobilized the spin label. This gel phase slowly tr ansformed over a wide temperature range to a double chain partially in terdigitated gel phase in which the spin labels had much more motion. This transformation could be detected as a broad low enthalpy transiti on by differential scanning calorimetry. In all cases the presence of phospholipid destabilized the mixed interdigitated phase. Stabilizatio n of the partially interdigitated bilayer by intermolecular hydrogen b onding interactions must outweigh the destabilizing forces caused by d isruptions in packing and van der Waals interactions between CBS molec ules resulting from insertion of molecules of phospholipid into this t ype of bilayer. Differences in the stability of different lipid combin ations in the mixed interdigitated phase are discussed in terms of the relative compatibility of their chain lengths.