Dipalmitoylphosphatidylcholine membranes modified with carotenoid pigment lutein: experiment versus Monte Carlo simulation study of the membrane organization.

Organization of bilayer lipid membrane formed with dipalmitoylphosphatidylcholine and containing a xanthophyll pigment lutein was studied by both the Monte Carlo simulation and UV-Vis absorption spectroscopy. The simulations were based on ten-state Pink model. The proposed model consisted of two monolayers represented by a two-dimensional triangular lattice with vacancies. The orientation and aggregation state of lutein, obtained from the analysis of the spectroscopic measurements, were used to calibrate intermolecular interactions in the model. In accordance with the experimental data, the model allows two orientations of lutein molecules: one spanning the membrane and the other parallel to its plane. The influence of the intermolecular interactions on the main phase transition as well as on the aggregation of lutein molecules is discussed. The analysis of the model enables us to learn about molecular mechanisms that govern the effects of lutein on the membrane properties as well as the effects of the lipid matrix on lutein organization in the membrane. A concept is discussed according to which increasing domination of parallel lutein orientation, observed at high temperatures, can protect the membrane against penetration by water molecules and reactive oxygen species and against loss of the membrane compactness, especially in the regions of oxidized acyl chains.