title: Competitive ion-exchange adsorption of proteins: competitive isotherms with controlled competitor concentration.
abstract: The competitive adsorption processes inevitably present in chromatographic separations of complex mixtures have not been extensively studied. This is partly due to the difficulty of measuring true competitive isotherms, in which all system parameters (including competitor concentrations) are held constant. We report a novel approach to determining competitive protein adsorption isotherms in which the competitor concentration is held constant across the entire isotherm. By using the heme prosthetic group in cytochrome b5 as a quantitative spectrophotometric label, competitive isotherms between cytochrome b5 and alpha-lactalbumin can be constructed. Similarly, manganese-substituted protoporphyrin IX heme replacement allows the non-perturbing labeling of individual cytochrome b5 conservative surface charge mutants by replacement of a single atom in the interior of the protein. This labeling allows the study of competition between cytochrome b5 charge mutants of identical size and shape, which differ only in charge arrangement. Using these techniques, the effect of competing species on equilibrium behavior and the apparent heterogeneity of anion-exchange adsorbents in the presence of competitors can be quantitatively studied by fitting the data to two popular single-component binding models, the Temkin and the Langmuir-Freundlich (L-F) isotherms.
authors.affiliation: Department of Chemical Engineering, University of Houston, 4800 Calhoun Avenue, Houston, TX 77204-4004, USA. tonycano@gmail.com
authors.forename: Tony
authors.initials: T
authors.lastname: Cano
authors.forename: Natalie D
authors.initials: ND
authors.lastname: Offringa
authors.forename: Richard C
authors.initials: RC
authors.lastname: Willson
mesh_terms: D000327:Adsorption; D000837:Anion Exchange Resins / Q000737:chemistry*; D001667:Binding, Competitive; D015786:Cytochromes b5 / Q000032:analysis / Q000737:chemistry* / Q000235:genetics; D018698:Glutamic Acid / Q000235:genetics; D005973:Glutamine / Q000235:genetics; D007474:Ion Exchange; D007700:Kinetics; D007768:Lactalbumin / Q000032:analysis / Q000737:chemistry; D008956:Models, Chemical; D009154:Mutation; D013816:Thermodynamics
chemical_list: D000837:Anion Exchange Resins; D005973:Glutamine; D018698:Glutamic Acid; D007768:Lactalbumin; D015786:Cytochromes b5