title: Volumetric interpretation of protein adsorption: ion-exchange adsorbent capacity, protein pI, and interaction energetics.
abstract: Adsorption of lysozyme (Lys), human serum albumin (HSA), and immunoglobulin G (IgG) to anion- and cation-exchange resins is dominated by electrostatic interactions between protein and adsorbent. The solution-depletion method of measuring adsorption shows, however, that these proteins do not irreversibly adsorb to ion-exchange surfaces, even when the charge disparity between adsorbent and protein inferred from protein pI is large. Net-positively-charged Lys (pI=11) and net-negatively-charged HSA (pI=5.5) adsorb so strongly to sulfopropyl sepharose (SP; a negatively-charged, strong cation-exchange resin, -0.22 mmol/mL exchange capacity) that both resist displacement by net-neutral IgG (pI=7.0) in simultaneous adsorption competition experiments. By contrast, IgG readily displaces both Lys and HSA adsorbed either to quaternary ammonium sepharose (Q; a positively-charged, strong anion exchanger, +0.22 mmol/mL exchange capacity) or to octadecyl sepharose (ODS; a neutral hydrophobic resin, 0 mmol/mL exchange capacity). Thus it is concluded that adsorption results do not sensibly correlate with protein pI and that pI is actually a rather poor predictor of affinity for ion-exchange surfaces. Adsorption of Lys, HSA, and IgG to ion-exchange resins from stagnant solution leads to adsorbed multi-layers, into or onto which IgG adsorbs in adsorption competition experiments. Comparison of adsorption to ion-exchange resins and neutral ODS leads to the conclusion that the apparent standard free-energy of adsorption Delta Gads( degrees ) of Lys, HSA, and IgG is not large in comparison to thermal energy due to energy-compensating interactions between water, protein, and ion-exchange surfaces that leaves a small net Delta Gads( degrees ). Thus water is found to control protein adsorption to a full range of substratum types spanning hydrophobic (poorly water wettable) surfaces, hydrophilic surfaces bearing relatively-weak Lewis acid/base functionalities that wet with (hydrogen bond to) water but do not exhibit ion-exchange properties, and surfaces with strong Lewis acid/base functional groups that exhibit ion-exchange properties in the conventional chemistry sense of ion-exchange.
authors.affiliation: Department of Materials Science and Engineering, Huck Institute of Life Sciences and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Hyeran
authors.initials: H
authors.lastname: Noh
authors.forename: Stefan T
authors.initials: ST
authors.lastname: Yohe
authors.forename: Erwin A
authors.initials: EA
authors.lastname: Vogler
mesh_terms: D000327:Adsorption; D006801:Humans; D007074:Immunoglobulin G / Q000737:chemistry* / Q000302:isolation & purification; D007477:Ions / Q000737:chemistry; D009113:Muramidase / Q000737:chemistry* / Q000302:isolation & purification; D012709:Serum Albumin / Q000737:chemistry* / Q000302:isolation & purification; D012996:Solutions; D013499:Surface Properties
chemical_list: D007074:Immunoglobulin G; D007477:Ions; D012709:Serum Albumin; D012996:Solutions; D009113:Muramidase