Conformational changes of β-carotene and zeaxanthin immersed in a model membrane through atomistic molecular dynamics simulations

In this work, we investigate systems formed by β-carotene and zeaxanthin embedded separately in a model lipid bilayer of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) through molecular dynamics (MD) simulations. The study is conducted using an all-atoms model and by analyzing the structural changes that occur at both the carotenoid molecule and the membrane during the simulations. We concentrate specifically on the conformation of the conjugated chain, given the relevance that this feature has in modulating the spectroscopic and antioxidant properties of the carotenoids. The force fields of the carotenoids are parametrized accordingly in order to reproduce the rotation potentials of the conjugated chains calculated using quantum DFT methods. A model to quantify the effective conjugated chain length is presented. The MD simulations are carried out using the parameters adjusted for the carotenoids along with those provided by the CHARMM36 force field for the lipids of the membrane. A differentiating dynamic behavior of β-carotene and zeaxanthin within the bilayer is observed in the simulations, which is analyzed in detail through umbrella sampling techniques. This behavior is driven basically by the interactions of the lipid polar heads with the hydroxyl groups of zeaxanthin, which are absent in β-carotene. These interactions influence the carotenoid orientation, modify the conformational distribution of the dihedral angles of the conjugated chain significantly, and specifically distort the membrane structure.