Lipid-cholesterol interactions in the P beta' phase. Application of a statistical mechanical model

Molecular modelling of membrane sterols with the use of the GROMOS 96 force field

Membrane located sterols determine the structure and function of eucariotic cell membranes. Moreover, they are targets for important antifungal antibiotic amphotericin B. Knowledge about the geometry and dynamics of sterols in the environment of lipidic membranes is necessary to understand their functions. However, due to the dynamic character of the membrane, no experimental data about sterol behaviour on the molecular level is available. Hence molecular modelling simulations could be a source of useful information. The main goal of this paper is to prove the adequacy of the GROMOS 96 force field for molecular simulations of membrane sterols. We focused our attention on the reproduction of characteristic geometrical features observed in the crystal of cholesterol hemiethanolate by molecular dynamics simulations. The results presented clearly indicate that the GROMOS 96 force field can be a useful tool to simulate the highly lipophilic systems. Moreover, interactions responsible for the stability of such systems can also be recognised.

A model for the lipid pretransition: coupling of ripple formation with the chain-melting transition

Below the thermotropic chain-melting transition, lipid membrane c(P) traces display a transition of low enthalpy called the lipid pretransition. It is linked to the formation of periodic membrane ripples. In the literature, these two transitions are usually regarded as independent events. Here, we present a model that is based on the assumption that both pretransition and main transition are caused by the same physical effect, namely chain melting. The splitting of the melting process into two peaks is found to be a consequence of the coupling of structural changes and chain-melting events. On the basis of this concept, we performed Monte Carlo simulations using two coupled monolayer lattices. In this calculation, ripples are considered to be one-dimensional defects of fluid lipid molecules. Because lipids change their area by approximately 24% upon melting, line defects are the only ones that are topologically possible in a triangular lattice. The formation of a fluid line defect on one monolayer leads to a local bending of the membrane. Geometric constraints result in the formation of periodic patterns of gel and fluid domains. This model, for the first time, is able to predict heat capacity profiles, which are comparable to the experimental c(P) traces that we obtained using calorimetry. The basic assumptions are in agreement with a large number of experimental observations.

Cholesterol effects on the phosphatidylcholine bilayer polar region: a molecular simulation study

A molecular dynamics (MD) simulation of a fully hydrated, liquid-crystalline dimyristoylphosphatidylcholine (DMPC)-Chol bilayer membrane containing approximately 22 mol% Chol was carried out for 4.3 ns. The bilayer reached thermal equilibrium after 2.3 ns of MD simulation. A 2.0-ns trajectory generated during 2.3-4.3 ns of MD simulation was used for analyses to determine the effects of Chol on the membrane/water interfacial region. In this region, 70% of Chol molecules are linked to DMPC molecules via short-distance interactions, where the Chol hydroxyl group (OH-Chol) is 1) charge paired to methyl groups of the DMPC choline moiety ( approximately 34%), via the hydroxyl oxygen atom (Och); 2) water bridged to carbonyl ( approximately 19%) and nonester phosphate ( approximately 14%) oxygen atoms, via both Och and the hydroxyl hydrogen atom (Hch); and 3) directly hydrogen (H) bonded to carbonyl ( approximately 11%) and nonester phosphate ( approximately 5%) oxygen atoms, via Hch ( approximately 17% of DMPC-Chol links are multiple). DMPC's gamma-chain carbonyl oxygen atom is involved in 44% of water bridges and 51% of direct H bonds formed between DMPC and Chol. On average, a Chol molecule forms 0.9 links with DMPC molecules, while a DMPC molecule forms 2.2 and 0.3 links with DMPC and Chol molecules, respectively. OH-Chol makes hydrogen bonds with 1.1 water molecules, preferentially via Hch. The average number of water molecules H bonded to the DMPC headgroup is increased by 7% in the presence of Chol. These results indicate that inclusion of Chol decreases interlipid links and increases hydration in the polar region of the membrane.

Progress in high resolution atomic force microscopy in biology

The atomic force microscope (AFM) was invented by Binnig, Quate and Gerber less than 10 years ago (Binniget al. 1986). In their first prototype, a piece of goldfoil was used as the cantilever, with a crushed diamond tip mounted at the end. On the back of the cantilever, a tunnelling junction was used to monitor the deflection of the cantilever (the gold-foil) when the specimen was scanned with the tip in contact with the surface. Thus, the surface topography of the specimen was obtained with a resolution critically dependent on the sharpness of the tip provided the deformation of the specimen was not serious. Even with such a crude set-up, they managed to obtain a lateral resolution of ˜ 30 Å and a vertical resolution of better than 1 Å on an amorphous A12O3surface. The operating principle of such an instrument is deceptively simple. However, such an arrangement was inconvenient for routine operations and unsuitable for imaging hydrated specimens, because the tunnelling junction is easily contaminated in air and works poorly in aqueous solutions (Alexanderet al. 1989). As a result, the application of this type of AFM to biological samples was rare (Engel, 1991).

Behavior of cholesterol and its effect on head group and chain conformations in lipid bilayers: a molecular dynamics study

Cholesterol molecules were put into a computer-modeled hydrated bilayer of dimyristoyl phosphatidyl choline molecules, and molecular dynamics simulations were run to characterize the effect of this important molecule on membrane structure and dynamics. The effect was judged by observing differences in order parameters, tilt angles, and the fraction of gauche bonds along the hydrocarbon chains between lipids adjacent to cholesterol molecules and comparing them with those further away. It was observed that cholesterol causes an increase in the fraction of trans dihedrals and motional ordering of chains close to the rigid steroid ring system with a decrease in the kink population. The hydrogen-bonding interactions between cholesterol and lipid molecules were determined from radial distribution calculations and showed the cholesterol hydroxyl groups either solvated by water, or forming hydrogen bond contacts with the oxygens of lipid carbonyl and phosphate groups. The dynamics and conformation of the cholesterol molecules were investigated and it was seen that they had a smaller tilt with respect to the bilayer normal than the lipid chains and furthermore that the hydrocarbon tail of the cholesterol was conformationally flexible.