Protein crystallization: scaling of charge and salt concentration in lysozyme solutions

We studied the crystallization of lysozyme solutions by adding sodium chloride at pH = 4.5, 5.9 and 7.8. A universal crystallization boundary is found if data are scaled according to the salt concentration normalized by the square of the charge at the appropriate pH. Calculations show that this finding is consistent with recent attempts to rationalize protein crystallization using second virial coefficients.