Protein/Nanoparticle Interactions: A Calorimetric and Spectroscopic Study

Date of Completion

January 2011

Keywords

Chemistry, Biochemistry|Biophysics, General

Degree

Ph.D.

Abstract

Understanding protein/nanoparticle (NP) interactions is essential for the rational design of NPs for biological applications such as drug delivery, implant coatings, and biocatalysis. Yet, there are no quantitative models to explain or predict these interactions. We have chosen inorganic zirconium phosphate nanospheres (nZrP) as a model system to investigate protein/NP interactions using titration calorimetry. nZrP was synthesized by contacting a fine spray of 20 mM ZrOCl2 solution with a solution of 20 mM Na2HPO 4 at pH 10. nZrP particles, thus obtained, were characterized by scanning electron microscopy (SEM) and dynamic light scattering (DLS), which indicated spheres with hydrodynamic radius (RH) of 30 to 90 nm, depending on the synthesis conditions. Composition of nZrP, as determined by inductively coupled plasma mass spectrometry (ICP MS), indicated Zr:P:Na elemental composition ratio of 1:37:111. We have used met-hemoglobin (Hb), myoglobin (Mb) and lysozyme (Lys) as model proteins and evaluated the binding enthalpies (ΔH) and entropies (ΔS) with nZrP. The binding constants (Kb) of Hb, Mb and lysozyme (in 10 mM Na2HPO4, pH 7.2 at 298 K) obtained from the calorimetric data were 3.3 ± 0.7, 25.7 ± 2.0 and 8.4 ± 1.0 µM−1, respectively. The corresponding ΔH and ΔS values were −27.1 ± 0.7, −6.2 ± 0 and −1.7 ± 0.08 kcal/mol, and −60.8 ± 2.9, 3.9 ± 0.1, 26.1 ± 0.2 cal/mol·K, respectively. Interestingly, binding of the positively charged lysozyme to the negatively charged nZrP was endothermic and entirely entropy driven. In contrast, binding of negatively charged Hb to negatively charged nZrP was exothermic and entirely driven by enthalpy. These changes are explained by a model of counter ion sequestration or release from the protein-solid interface on complex formation. The curvature of the nZrP-binding surface, as opposed to that of planar ZrP nanodiscs (α-ZrP), did not influence the binding thermodynamics or affinities to any significant extent. The secondary structure of nZrP-bound Hb, Mb and lysozyme were significantly retained. Additionally, Hb/α-ZrP was stabilized by RNA mediation. ^

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