Synchrotron radiation small-angle X-ray scattering study of structural transition and gelation of hen egg-white lysozyme depending on pH and ionic strength under isothermal heating

We studied the tertiary structural transition and gelation of hen egg-white lysozyme (HEWL) depending on pH and ionic strength under isothermal heating by using time-resolved synchrotron radiation small-angle X-ray scattering; By annealing at 85 degrees C for 65 min, 5% w/v HEWL solutions with 0.1M NaCl formed a turbid/opaque gel at PH 5.1 similar to 7.7, and the tertiary structural transition was promoted by elevating pH from 3.2 to 7.7. On the other hand, the solutions without NaCl (salt free) did not set to gel at PH 3.0 similar to 7.5, and the tertiary structure did not reach a fully unfolded state. The present results suggest that the electrostatic repulsive interaction between HEWL molecules contributes to the stabilization of the tertiary structure of HEWL and the suppression of the aggregation.