title: pH effect on protein G orientation on gold surfaces and characterization of adsorption thermodynamics.
abstract: The pH effect on adsorbed antibody-binding protein (protein G) orientation on gold (Au) and its adsorption thermodynamic characteristics were investigated using quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS). The adsorbed protein G orientation was measured by binding response of two antibody-antigen systems: the model bovine serum albumin (BSA) and the foodborne pathogen  E. coli  O157:H7. Surface coverage was not significantly affected by pH, but its orientation was. The most properly oriented protein G for antibody binding was achieved at near-neutral pH. Adsorption was verified by XPS measurements using nitrogen (N) 1s, oxygen (O) 1s, and Au 4p peak heights. Adsorption energetics were determined by van't Hoff and Langmuir kinetic analyses of adsorption data obtained at 296, 303, and 308 K. Large characteristic entropy change of protein adsorption was observed (ΔS° = 0.52 ± 0.01 kcal/mol·K). The adsorption process was not classical physisorption but exhibited chemisorption characteristics based on significant enthalpy change (ΔH° = -25 ± 6 kcal/mol).
authors.affiliation: Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States.
authors.forename: Blake N
authors.initials: BN
authors.lastname: Johnson
authors.forename: Raj
authors.initials: R
authors.lastname: Mutharasan
mesh_terms: D000327:Adsorption; D000818:Animals; D000906:Antibodies / Q000276:immunology; D001426:Bacterial Proteins / Q000737:chemistry* / Q000276:immunology; D002417:Cattle; D019453:Escherichia coli O157 / Q000276:immunology; D006046:Gold / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D011487:Protein Conformation; D012710:Serum Albumin, Bovine / Q000276:immunology; D013499:Surface Properties; D013816:Thermodynamics
chemical_list: D000906:Antibodies; D001426:Bacterial Proteins; C034284:IgG Fc-binding protein, Streptococcus; D012710:Serum Albumin, Bovine; D006046:Gold