Effect of solvent polarity on the secondary structure, surface and mechanical properties of biodegradable kafirin films

Kafirin, the prolamin from sorghum was studied in kafirin only condition and was also dissolved in various concentrations of water/ethanol and water/tertiary butanol solutions with a wide range of polarities and then plasticized and cast into films in order to study the effect of solvent polarity on conformation, mechanical and surface properties of films. Characterization of amino acid sequence led to a kafirin composition consisting of 55% alpha 1, 24% alpha 2, 18%beta, 2% gamma kafirin sub-units. Further study of the conformation led to a prediction of overall 66.17% alpha-helix conformation and 30% solvent inaccessible residue for total kafirin protein. Based on the amino acid composition and the total hydrophobicity of each individual amino acids in kafirin, the water contact angle was predicted to be 60 degrees, which is within experimental values reported (55-70 degrees for kafirin films cast from ethanol, 58-82 degrees for films cast from tertiary butanol solvent). The FTIR results coupled with CD results showed that decreasing solvent polarities caused increasing alpha-helical structure. Kafirin-plasticizer interactions and the solvent inaccessible residue of kafirin influenced the effect of different solvent polarity on the alpha-helical structure. Films fabricated using water/t-butanol solvent showed higher tensile strength, and elongation at break compared to films fabricated from water/ethanol solvents.