Structure and interaction in protein solutions as studied by small-angle neutron scattering

Small-angle neutron scattering (SANS) measurements have been performed to compare the effect of the salts KF, KCl, and KBr on crystallization in aqueous solution of lysozyme protein. It is found that the propensity of the salt to crystallize protein follows the Hoffmeister series (KF < KCl < KBr) with marked differences in the effect of these salts. In pure protein solution, lysozyme macromolecules are prolate ellipsoidal with semimajor and semiminor axes as 22 and 13.5 A, respectively. SANS also gives that the effective (structural+counterion) charge (Z) on the protein as obtained by taking into account screened Coulomb interaction between the protein macromolecules is found to be much smaller than the structural charge. There is decrease in Z suggesting the higher counterion condensation on protein with the increase in the concentration. The counterion condensation seems to be responsible for the differences in the effect of different salts. It is also found that with the addition of salts, lysozyme macromolecules convert to dimers, and for the same salt concentration the comparative effect of different salts follows the Hoffmeister series. Time evolved measurements prior to and after the crystallization show that the protein solution mostly consists of monomers and dimers. Interestingly, higher-mers are not observed in these measurements as perhaps they are formed in very small numbers towards the process that leads to the crystallization. The time dependent data have been used to obtain the fraction of crystallization as a function of time.