The gel-fluid phase equilibrium in a two-component system formed from dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) was investigated using solid-state wide-line 2H NMR spectroscopy. Analysis of the spectral first moments and the quantitation of gel and fluid phases by means of difference spectroscopy provided the temperature-composition phase diagrams. Phase diagrams were constructed for mixtures of perdeuterated DMPC, DMPC-d54, with DSPC and for the complementary system comprised of DMPC and perdeuterated DSPC, DSPC-d70. The gel-fluid coexistence region was found to extend over a wider range of temperature and composition for the DMPC-d54-DSPC system than for the DMPC-DSPC-d70 system. Comparison of these data with the phase diagram for the DMPC-DSPC system showed that in the gel-fluid region the fraction of lipids in the fluid phase at a given temperature and system composition decreases for the three systems in the order DMPC-d54-DSPC greater than DMPC-DSPC greater than DMPC-DSPC-d70. While the fluid fraction varies by as much as 90% among the three systems, the composition of the fluid phase, i.e., the ratio of the concentrations of the two molecules in the fluid phase, varies by about 20% over the whole temperature and system composition range. The effective acyl chain lengths of the DMPC-d54 and DSPC-d70 molecules as a function of temperature and composition in the fluid phase, when the system is all fluid or is in the gel-fluid coexistence region, were calculated from the quadrupole splittings in the axially symmetric powder patterns obtained for the all-fluid phase. The magnitudes of the coefficient of thermal expansion for both the DMPC-d54 and the DSPC-d70 molecules were smaller in the fluid phase of binary mixtures than in one-component bilayers containing either DSPC-d70 or DMPC-d54 alone. In addition, at any given temperature in the fluid phase, the increase in the acyl chain length of DMPC-d54 with increasing DSPC content of the system was smaller than the concomitant increase in the length of DSPC-d70 in mixtures with DMPC. In the entire temperature and composition range when the binary mixtures are in the all-fluid or in the gel-fluid coexistence region, the largest value obtained for the DMPC-d54 molecule in the fluid phase was smaller than the smallest value obtained for the DSPC-d70 molecule in the fluid phase. The acyl chain lengths were used to calculate the effective weighted-average thickness, d, of the fluid phase bilayer.(ABSTRACT TRUNCATED AT 400 WORDS)

The myelin basic protein (MBP) from bovine spinal cord was cleaved at the single tryptophan residue to produce an N-terminal fragment ( F l ) of molecular weight 12.6K and a C-terminal fragment (F2) of molecular weight 5.8K. The interactions of the two fragments with bilayers of the acidic lipid dimyristoylphosphatidylglycerol (DMPG) were compared with those of the intact protein, by using both chemical binding assays and spin-label electron spin resonance spectroscopy. The saturation binding stoichiometries of the two fragments were found to sum to that of the MBP, having values of 1 1, 24, and 36 mol of DMPG/mol of protein for F2, F1, and the MBP, respectively. The strength of binding was found to increase in the order F2 < F1 < MBP, which follows that of the net charges on the different fragments.The ionic strength dependence of the protein binding indicated that the interaction is primarily of electrostatic origin. The efficiency of displacement of the proteins by salt was in the order F2 > F1 > MBP, which correlates with both the strength of binding and the net charge on the different protein fragments. Nitroxide derivatives of phosphatidylglycerol (PG) labeled on the sn-2 chain were used to probe the protein-induced changes in the acyl chain dynamics. Both the fragments and the MBP decreased the lipid chain mobility as recorded by the C-5 atom and (2-12 atom position nitroxide-PG spin-labels, in a manner which followed the protein binding curves. At saturation binding, the reduction in mobility recorded by the C-5 atom label was in the order M B P > F1 > F2. An additional population of lipids, whose chain motion was restricted relative to that of the bulk population of fluid lipids, was resolved in the case of the F1 fragment and the MBP, when using the C-12 atom position labeled PG. Approximately nine DMPG molecules per F1 fragment were found in this motionally restricted population, which was assigned to lipids in direct contact with partially penetrant sections of the protein, as opposed to a 18: 1 mol/mol lipid/protein stoichiometry found for the restricted component with the intact MBP. These results suggest that the principal sites of hydrophobic interaction of the protein with lipid bilayers are at least partly located in the 12.6-kDa fragment. The tryptophan residue at position 116 appears also to be important for the structural and functional properties of the intact bovine myelin basic protein.x e water-soluble peripheral myelin basic protein (MBP)' plays an important role in the structural organization of the myelin sheath [see Boggs and Moscarello (1978a,b) and Boggs et al. (1982)l. Induction and maintenance of the multilamellar structure of myelin are thought to be brought about by a bridging of apposing cytoplasmic surfaces. A hairpin structure of the protein caused by a tri-proline sequence approximately in the middle of the molecule is considered to cause interlamellar interactions (Eylar et al., 1971).