Monte Carlo studies of phospholipid lamellae. Effects of proteins, cholesterol, bilayer curvature, and lateral mobility on order parameters

In this paper we present the results of a Monte Carlo study of the effects of protein, cholesterol, bilayer curvature, and mobility on the chain order parameters of a lipid layer. The Monte Carlo method used is identical to the version developed earlier (Scott, Jr., H.L. (1977) Biochim. Biophys. Acta 469, 264–271). Simulations of protein and cholesterol effects are accomplished by insertion of a rigid stationary cylinder into the lipid matrix. The protein studies show the presence of boundary lipid (Jost, P., Griffith, O.H., Capaldi, R.H. and Vanderkooi, G. (1973) Biochim. Biophys. Acta 311, 141–152). The effect of cholesterol is dependent upon the length of the lipid hydrocarbon chains relative to the cholesterol depth of penetration. Our computer studies of bilayer curvature show the manner in which this curvature disrupts chain packing and are consistent with experimental results (Chrzeszczyk, A., Wishnia, A. and Springer, C.S. (1977) Biochim. Biophys. Acta, 470, 161–171). We also find that restricting lateral motion in chains, the simplest manner in which head group interactions can affect hydrocarbon chain order, does not measurably alter the order parameters. We argue that this provides some support for an earlier hypothesis by Scott (Scott, Jr., H.L. (1975) Biochim. Biophys. Acta 406, 329–346) regarding head group-chain interaction in monolayer experiments.