A variety of published physical measurements, computational algorithms, and structural modeling methods have been used to create a molecular model of 19 kDa alpha-zein (Z19). Zetaeins are water-insoluble storage proteins found in corn protein bodies. Analyses of the protein sequence using probability algorithms, structural studies by circular dichroism, infrared spectroscopy, small-angle X-ray scattering (SAXS), light scattering, proton exchange, NMR, and optical rotatory dispersion experiments suggest that Z19 has approximately 35-60% helical character, made up of nine helical segments of about 20 amino acids with glutamine-rich "turns" or "loops". SAXS and light-scattering experiments suggest that in alcohol/water mixtures alpha-zein exists as an oblong structure with an axial ratio of approximately 6:1. Furthermore, ultracentifugation, birefringence, dielectric, and viscosity studies indicate that alpha-zein behaves as an asymmetric particle with an axial ratio of from 7:1 to 28:1. Published models of alpha-zein to date have not been consistent with the experimental data, and for this reason the structure was re-examined using molecular mechanics and dynamics simulations creating a new three-dimensional (3D) structure for Z19. From the amino acid sequence and probability algorithms this analysis suggested that alpha-zein has coiled-coil tendencies resulting in alpha-helices with about four residues per turn in the central helical sections with the nonpolar residue side chains forming a hydrophobic face inside a triple superhelix. The nine helical segments of the 19 kDa protein were modeled into three sets of three interacting coiled-coil helices with segments positioned end to end. The resulting structure lengthens with the addition of the N- and C-terminal sections, to give an axial ratio of approximately 6 or 7:1 in agreement with recent experiments. The natural carotenoid, lutein, is found to fit into the core of the triple-helical segments and help stabilize the configuration. Molecular dynamics simulations with explicit methanol/water molecules as solvent have been carried out to refine the 3D structure.

Cereal Chem. 84(3):265-270Circular dichroism studies were performed on zein to determine how the secondary and tertiary structure changes with different solvents, temperatures, or pH. Alcoholic solvent type and common denaturants such as SDS and low amounts of urea had little effect on the secondary structure of zein. Utilization of dimethylformamide or acetic acid as solvent gave changes in tertiary structure. Solutions of zein in 8M urea produced solutions with large changes in tertiary structure. The dissolution of zein in 50 mM sodium hydroxide produces a zein with large changes in secondary and tertiary structure and little loss in primary structure. Increasing the temperature of zein to 70°C in 80% ethanol-water gave reversible changes in the primary structure (20% reduction in absolute magnitude of [θ] λ at 208 and 222 nm) and tertiary structure (40% reduction in absolute magnitude of [θ] λ at 268 nm).

To improve the mechanical properties of zein films, a commercial white zein was reacted in glacial acetic acid (HAc) with glutaraldehyde (GDA) from 0.5 to 16% by weight based on the weight of zein. Amounts of 4% or higher GDA-modified zein, when cured in a closed system, generated a gel. That gel was insoluble in organic solvents known to solubilize zein. Zein solutions cast within a silicon rubber gasket sealed onto Teflon-coated plates, when air dried, generated 0.7-0.9-mm thick films. Those films from 8% GDA-modified zein had a water vapor permeability of 5.9 Â 10 À8 g cm/Pa s m 2 . Overall, results indicate that our thicker films gave higher water vapor permeability than values reported in the literature. To achieve consistent and comparable results with differential scanning calorimetry (DSC) and dynamic mechanical analyses (DMA), film samples had to be heated to remove residual HAc. An increase in the glass transition temperatures for those films from GDA-modification of zein, observed with both DSC and DMA, was attributed to the crosslinking of zein. Physical testing of tensile bars cut from GDA-modified zein films showed minor enhancement of tensile strength, percent elongation, and Young's modulus in the 0.5% GDA-modified zein films, which increased and leveled off with 4% or higher amounts of GDA for the modifications. When the 4 to 16% GDA results are averaged, the tensile strength for the modified zein films increased 1.8-fold when compared with unmodified zein control films. On the same note, percent elongation showed a 1.8-fold increase and Young's modulus showed a 1.5-fold increase. GDA reaction with zein generated films with improved tensile strength, ductility, and stiffness when compared with respective zein control films that retained their integrity when subjected to either boiling water or extensive soaking.