Surface-mediated retention effects of subtilisin site-specific variants in cation-exchange chromatography

Wild-type subtilisin and several site-specific variants were resolved on a strong cation-exchange column by isocratic elution and a series of sodium chloride concentrations. Changes in primary sequence at the protein surface have an observable effect on the chromatographic behavior of subtilisin. This supports the concept that three-dimensional structure determines which biopolymer surface residues are in position to interact with the stationary phase surface. The retention data fit the stoichiometric displacement model (SDM) of retention. Plots of ln k' vs. ln 1/[NaCl] yield values for the average number of ionic groups (Z) on the protein that interact with the support matrix. Application of the SDM to the chromatographic retention of the variants has uncovered an unusual phenomenon at the protein surface at low ionic strength. A SDM plot normally provides a linear relationship between ln k' and ln 1/[NaCl] with the slope corresponding to the Z number. This study revealed two lines differing in slope and intercept, indicating that the Z number of subtilisin changes at some intermediate ionic strength of the eluent. These results are attributed to some salt-induced protein surface event that triggers a change in structure. Chromatographic detection of this occurrence reflects the connection between the surface-mediated event and mobile phase ionic strength.