Characterization of zein assemblies by ultra-small-angle X-ray scattering†
Zein, a major corn protein, has an amphiphilic molecule capable of self-assembling into distinctly different structures, i.e., rods, sheets, and spheres. In this work, ultra-small-angle X-ray scattering (USAXS) was applied to investigate the formation of self-assembled zein structures in binary solvent systems of ethanol and water. The study included observing structural changes due to aging. Three distinctive regions, each corresponding to different co-existing structures having a hierarchical organization, were observed in zein-solvent systems. Rod shaped (Rg = 1.5–2.5 nm, P = 1) primary structural units were identified, believed to be molecular zein. Two-dimensional sheet-like structures (Rg = 80–200 nm, 2 < P < 3) were observed, believed to be formed by primary units first assembled into one-dimensional fibers and then into 2D sheet structures. Also, large three-dimensional spherical aggregates were observed (Rg > 1000 nm, P = 4), believed to have assembled from two-dimensional sheet structures. Aging did not change the size or the shape of the primary units, but USAXS detected changes in Rg and P values of the intermediate structures, pointing to a further level of self-assembly where proteins develop a more regular and organized structure. The viscoelastic moduli (G′ and G′′), the consistency index (K) and the flow behavior index (n), were also measured to investigate the effect of zein structural development by self-assembly on rheological behavior. Samples became more solid-like with aging. Raman spectra suggested that zein underwent secondary structure transformations from α-helix to β-sheets, which influenced the size and morphology of molecular assemblies and ultimately the rheological properties of zein solutions.