Radiotracer measurements of protein mass transfer: kinetics in ion exchange media

We describe a method to measure protein mass transfer kinetics in ion exchange adsorbents for preparative chromatography based on the use of radioactively labeled protein. The method was developed and evaluated using lysozyme as a test protein with the three commercial strong-acid cation exchangers SP-Sepharose-FF, SP-Sepharose-XL, and S-HyperD. Iodination with 125I was used to label the protein, which was added in trace amounts (approximately 0.1%) to an unlabeled protein solution. The solution was recirculated through a shallow bed of the adsorbent particles and the radioactivity accumulated in the bed measured with a gamma-counter as a function of time. Radiotracer-based kinetics measurements were found to be in good agreement with results obtained with a conventional shallow-bed technique, provided that freshly labeled protein solutions were used. The method has advantages in terms of simplicity, ability to deal with adsorption from complex mixtures, and the potential for measurements under tracer diffusion conditions. Kinetics results obtained for the three different stationary phases were generally consistent with previous studies. Protein mass transfer can be described by a pore diffusion model with a nearly salt-independent pore diffusivity for SP-Sepharose-FF and by a homogeneous diffusion model with a saltindependent adsorbed phase diffusivity for S-HyperD. However, it appears that a more complex model, accounting for parallel pore and surface diffusion, is needed to describe protein mass transfer in SP-Sepharose-XL. The modeling results were found to be correlated with the apparent pore sizes determined by inverse SEC.