Dipalmitoylphosphatidylcholine membranes modified with zeaxanthin: numeric study of membrane organisation.

The model of a dipalmitoylphosphatidylcholine (DPPC) bilayer containing a xanthophyll pigment zeaxanthin (ZEA) is proposed. The model is based on the ten-state Pink-Green-Chapman model of a lipid monolayer. The Monte Carlo method of computer simulation has been applied. Our model of the lipid membrane consists of two lipid monolayers with ZEA molecules spanning the lipid bilayer. The concentration of ZEA molecules is assumed to be conserved. Within the model, the interactions between lipid monolayers in a bilayer exist through ZEA molecules only. The experimental data concerning the aggregation of ZEA in DPPC from the literature and from our research were applied as a criterion to fit the model parameters. The model gives the dependences of the main phase transition temperature on ZEA/DPPC molar ratio, the percentage of ZEA in a monomeric form on ZEA/DPPC molar ratio and on temperature. The dependences obtained within the model and the experimental ones are in qualitative agreement. The influence of intermolecular interaction parameters on ZEA aggregation has been discussed. The differences between the model and the experimental results concerning mainly the pattern of ZEA aggregation have been discussed. Analyses of the lipid microconfiguration allow to advance the hypothesis concerning the influence of ZEA on the membrane permeability.