Lipid storage in plants is achieved among all plant species by formation of oleosomes, enclosing oil (triacylglycerides) in small subcellular droplets. Seeds are rich in this pre-emulsified oil to provide a sufficient energy reservoir for growing. The triacylglyceride core of the oleosomes is surrounded by a phospholipid monolayer containing densely packed proteins called oleosins. They are anchored in the triacylglycerides core with a hydrophobic domain, while the hydrophilic termini remain on the surface. These specialized proteins are expressed during seed development and maturation. Particularly, they play a major role in the stabilization and function of oleosomes. To better understand the importance of oleosins for oleosome stabilization, enzymatic digestion of oleosins was performed. This made it possible to compare and correlate changes in the molecular structure of oleosins and changing macroscopic properties of oleosomes. Tryptic digestion cleaves the hydrophilic part of the oleosins, which is accompanied by a loss of secondary structures as evidenced by Fourier-transform infrared and sum frequency generation spectra. After digestion, the ability of oleosins to stabilize oil-water or air-water interfaces was lost. The surface charge and the associated aggregation behavior of oleosomes are governed by interactions typical of proteins before digestion and by interactions typical of phospholipids after digestion.

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The influence of the polysaccharide charge distribution on the structure, thickness, and charge reversal of the interfacial layers, formed by adsorbed positively charged protein and oppositely charged polysaccharide, has been investigated using lattice-based self-consistent field (SCF) approach. We compare the adsorption behaviour of a uniformly charged polysaccharide model with that consisting of a short and a long block carrying different charge densities. For homogeneously charged 10 polysaccharide we observe a resulting interfacial layer that is closer to a mixed protein + polysaccharide film, rather than a multi-layer. We also find that the maximum adsorption of polysaccharide occurs at an optimal value of its charge, above and below which the adsorbed amount decreases. In contrast, for heterogeneously charged chains, as their charge is increasingly located on the shorter block, a much thicker interfacial layer results. In such cases the weakly charged longer blocks extend well away from 15 the surface into the solution. The interfacial film begins to resemble a multilayer with a primary protein and a distinct secondary polysaccharide layer. When the weakly charged long blocks still have a sufficient amount of negative charge, we also observe a reversal of the sign of surface potential from positive to a negative value. Our SCF calculated values for the reversed surface potential are of the order of −25 mV, in good agreement with several experimental results involving -potential measurements on 20 particles covered with such protein + polysaccharide films.

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