Mixtures of the sterol ester gamma-oryzanol (ORY) and the sterol beta-sitosterol (SIT) form helical tubules when mixed together in triglyceride oil. AFM imaging shows a hierarchical structure in the system. Initially, SIT and ORY form dimers, stabilised by a hydrogen bond, although there is evidence of a substantial energy barrier to nucleation. Molecular docking and FTIR confirms the H-bond and that the dimer adopts a staggered conformation due to the presence of an orthoganol methyl group on ORY that prevents parallel alignment of the sterane cores. This dimer self-assembles into a helical tubule, but it is unclear what drives the further self-association. The wedge shape, we believe, directs formation of a helical tubule as it creates a twist when consecutive dimers align. De-novo synthesised sterol ester analogues of ORY that lack both the orthogonal methyl group and the hydrogen bond form a parallel dimer and do not gel. Once formed tubules further associate into bundles. Raman, UV-Vis and CD spectroscopy show bundle formation is associated with vibrational modes in ferulic acid ester groups attached to ORY suggesting these start to aggregate. MD simulations of two tubules shows that ferulate groups of ORY protrude from the surface of tubules, and that π-π-stacking, vdW and H-bond interactions between the ferulates stick tubules together. A combination of experimental and simulation techniques allows elucidation of molecular features controlling phytosterols to act as oleogelators. With this knowledge we have begun to design and synthesise new olegelators de-novo to allow control over oleogel structure and properties.
