J. Phys. Chem. B 2009, 113, 15083–15089
15083
Molecular Signatures of Enzyme-Solid Interactions: Thermodynamics of Protein Binding to r-Zr(IV) Phosphate Nanoplates Michael R. Duff, Jr. and Challa V. Kumar* Department of Chemistry, UniVersity of Connecticut, Storrs, Connecticut 06269-3060 ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: September 4, 2009
Isothermal titration calorimetry (ITC) was used to determine the thermodynamics of protein binding to the nanoplates of R-Zr(HPO4)2 · H2O (R-ZrP). The binding constants (Kb) and ∆G, ∆H, and ∆S have been evaluated for a small set of proteins, and Kb values are in the range of 2-760 × 105 M-1. The binding of positively charged proteins to the negatively charged R-ZrP was endothermic, while the binding of negatively charged proteins was exothermic, and these are contrary to expectations based on a simple electrostatic model. The binding enthalpies of the proteins varied over a range of -24 to +25 kcal/mol, and these correlated roughly with the net charge on the protein (R2 ) 0.964) but not with other properties such as the number of basic residues, polar residues, isoelectric point, surface area, or molecular mass. Linear fits to the enthalpy plots indicated that each charge on the protein contributes 1.18 kcal/mol toward the binding enthalpy. Binding entropies of positively charged proteins were favorable (>0) while the binding entropies of negatively charged proteins were unfavorable (
