Computer simulations: molecular dynamics simulations

The structure and stability of oleogels made from the sterol β-sitosterol and sterol ester γ-oryzanol are studied by a combination of molecular docking and molecular dynamics (MD) simulation. Molecular docking reveals the stacking motif of the individual sterols in dimer form and a model tubule, highlighting two structural features. First, a hydrogen bond between the phenol group of the sterol and carbonyl of the ester bond of oryzanol appears to be important in selecting for a parallel head-to-head dimer conformation. Second, a methyl group attached to the C14 of the oryzanol introduces a steric hindrance to stacking of the steroid cores making them adopt a wedge shape. It is believed the wedge shape leads to helical packing of dimers in the tubule. MD allows us to probe interactions between tubules in the network gel structure. Simulations of two interacting tubules reveal the importance of van der Waals, H-bonding, dispersion interactions, and π-π stacking of ferulate moieties of oryzanol on association of tubules. Finally, we investigate the instability of phytosterol tubules in water, showing that the H-bond in the dimer is unstable in water, leading to tubule disruption. Mechanisms for the water stability of oleogels in the presence of glycerol and lecithin are proposed.