Structural modifications of an amaranth globulin induced by pH and NaCl

The influence of pH and NaCl on the structure of globulin-P, the polymerizable amaranth 11S type globulin, was studied by differential scanning calorimetry, gel filtration, and gradient sedimentation. At mu = 0.54, the protein is stable for pH ranging from 5 to 9 but becomes rapidly unfolded as pH decreases below 5. For pH values above.9, globulin-P denatures more gradually than in acidic medium, and it also dissociates into subunits, which are possibly less thermostable. At pH 6.5 or 8.5 and low sodium chloride concentrations (mu less than or equal to 0.01), dialyzed globulin-P destabilizes, yielding species of lower thermal stability. The increase in NaCl concentration up to 0.1 hi induces folding of globulin-P toward a more stable structure. Above 0.1 M NaCl, increasing the ionic strength up to mu = 0.5 elevates the denaturation temperature (T-d) and denaturation enthalpy (Delta H). From mu = 0.1 to 0.5 the content of soluble globulin-P polymers decreases, possibly owing to protein insolubilization. Above 0.5 M, NaCl shows a stabilizing effect reflected by increasing T-d, whereas Delta H stays constant; this effect is similar to that found by other authors in some storage proteins.