Unperturbed hydrocarbon chains and liquid phase bilayer lipid chains: a computer simulation study

Abstract

In this work, the properties of saturated and unsaturated fatty acid acyl chains 16:0, 18:0, 18:1(n-9)cis, 18:2(n-6)cis, 18:3(n-3)cis, 18:4(n-3)cis, 18:5(n-3)cis, 20:4(n-6)cis, 20:5(n-3)cis and 22:6(n-3)cis in a bilayer liquid crystalline state and similar hydrocarbon chains (with CH[Formula: see text] terminal groups instead of C=O groups) in the unperturbed state characterised by a lack of long-range interaction were investigated. The unperturbed hydrocarbon chains were modelled by Monte Carlo simulations at temperature [Formula: see text] K; sixteen fully hydrated homogeneous liquid crystalline phosphatidylcholine bilayers containing these chains were studied by molecular dynamics simulations at the same temperature. To eliminate effects of the simulation parameters, the molecular dynamics and Monte Carlo simulations were carried out using the same structural data and force field coefficients. From these computer simulations, the average distances between terminal carbon atoms of the chains (end-to-end distances) were calculated and compared. The trends in the end-to-end distances obtained for the unperturbed chains were found to be qualitatively similar to those obtained for the same lipid chains in the bilayers. So, for understanding of a number of processes in biological membranes (e.g., changes in fatty acid composition caused by environmental changes such as temperature and pressure), it is possible to use, at least as a first approximation, the relationships between the structure and properties for unperturbed or isolated hydrocarbon chains.

Keywords: Biomembranes; Lipid bilayers; Molecular dynamics; Monte Carlo; Unsaturated hydrocarbon chains.

Fig. 1

Structures, from top to bottom, of sn-2 lipid chains (fatty acid acyls) of 18:1(n-9)cis, 18:2(n-6)cis, 18:3(n-3)cis, 18:4(n-3)cis, 18:5(n-3)cis, 20:4(n-6)cis and 20:5(n-3)cis; phosphatidylcholine molecule of 18:0/22:6(n-3)cis PC showing structures of sn-2 22:6(n-3)cis acyl chain and sn-1 18:0 acyl chain; structure of the possible sn-1 16:0 acyl chain

Fig. 2

Structures, from top to bottom, of hydrocarbon chains alk-16:0, alk-18:0, alk-18:1(n-9)cis, alk-18:2(n-6)cis, alk-18:3(n-3)cis, alk-18:4(n-3)cis, alk-18:5(n-3)cis, alk-20:4(n-6)cis, alk-20:5(n-3)cis and alk-22:6(n-3)cis studied by Monte Carlo simulations. Such names of the hydrocarbon chains are used to stress the chain and corresponding FA acyl (Fig. 1) structural similarity

Fig. 3

Time evolution of the average area per lipid $A_{\mathrm{pl}}$ of the PC bilayers with sn-1 chain 16:0 (a) and 18:0 (b)

Fig. 4

Sixteen structural units; to construct a linear hydrocarbon (n-alkane or n-alkene) chain of the structure, e.g., as in Fig. 2, that is typical for the biomembrane phospholipid chain structure (Fig. 1), and calculate the energy U according to expression (4), several of the presented units should be properly combined. Three variable torsions in each unit are marked by red arrows. The number at the bottom right of the unit is the unit’s number

Fig. 5

Six two-dimensional energy maps of structural unit 9 from Fig. 4. The value 0${}^{\circ }$ of any torsions (angles ${\mathit{\phi }}_1$, ${\mathit{\phi }}_2$ and ${\mathit{\phi }}_3$) corresponds to the eclipsed conformation. The numbers near equienergetic contours are energies (kJ/mol). The energy of each map is measured from the global energy minimum of structural unit 9

Fig. 6

Average end-to-end distances ${⟨h⟩}_{\mathrm{\Theta }}$, [nm], for unperturbed hydrocarbon chains obtained by Monte Carlo (MC) simulations, and ${⟨h⟩}_{\mathrm{bil}}$, [nm], for the same acyl chains in liquid crystalline phosphatidylcholine (PC) bilayers obtained by molecular dynamics (MD) simulations (triangles for the marked chains in 18:0/... PC bilayers, circles for the marked chains in 16:0/... PC bilayers). The ranges for saturated acyl chains 16:0 and 18:0 are obtained for eight appropriate mixed-chain PC bilayers. Computer simulations of the present work, $T=303$ K. Arrows show qualitatively trends in ${⟨h⟩}_{\mathrm{\Theta }}$ and ${⟨h⟩}_{\mathrm{bil}}$. To compare the obtained trends, the same names used here both for acyls (ordinate axis) and hydrocarbon chains (abscissa axis)