Interplay of hydrophobic and electrostatic interactions in biopolymer chromatography. Effect of salts on the retention of proteins.
The effect of salt on the retention behavior of proteins in electrostatic and hydrophobic interaction chromatography is described by a three-parameter equation, log k' = A - B log ms + Cms, where k' is the retention factor and ms is the molality of the salt in the eluent. Parameter B, termed the electrostatic interaction parameter, depends on the characteristic charge of the protein and the salt counterion and governs the change of retention with the salt concentration in ion-exchange chromatography. According to the model the magnitude of the hydrophobic interaction parameter C is determined by the hydrophobic contact area upon protein binding at the stationary phase surface and the properties of the salt as measured by its molal surface tension increment. Retention data measured at different salt concentrations in the eluent on a variety of ion exchangers can be fitted to the above equation which yields U-shaped plots of log k' against log ms. The limiting slopes of the appropriate plots at sufficiently low and high salt concentrations can be used to evaluate the electrostatic and hydrophobic interaction parameters, respectively. The approach, which is based on a combination of established treatments of electrostatic and hydrophobic interactions offers a convenient framework for analyzing retention data in biopolymer high-performance liquid chromatography and for the characterization of stationary phases. Furthermore, it may facilitate some characterization of protein molecules on the basis of their retention behavior as a function of the concentration and nature of the salt in the eluent. In the treatment of electrostatic interactions use is made of the counterion condensation theory that is believed to make possible a more comprehensive analysis than the traditional stoichiometric ion-exchange model which assumes binding of the proteins by coulombic interactions at discrete sites. The treatment of hydrophobic interactions is based on an adaptation of the solvophobic theory which predicts that the hydrophobic portion of the free energy of binding is proportional to the hydrophobic contact area and the microthermodynamic surface tension of the aqueous salt solution. Despite its simplicity the theory was successful in explaining the observed effect of the nature and concentration of salt in the eluent, the pH and the effect of the density of fixed charges at the surface of the stationary phase in the absence of specific salt effects.