Restatement of order parameters in biomembranes: calculation of C-C bond order parameters from C-D quadrupolar splittings

Abstract

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.

Intramolecular and Lattice Melting inn-Alkane Monolayers: An Analog of Melting in Lipid Bilayers

“…The footprint reduction mechanism operative at melting involves a collective structural collapse of the molecules from a linear to a more globular shape. Thus the monolayer melting transition is qualitatively similar to the well-known gel-to-fluid transition in bilayer lipid membranes [14][15][16] in which intrachain and lattice melting occur simultaneously.…”

Experimental and theoretical studies of emodin interacting with a lipid bilayer of DMPC

“…In all three cases, the |S CD | profiles exhibit values lower than 0.23, indicating a greater disorder of the hydrocarbon chains. In the case of pure-DMPC, our simulations reproduced the experimental (Petrache et al 2000;Vermeer et al 2007;Douliez et al 1995) and theoretical Jämbeck and Lyubartsev 2012) |S CD | values of the sn-1 and sn-2 reported previously, with |S CD | profiles presenting an increase in the |S CD | values over the first five carbons of sn-1 and a plateau in the values of the correspondent atoms in the sn-2, followed by a decrease in magnitude of these parameters for the last six carbons of both the sn-1 and sn-2 acyl chains.…”

A Quantitative Coarse-Grain Model for Lipid Bilayers

“…[22][23][24] The only significant difference involves the central region, corresponding to the terminal methyl groups, where the density of our model is slightly higher than that obtained from experiment. The pronounced central trough in the electron density is possibly determined by the "curling up" of the terminal methyl segment; experimental order parameters 5 indeed indicate that the terminal methyl segment is significantly tilted with respect to the neighboring segment. Our model (by a Subscripts C, P, G, T, and W stand for the site types choline, phosphate, glycerol, tail, and water, respectively.…”

“…We calculated an average segmental order parameter for the entire tail region of S mol ) 0.36 ( 0.01, consistent with the value of 0.38 deduced from the experimental data. 5,79 Standard errors have been estimated using the block averaging method 72 using 10 consecutive 10 ns blocks.…”