Structural modifications of an amaranth globulin induced by pH and NaCl

The influence of pH and NaCl on the structure of globulin-P, the polymeriza ble amaranth 11S type globulin, was studied by differential scanning calori metry, gel filtration, and gradient sedimentation. At mu = 0.54, the protei n is stable for pH ranging from 5 to 9 but becomes rapidly unfolded as pH d ecreases below 5. For pH values above.9, globulin-P denatures more graduall y than in acidic medium, and it also dissociates into subunits, which are p ossibly less thermostable. At pH 6.5 or 8.5 and low sodium chloride concent rations (mu less than or equal to 0.01), dialyzed globulin-P destabilizes, yielding species of lower thermal stability. The increase in NaCl concentra tion up to 0.1 hi induces folding of globulin-P toward a more stable struct ure. Above 0.1 M NaCl, increasing the ionic strength up to mu = 0.5 elevate s the denaturation temperature (T-d) and denaturation enthalpy (Delta H). F rom mu = 0.1 to 0.5 the content of soluble globulin-P polymers decreases, p ossibly owing to protein insolubilization. Above 0.5 M, NaCl shows a stabil izing effect reflected by increasing T-d, whereas Delta H stays constant; t his effect is similar to that found by other authors in some storage protei ns.