Determination of Chemical Activity of Soybean Proteins and Amino Acid Model Systems From NMR Relaxation Data in Relation to Water Sorption Isotherms

253 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1994.Hydration, aggregation, ion binding, and water sorption properties of soy proteins were determined by $\sp $O, $\sp2$H and $\sp1$H nuclear magnetic resonance (NMR), combined with rheological and computational techniques. Selected amino acids were employed as model systems for soy protein hydration, activity, and water sorption. It was found that aspartic acid, glutamic acid, lysine, and arginine in concentrated aqueous D$\sb2$O solutions behave nonideally and that their dynamics is dominated by repulsive charge-charge interactions. On the other hand, the nonideal behavior of glycine, betaine, and proline in concentrated aqueous solutions is dominated by amino acid aggregation.Nonlinear regression analysis of $\sp $O NMR data for soy proteins indicates that the transverse relaxation of soy proteins at alkaline pH depends on the protein activity coefficients, related to the repulsive charge-charge interactions between soy proteins. At neutral pH, the transverse relaxation of soy globulin fractions in solution containing 0.4 M NaCl showed that the protein activity coefficients are negative ($\cong{-}1.2),$ indicating the presence of soy globulin self-association. The relative hydration of 7S soy globulin was 1.3 times higher than that of 11S soy globulin. A new approach to water vapor isotherms of selected amino acids, as well as the 7S and 11S soy globulins, using nonlinear regression analysis, shows that the relative vapor pressure is thermodynamically linked to the amino acid clustering or protein aggregation. Results of nonlinear regression analysis with a six-parameter model showed that a one-step protein aggregation process dominated in both hydrated 7S and 11S soy globulins. These findings are potentially important for controlling the stability and shelf-life of soy protein-based food products.Nuclear spin retaxation and rheological results at pH 7.3 for soy protein solutions are consistent with an ion binding/isoelectric precipitation model. Ion binding occurs in the order of the Hofmeister or lyotropic series (e.g., Li$\sp+ >$ Na$\sp+ >$ Cs$\sp+$; SO$\sb4\sp{2-} >$ Cl$\sp- >$ Br$\sp- >$ I$\sp-$). These findings are therefore potentially important for formulation of new foods and the quality control of food products based on soy proteins.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD