Adsorption heterogeneity and thermodynamic driving forces in anion exchange equilibria of cytochrome b₅

Batch adsorption studies of recombinant soluble core of rat cytochrome b₅ on the polymeric strong anion exchanger, Mono Q, show non-Langmuirian adsorption isotherms. Adsorption data are well fit by the Hill equation. Concave-upward Scatchard plots, and values of the Hill coefficient below unity, imply heterogeneous adsorption due to negative lateral interaction between adsorbed molecules and/or nonuniform binding affinities of adsorbent sites. Adsorption isotherms from the batch studies can be interpreted in terms of a stoichiometric displacement model for obtaining the apparent number of binding interactions (Z) between the protein and the adsorbent surface. Z values obtained are low and fractional and represent an average over diverse binding forms arising from surface heterogeneity, van't Hoff plots of the adsorption data vary in slope over the experimental temperature range. The positive enthalpies at low temperatures indicate a dominant entropic contribution to the driving force, possibly arising from the liberation of bound ions and waters of solvation upon adsorption.