Molecular structure, dynamics and hydration studies of soybean storage proteins and model systems by nuclear magnetic resonance

The potential of high-resolution 13C NMR for the characterization of soybean storage proteins was explored. The spectra of a commercial soy protein isolate as well as those of alkali-denatured 7S and 11S soybean globulins were well resolved and tentatively assigned. Relaxation measurements indicated fast motion for several side chains and the protein backbone. Protein fractions (11S and 7S) were also investigated at various states of molecular association. The large size of the multisubunit soybean storage proteins affected adversely both the resolution and the sensitivity of their 13C NMR spectra. A comparison of 17O and 2H NMR relaxation rates of water in solutions of lysozyme (a model system) as a function of concentration, pH and magnetic field suggested that only 17O monitors directly the hydration of lysozyme. Analysis of 17O NMR lysozyme hydration data in terms of a two-state, fast-exchange, anisotropic model resulted in hydration parameters which are consistent with the protein's physico-chemical properties. The same model was applied to the calculation of the amount and mobility of bound water in soy protein dispersions by means of 17O NMR relaxation measurements as a function of protein concentration. The protein concentration dependences of 1H transverse NMR relaxation measurements at various pH and ionic strength values were fitted by a viral expansion. The interpretation of the data was based on the effects of protein aggregation, salt binding and protein group ionization on the NMR measurements. In all cases, relaxation rates showed a linear dependence on protein activity