The Aggregation of Lutein in Lipid Membranes the Monte Carlo Simulation

Lutein is the one of the main xanthopyll pigments found in the membranes of the macula lutea of the primate vision apparatus. The two functions of lutein have been studied experimentally: protection against oxidative damage of the lipid matrix [1] and screening against excess radiation [2]. This latter function depends on the molecular organisation, i.e. orientation and aggregation of lutein. The pigment incorporated into the dipalmitoylphosphatidylcholine (DPPC) liposomes forms H-type aggregates manifested by a shift of the maximum absorption wavelength to the ultraviolet region. In this study, the aggregation of lutein in the model DPPC bilayer is investigated theoretically via computer simulation. The main aim is to propose theoretical intermolecular interactions which would concur with the experimentallyfound degree of lutein aggregation. The model is based on the ten-state PinkGreen-Chapman model of a lipid bilayer. The mismatch intermolecular interaction and hydrogen bonding of lutein molecules are included. The increase in the membrane surface area caused by the aggregation of lutein weakens the attraction of lutein molecules. The following features of the model are in qualitative agreement with the experimental dependiences: the percentage of lutein in monomeric form increases and in aggregated (except dimeric) form decreases upon an increase in temperature, an increase in temperature leads to an increase in the percentage of lutein in dimeric form due to luteinń lutein hydrogen bonding, which is more probable in a more fluid surrounding, lutein slightly reduces the temperature of the main phase transition.