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Article

Polyoxometalate Binding to Human Serum Albumin: A Thermodynamic and Spectroscopic Approach

Guangjin Zhang,^† Bineta Keita,^† Constantin T. Craescu,^‡ Simona Miron,^‡ Pedro de Oliveira,^† and Louis Nadjo*^†

Laboratoire de Chimie Physique, Equipe d'Electrochimie et Photolectrochimie, UMR 8000, CNRS, Universit Paris-Sud 11, Btiment 350, 91405 Orsay Cedex, France, and INSERM U759/Institut Curie-Recherche, Centre Universitaire Paris-Sud 11, Btiment 112, 91405 Orsay Cedex, France

J. Phys. Chem. B, 2007, 111 (38), pp 11253–11259

DOI: 10.1021/jp072947u

Publication Date (Web): September 5, 2007

†

Université Paris-Sud 11.

‡

Centre Universitaire Paris-Sud 11.

To whom correspondence should be addressed. Phone: 33 1 69 15 77 51. Fax: 33 1 69 15 43 28. E-mail: nadjo@lcp.u-psud.fr.

Abstract

The molecular recognition of polyoxometalates by human serum albumin is studied using two different polyoxometalates (POMs) at pH 7.5. The results are compared with those obtained at pH 3.5 and 9.0. At pH 7.5, both POMs strongly interact with the protein with different binding behaviors. The Keggin shaped POM, [H₂W₁₂O₄₀]^6- (H2W12), specifically binds the protein, forming a complex with a 1:1 stoichiometry with K_a = 2.9 × 10⁶ M^-1. The binding constant decreased dramatically with the increase of the ionic strength, thus indicating a mostly electrostatic binding process. Isothermal titration calorimetry (ITC) experiments show that the binding is an enthalpically driven exothermic process. For the wheel shaped POM [NaP₅W₃₀O₁₁₀]^14- (P5W30), there are up to five binding sites on the protein. Increasing the ionic strength changes the binding behavior significantly, leading to a simple exothermic process, with several binding sites. Competitive binding experiments indicate that the two POMs share one common binding site. In addition, they show the existence of another important binding site for P5W30. The two POMs exhibit different binding dependences on the pH. The combination of the experimental results with the knowledge of the surface map of the protein in its N−B conformation transition domain leads to the proposal for the probable binding site of POMs. The present work reveals a protein conformation change upon P5W30 binding, a new feature not explicitly documented in previous studies.