Effect of ionic strength on the molecular weight and conformation of wheat gluten proteins in 3 M urea solutions

Abstract

1. 1.|The influence of ionic strength upon the physicochemical properties of wheat gluten proteins in 3 M urea solution was studied by osmotic pressure, optical rotatory dispersion, intrinsic viscosity, and sedimentation velocity measurements. The number-average molecular weights of wheat gluten and gliadin by osmotic pressure at 25° between pH 4.7 and 7.5 are 67300 and 46600, respectively, and are constant with changes in ionic strength between 0.0025 and 0.15. Gluten and gliadin are considerably aggregated in aluminum lactate buffer of pH 3.2–3.4 and have molecular weights of 224000 and 81200, respectively, but in 3 M urea plus aluminum lactate buffer the molecular weight of gluten is not much higher than 67300. 2. 2.|The sedimentation coefficient of glutenin and gliadin in 3 M urea plus KCl is not altered by changes in ionic strength between 0.0025 and 0.5 (pH 4.8–5.8). Because the osmotic molecular weight and sedimentation coefficient are constant, other physical changes noted are caused by structural changes other than aggregation. Both proteins appear to contain a mixture of α-helix and random-coil structure based on the interpretation of optical rotatory dispersion data in terms of Moffitt—Yang, Cotton effect, and Shechter—Blout calculations. Gliadin contains more helix than glutenin. Based on these measurements there appears to be an increase of helical content in glutenin with an increase in ionic strength but with no significant change in gliadin. Intrinsic viscosities measured in these solvents suggest that glutenin and gliadin molecules have relatively high axial ratios. The intrinsic viscosities decrease with increasing ionic strength and the change is greater with glutenin. These observations are consistent with a more constrained structure for gliadin as compared with glutenin.