Mixtures of semisynthetic species of cerebroside sulfate with dipalmitoyl phosphatidylcholine. Thermotropic phase behavior and permeability

The phase behavior of mixtures of dipalmitoyl phosphatidylcholine (DPPC) with semisynthetic species of cerebroside sulfate (CBS) containing palmitic acid (C16:0-CBS) or lignoceric acid (C24:0-CBS) in 0.1 M KCl was studied using differential scanning calorimetry. DPPC and C16:0-CBS were miscible in all proportions in the gel phase above 10 mol% CBS and in the liquid-crystalline phase. However, C24:0-CBS was less miscible with DPPC over a wide concentration range in the gel phase. At high CBS concentrations it was probably also not entirely miscible with DPPC in the liquid-crystalline phase. Small amounts of both species of CBS lowered the transition temperature and enthalpy of DPPC, suggesting that they are more soluble in the liquid-crystalline phase of DPPC than the gel phase. The transition temperature at higher CBS concentrations was also less than expected, especially after cycling through the phase transition in the case of C24:0-CBS, suggesting that mixing with DPPC inhibited the intermolecular hydrogen bonding interactions and dehydration of CBS. In C24:0-CBS–DPPC mixtures several populations were present over a wide compositional range, including two solid-solid solutions of fixed composition. At high C24:0-CBS concentrations some C24:0-CBS also phase separated out of the mixture. Structural considerations suggested that the C24:0-CBS which is mixed with DPPC must be interdigitated into the DPPC bilayer. Other populations that are present may have a different structural organization. A fatty acid spin label in these mixtures was a little less ordered than in either lipid by itself. The permeability of these lipids, as well as the two asymmetric species 1-stearoyl-2-caproyl phosphatidylcholine and 1-stearoyl-2-myristoyl phosphatidylcholine (18:10PC and 18:14PC), to a water-soluble spin label tempocholine chloride was also measured. The studies with 18:10PC and 18:14PC indicated that both triple-chain mixed interdigitated bilayers and double-chain partially interdigitated bilayers can trap water-soluble substances and have low permeability. Both species of CBS could also entrap the spin label and had low permeability at 4 °C. However, they rapidly lost the entrapped compound when they transformed into their stable dehydrated phases or into the liquid-crystalline phase. Mixing with DPPC prevented both of these losses. These studies supported the conclusion that a significant amount of the CBS was mixed with the DPPC and that this mixing prevented the dehydration changes which CBS undergoes by itself. They also suggested that the C24:0-CBS can pack with the DPPC in the liquid-crystalline phase in a regular way which must involve inter-digitation of the long C24:0 chain into the DPPC bilayer.Key words: sulfatide, phosphatidylcholine, phase diagram, differential scanning calorimetry, interdigitated bilayer.