An expression for the C-C bond order parameter, SCC, of membrane hydrocarbon chains has been derived from the observed C-D bond order parameters. It allows calculation of the probability of each of the C-C bond rotamers and, consequently, the number of gauche defects per chain as well as their projected average length onto the bilayer normal, thus affording the calculation of accurate hydrophobic bilayer thicknesses. The effect of temperature has been studied on dilauroyl-, dimyristoyl-, and dipalmitoylphosphatidylcholine (DLPC, DMPC, DPPC) membranes, as has the effect of cholesterol on DMPC. The salient results are as follows: 1) an odd-even effect is observed for the SCC versus carbon position, k, whose amplitude increases with temperature; 2) calculation of SCC, from nonequivalent deuterons on the sn-2 chain of lipids, SCC2, leads to negative values, indicating the tendency for the C1-C2 bond to be oriented parallel to the bilayer surface; this bond becomes more parallel to the surface as the temperature increases or when cholesterol is added; 3) calculation on the sn-2 chain length can be performed from C1 to Cn, where n is the number of carbon atoms in the chain, and leads to 10.4, 12.2, and 13.8 A for DLPC, DMPC, and DPPC close to the transition temperature, TC, of each of the systems and to 9.4, 10.9, and 12.6 for T-TC = 30-40 degrees C, respectively; 4) separation of intra- and intermolecular motions allows quantitation of the number of gauche defects per chain, which is equal to 1.9, 2.7, and 3.5 for DLPC, DMPC, and DPPC near TC and to 2.7, 3.5, and 4.4 at T-TC = 30-40 degrees C, respectively. Finally, the validity of our model is discussed and compared with previously published models.

The ordering properties of sn-1 and sn-2 dimyristoylphosphatidylcholine (DMPC) chains are investigated by NMR and neutron diffraction in order to relate membrane defects to bilayer thickness. First, the effect of temperature and 30 mol % cholesterol on DMPC membranes perdeuterated on the sn-1 acyl chain is studied by solid state 2 H-NMR. Using the concept of C-C bond order parameters, S CC , a comparison is made between both lipid chains according to our previously published data on sn-2 chains (Douliez; et al. Biophys. J. 1995 et al. Biophys. J. , 68, 1727 et al. Biophys. J. -1739). An odd-even effect is observed when plotting S CC versus carbon position, k, the amplitude of which is constant when increasing temperature, in the presence or absence of cholesterol. Such an effect can be related to different dynamics of conformers used to describe C-C bond isomerization. It is also found that S odd CC < S even CC for sn-1 chains and S odd CC > S even CC for sn-2 chains. Such an inversion in parity is accounted for by the peculiar conformational dynamics occurring around C 1 -C 2 in the sn-2 chain. Calculation of the carbonyl C 1 -C 2 bond order parameter indicates that the beginning of the sn-1 chain tends to be perpendicular to the plane of the bilayer, i.e. conversely to what has been found for the C 1 -C 2 bond of the sn-2 chain which orients parallel to the bilayer surface. The presence of cholesterol does not markedly modify the dynamics of the C 1 -C 2 bond of the sn-2 chain, whereas it does restrict by 70% the conformational freedom of the beginning of the sn-1 chain. The number of gauche defects for the sn-1 chain increases from 3 to 4.3 on going from 25 to 60°C and is greater by 0.5-0.7 unit than that of the sn-2 chain, at corresponding temperatures. The average length of the sn-1 chain is found to be 12.7 Å at 25°C and 11.0 Å at 50°C, i.e. it is greater than that of the sn-2 chain by 0.7 Å close to the main transition temperature, T C , whereas both chains have the same length for T > T C + 15°C. At corresponding temperatures, addition of cholesterol results in a 3.5 Å increase in the sn-1 chain length. Second, measurement of the peak-to-peak distance in the one-dimensional neutron-scattering amplitude density of the membrane leads to a bilayer hydrophobic thickness, d h , of 29.3 and 33.2 Å at 50°C, in the absence and presence of 30 mol % cholesterol, indicating a large apparent discrepancy between d h and twice the acyl chain length as calculated from NMR. However, comparison of chain length determination by NMR and neutron diffraction using deuterium labels at positions 5,15 and 4,14 on one of the dipalmitoylphosphatidylcholine acyl chains shows that both methods measure the same quantity within 0.4 Å. Hence, the marked differences observed between neutron diffraction and solid state NMR in bilayer hydrophobic thickness determination are interpreted in terms of protrusion of molecules above the average bilayer surface. This protrusion is estimated to be 2.1 Å for DMPC at 50°C and is reduced to 0.7 Å...

No abstract

No abstract