A PHASE-BEHAVIOR STUDY OF MIXTURES OF SPHINGOSINE WITH ZWITTERIONIC PHOSPHOLIPIDS

The interactions of sphingosine (SPH) with dipalmitoylphosphatidylchol ine (DPPC) and dielaidoylphosphatidylethanolamine (DEPE) have been stu died by means of differential scanning calorimetry (DSC) and P-31-nucl ear magnetic resonance (P-31-NMR). Experiments were carried out with t he fully protonated form of SPH, at pH 6.0. DSC studies showed that th e main T-c transition temperature of DPPC was perturbed by the presenc e of SPH so that T-c of the mixture was higher than those of pure comp onents at concentrations of SPH up to 50 mol%, with an azeotropic poin t at 30 mol% of SPH. At higher concentrations solid phase separations were observed from 70 to 95 mol% of SPH with an eutectic point at 90 m ol% of SPH. P-31-NMR showed lamellar phases in DPPC/SPH mixtures, at a ll the range of concentrations. The behavior of DEPE/SPH mixtures was somewhat different since no azeotropic point was detected, the gel to liquid-crystalline transition being depressed by the presence of SPH, and an eutectic point was detected at 60 mol%. Solid phase immiscibili ties were present between 50 mol% and 85 mol% of SPH. It is also remar kable that the liquid-crystalline to hexagonal H-II phase transition o f DEPE was only slightly shifted to lower temperatures at concentratio ns of SPH lower than 33 mol% of SPH but, this transition disappeared a t concentrations of SPH higher than 33 mol% of SPH, so that isotropic phases were formed instead, as seen through P-31-NMR. The present resu lts show the importance of taking into account the effects appearing i n mixtures of SPH with zwitterionic phospholipids when considering the ir influence on the organization of biomembranes.