Elucidation of Metal‐Ion Accumulation Induced by Hydrogen Bonds on Protein Surfaces by Using Porous Lysozyme Crystals Containing Rh^IIIIons as the Model Surfaces

Abstract

Metal-ion accumulation on protein surfaces is a crucial step in the initiation of small-metal clusters and the formation of inorganic materials in nature. This event is expected to control the nucleation, growth, and position of the materials. There remain many unknowns, as to how proteins affect the initial process at the atomic level, although multistep assembly processes of the materials formation by both native and model systems have been clarified at the macroscopic level. Herein the cooperative effects of amino acids and hydrogen bonds promoting metal accumulation reactions are clarified by using porous hen egg white lysozyme (HEWL) crystals containing Rh(III) ions, as model protein surfaces for the reactions. The experimental results reveal noteworthy implications for initiation of metal accumulation, which involve highly cooperative dynamics of amino acids and hydrogen bonds: i) Disruption of hydrogen bonds can induce conformational changes of amino-acid residues to capture Rh(III) ions. ii) Water molecules pre-organized by hydrogen bonds can stabilize Rh(III) coordination as aqua ligands. iii) Water molecules participating in hydrogen bonds with amino-acid residues can be replaced by Rh(III) ions to form polynuclear structures with the residues. iv) Rh(III) aqua complexes are retained on amino-acid residues through stabilizing hydrogen bonds even at low pH (approximately 2). These metal-protein interactions including hydrogen bonds may promote native metal accumulation reactions and also may be useful in the preparation of new inorganic materials that incorporate proteins.

Porous Protein Crystals as Reaction Vessels for Controlling Magnetic Properties of Nanoparticles

“…Thus, porous protein crystals have been utilized as nanosized porous biomaterials for applications in separation processes, heterogeneous catalysis, and drug delivery systems 14. These crystals are able to capture and deposit metal ions and metal complexes within solvent channels when the crystals are soaked in the solutions containing the metal ions because the amino acid side chains responsible for coordinating the metal ions are periodically aligned on the surface of solvent channels of the crystal lattices 17–22. We have demonstrated the accumulation of functional molecules such as metal complexes and metal clusters into the solvent channels of porous myoglobin crystals by chemical conjugation and coordination interactions 23, 24.…”

“…HEWL crystals are exploited as templates for the synthesis because i) the crystals are known to have the capacity to accumulate various metal ions and metal complexes, ii) these crystals can form various polymorphic structures indicating tetragonal, orthorhombic, and monoclinic structures by changing crystallization conditions, and iii) HEWL is easily and abundantly crystallized. Quantities of more than several hundred milligrams can be routinely obtained 17–21, 31–35. We have synthesized CoPt‐NPs within the solvent channels of orthorhombic‐, tetragonal‐, and monoclinic‐HEWL crystals (O‐, T‐, and M‐HEWL, respectively) by chemical reduction of Co 2+ and Pt 2+ ions pre‐organized on the interior surface of the solvent channels ( Figure ).…”

“…There are small clefts with a width of 1.6 nm inside M‐HEWL (Figure 1d). The surfaces of the solvent channels are expected to capture metal ions because they mainly consist of coordinating amino acid residues, such as histidine, lysine, aspartic, and glutamic acid 17–21. These crystals were reinforced by cross‐linking with 1% glutaraldehyde to maintain the crystal lattice structures under various conditions 36.…”

Porous Protein Crystals as Reaction Vessels

“…c) Snapshot analyses of Rh ion binding to Site B. Each structure obtained at a different concentration of RhCl 3 in sodium acetate buffer at pH 4.5 23…”

“…[11,13,14] Crystals of HEWL have several important advantages; 1) they can be routinely obtained in quantities exceeding several hundred milligrams overnight, 2) various metal ions and metal complexes can be accumulated in solvent channels of the crystals, and 3) HEWL can be crystallized in tetragonal, orthorhombic, and monoclinic lattices by varying the crystallization conditions ( Figure 3). [15][16][17][18][19][20][21][22][23][24] The solvent channels of the protein crystals are composed of amino acid residues periodically positioned on their surfa-Abstract: Porous protein crystals have the potential to provide new porous materials due to their unique chemical environments composed of amino acid residues periodically exposed at the surface of the solvent channels in the crystal lattice. This enables accumulation of external compounds in special arrangements by metal coordination interactions or by chemical modifications.…”