Effect of cholesterol on the structure of a phospholipid bilayer

Abstract

Cholesterol plays an important role in regulating the properties of phospholipid membranes. To obtain a detailed understanding of the lipid-cholesterol interactions, we have developed a mesoscopic water-lipid-cholesterol model. In this model, we take into account the hydrophobic-hydrophilic interactions and the structure of the molecules. We compute the phase diagram of dimyristoylphosphatidylcholine-cholesterol by using dissipative particle dynamics and show that our model predicts many of the different phases that have been observed experimentally. In quantitative agreement with experimental data our model also shows the condensation effect; upon the addition of cholesterol, the area per lipid decreases more than one would expect from ideal mixing. Our calculations show that this effect is maximal close to the main-phase transition temperature, the lowest temperature for which the membrane is in the liquid phase, and is directly related to the increase of this main-phase transition temperature upon addition of cholesterol. We demonstrate that no condensation is observed if we slightly change the structure of the cholesterol molecule by adding an extra hydrophilic head group or if we decrease the size of the hydrophobic part of cholesterol.

Fig. 1.

Schematic drawing of the mesoscopic models that are studied in this work. (A and B) Figure represents DMPC (A) and cholesterol (B). The model contains hydrophobic (white) and hydrophilic (black) beads that are connected with springs and bond-bending potentials. The model contains explicit water molecules that are modeled as hydrophilic beads. To study the effect of change in the chemical structure of cholesterol we introduce three “new” molecules in which we change the hydrophobic–hydrophilic balance of cholesterol. (C) Cholesterol with a shorter tail length. (D) Cholesterol that is more hydrophilic. (E) Cholesterol that is less hydrophobic.

Fig. 2.

Phase diagram and the structure of the various phases. (Left) Computed-phase diagram as a function of temperature (in degrees centigrade) and cholesterol concentration. The black lines give the phase boundaries. The color coding gives the condensation effect at a given state point, where blue indicates very little condensation and orange a large condensation effect. (Right) Schematic drawing of the various phases. L_α, lipids in the liquid phase; P′_β, ripple phase; L′_β, gel phase with tilted lipid chains; L′_c, gel phase with lipid chains not tilted; L_II, gel phase, similar to L′_c, containing small cholesterol clusters; L_o, liquid-ordered phase. The condensation effect is defined as the difference, in Ų, between A_{M, sim} and A_{M, ideal}.

Fig. 3.

Area per molecule as a function of the cholesterol concentration for the molecules shown in Fig. 1. Data for cholesterol (A) and for the modified cholesterol (B) molecules shown in Fig. 1 C–E. (A) We compare experimental data of Hung et al. (21) with our simulation results and the ideal mixing estimates. This estimate is given by A_M,mix = (1 − x_c)A_L + x_cA_C, with x_c as the mole fraction of cholesterol. A_L and A_C are the pure-component area per lipid and the area per cholesterol, respectively. The experimental data and simulations were both at T = 30 °C. (B) Effect of changes of the cholesterol hydrophobic–hydrophilic balance; the circles are for cholesterol in which the hydrophilic part is increased, the squares are for cholesterol with a decreased hydrophobic part, and the triangles are for cholesterol with a shorter tail length (see Fig. 1).

Fig. 4.

The relative bilayer thickness (A) and order parameter (B) of the DMPC–cholesterol system as a function of cholesterol concentration. (A) We compare the experimental data of Pan et al. (30) and Hung et al. (21) with the results of our simulation. The relative bilayer thickness swelling is defined as d/d₀, with d the phosphorus-to-phosphorus distance in the electron density profile and d₀ the thickness of the pure bilayer. (B) Experimental data are from Pan et al. (30) and Mills et al. (31). The orientational lipid tail order parameter, S_NMR, is defined as S_NMR = 0.5 〈3 cos θ² − 1〉, where θ is defined as the angle between the orientation of the vector along two beads in the chain and the normal to the bilayer plane, and the average is taken of the ensemble average over all beads. S_X-ray quantifies the average tilt of the chain of the lipids by using the same formula where the angle θ is between the orientation of the vector along the first and the last tail beads and the normal to the bilayer plane. The experimental data and simulations were both at T = 30 °C.

Fig. 5.

Snapshots of a side view of the bilayer. (A) L_α phase for 10% cholesterol at T = 37 °C. (B) L₀ phase for 40% cholesterol at T = 37 °C. (C) Ripple (P′_β) phase for 5% cholesterol at T = 20 °C. (D) L′_β phase for 5% cholesterol at T = 5 °C. (E) L′_c phase for 15% cholesterol at T = 5 °C. (F) L_II phase for 40% cholesterol at T = 5 °C. The hydrophilic and the hydrophobic beads of the phospholipids are depicted in dark blue and in light blue, respectively. The end beads of the lipid tails are depicted in gray. The cholesterol headgroup is depicted in yellow, the cholesterol tetrameric ring and tail beads are depicted in red. For clarity, water beads are not shown. The difference in the width of the bilayers illustrates the condensation effect nicely.