The behaviour of β-carotene in the phosphatidylcholine bilayer as revealed by a molecular simulation study

A molecular dynamics (MD) simulation of the fully hydrated bilayer made of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and containing β-carotene (β-Car) molecules was carried out as a complementary approach to experimental techniques to investigate the orientation of β-Car in the lipid membrane as well as its influence on the bilayer properties. The bilayer reached thermal equilibrium after 1200 ps of MD simulation and the productive run was carried out for 2800 ps. The results indicate that the carotene rings are located in the region occupied by the carbonyl groups of the POPC γ-chain with no trace of penetration towards the centre of the bilayer. Carotene exhibits an ordering effect on both the β- and the γ-chain. While the fully saturated γ-chain is affected evenly along, the order of the mono-unsaturated β-chain is modified mainly below the double bond. In general, a high value of the order parameter and the chain tilt in the range from 11.4° to 26.7° were observed for the β-Car molecules. However, for chain segment adjacent to methyl groups the value of the order parameter is low and the tilt angle is close to 75°. Moreover, the probability of trans conformation being generally close to 1.0 along the β-Car chain is reduced for these segments. Our MD simulation study suggests two pools of the preferential orientation of β-Car: a slightly bent structure corresponding to a small chain tilt angle and a rather stretched structure that corresponds to a higher chain tilt. The results are discussed in the light of experimental findings.