title: Adsorption of RNase A on cationic polyelectrolyte brushes: a study by isothermal titration calorimetry.
abstract: We present a study of the adsorption of a positively charged protein to a positively charged spherical polyelectrolyte brush (SPB) by isothermal titration calorimetry (ITC). ITC is used to determine the adsorption isotherm as a function of temperature and of salt concentration (at physiological pH 7.2). At low ionic strength, RNase A is strongly adsorbed by the SPB particles despite the fact that both the SPB particles and the protein are positively charged. Virtually no adsorption takes place when the ionic strength is raised through added salt. This is strong evidence for counterion release as the primary driving force for protein adsorption. We calculated that ~2 counterions were released upon RNase A binding. The adsorption of RNase A into like-charged SPB particles is entropy-driven, and protein protonation was not significant. Temperature-dependent measurements showed a disagreement between the enthalpy derived via the van't Hoff equation and the calorimetric enthalpy. Further analysis shows that van't Hoff analysis leads to the correct enthalpy of adsorption. The additional contributions to the measured enthalpy are potentially sourced from unlinked equilibria such as conformational changes that do not contribute to the binding equilibrium.
authors.affiliation: Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Alisa L
authors.initials: AL
authors.lastname: Becker
authors.forename: Nicole
authors.initials: N
authors.lastname: Welsch
authors.forename: Christian
authors.initials: C
authors.lastname: Schneider
authors.forename: Matthias
authors.initials: M
authors.lastname: Ballauff
mesh_terms: D000327:Adsorption; D000465:Algorithms; D002021:Buffers; D002151:Calorimetry; D002412:Cations; D004800:Enzymes, Immobilized / Q000737:chemistry*; D009994:Osmolar Concentration; D011108:Polymers / Q000737:chemistry*; D011485:Protein Binding; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*; D013816:Thermodynamics; D017186:Titrimetry
chemical_list: D002021:Buffers; D002412:Cations; D004800:Enzymes, Immobilized; D011108:Polymers; D000644:Quaternary Ammonium Compounds; D012259:Ribonuclease, Pancreatic