Physical characterization and in silico modeling of inulin polymer conformation during vaccine adjuvant particle formation

• Physical analyses and modeling reveal key features of inulin particle formation.
• Inulin polysaccharide chains adopt random coil in solution.
• Glucose end groups contribute to organized self-assembly of inulin particles.
• Inulin fibers comprise inulin helices perpendicular to the long axis.
• γ-Inulin particles comprise 11 nm semi-crystalline layers with 78% crystallinity.

This study combined physical data from synchrotron SAXS, FTIR and microscopy with in-silico molecular structure predictions and mathematical modeling to examine inulin adjuvant particle formation and structure. The results show that inulin polymer chains adopt swollen random coil in solution. As precipitation occurs from solution, interactions between the glucose end group of one chain and a fructose group of an adjacent chain help drive organized assembly, initially forming inulin ribbons with helical organization of the chains orthogonal to the long-axis of the ribbon. Subsequent aggregation of the ribbons results in the layered semicrystalline particles previously shown to act as potent vaccine adjuvants. γ-Inulin adjuvant particles consist of crystalline layers 8.5 nm thick comprising helically organized inulin chains orthogonal to the plane of the layer. These crystalline layers alternate with amorphous layers 2.4 nm thick, to give overall particle crystallinity of 78%.