Conformational Order of DMPC sn-1 versus sn-2 Chains and Membrane Thickness:  An Approach to Molecular Protrusion by Solid State ²H-NMR and Neutron Diffraction

Abstract

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...

Long-timescale motions in glycerol-monopalmitate lipid bilayers investigated using molecular dynamics simulation

“…This alternating pattern is also visible in the S CH (C n ) curves for these two phases, most pronounced for the LC phase. A similar pattern has been reported as the odd-even effect in the context of carbon order parameters S CC for the DPPC system [29] and for other lipids [99,102]. A decrease of the odd-even effect toward the end of the tail [29], as well as an increase of this effect with increasing temperature [99,102] were also reported.…”

“…A similar pattern has been reported as the odd-even effect in the context of carbon order parameters S CC for the DPPC system [29] and for other lipids [99,102]. A decrease of the odd-even effect toward the end of the tail [29], as well as an increase of this effect with increasing temperature [99,102] were also reported. The effect was explained in these studies as predominantly arising from the alternation of bond-vector orientations relative to the bilayer normal in a nearly all-trans conformation.…”

Polyunsaturated phosphatidylinositol and diacylglycerol substantially modify the fluidity and polymorphism of biomembranes: A solid‐state deuterium NMR study

“…We deduce from our data that these biological vesicles are very fluid even at low temperatures. PI provides a greater chain disorder of the membrane, which may result in a reduction of the membrane thickness (21,22). This effect could be promoted both by the PI head group and chains.…”

Sterols and membrane dynamics

“…Because the membrane ordering is directly linked to bond or molecule space averages, it can be translated, using an appropriate theory, to average length of a molecule in a membrane and hence to membrane thickness (Fig. 2d) [13,14]. It is shown that increase/ decrease in bilayer order means increase/decrease in membrane thickness.…”