Intrinsically fluorescent gold nanoclusters stabilized within a copper storage protein that follow the Irving-Williams trend in metal ion sensing.

Creating new environmentally friendly and non-toxic biomaterials with novel properties is required for numerous applications in healthcare and sensing. Protein bound gold nanoclusters constitute one such class of materials that offer promise in fluorescence imaging and sensing applications. However, unlike alkane thiol-protected gold nanoclusters, the number of protein-templated gold nanoclusters with such properties is limited and there is a need to expand the repertoire of such attractive hybrid quantum clusters. Herein, we report the synthesis, characterization, and applications of new fluorescent gold nanoclusters with tunable emission properties including blue, orange, and red, within a four-helix bundle copper storage protein (Csp1). The template protein consists of 13 cysteines along the length of the helix, which are suitable ligands to template Au and stabilize the resulting 14-19 atom clusters within the protein. The resulting clusters were extensively characterized by employing spectroscopic, microscopic and other analytical methods. The optical emission, relative quantum yields, and the excited state lifetime of the clusters are shown to depend on synthetic conditions. The clusters were found to be sensitive to the ppm level of transition metal ions with the quenching capabilities following the Irving-Williams series of metals (Co2+ < Ni2+ < Cu2+), which is rationalized based on the relative affinities of transition metals for a given set of ligands. The clusters were also found to be stable across the pH range 4-8.5 which, along with tunable emission properties paves the path for live bio-imaging and bio-sensing applications under physiological conditions.

The oxygen reactivity of an artificial hydrogenase designed in a reengineered copper storage protein.

The O2 reactivity of an artificial biomolecular hydrogenase, the nickel binding protein (NBP) is investigated. Kinetic analyses revealed a complete 4e- reduction of O2 to H2O under catalytic conditions with associated k0 for ET in the order of 10-6 cm s-1. Protein destabilization and S oxygenation are contributing factors to the deactivation of NBP under oxic conditions. Computational studies provided insight into the S oxygenation and the reaction intermediates of a proposed mechanistic pathway for O2 activation by NBP.

Redesign of a Copper Storage Protein into an Artificial Hydrogenase.

We report the construction of an artificial hydrogenase (ArH) by reengineering a Cu storage protein (Cspl) into a Ni-binding protein (NBP) employing rational metalloprotein design. The hypothesis driven design approach involved deleting existing Cu sites of Csp1 and identification of a target tetrathiolate Ni binding site within the protein scaffold followed by repacking the hydrophobic core. Guided by modeling, the NBP was expressed and purified in high purity. NBP is a well-folded and stable construct displaying native-like unfolding behavior. Spectroscopic and computational studies indicated that the NBP bound nickel in a distorted square planar geometry that validated the design. Ni(II)-NBP is active for photo-induced H2 evolution following a reductive quenching mechanism. Ni(II)-NBP catalyzed H+ reduction to H2 gas electrochemically as well. Analysis of the catalytic voltammograms established a proton-coupled electron transfer (PCET) mechanism. Electrolysis studies confirmed H2 evolution with quantitative Faradaic yields. Our studies demonstrate an important scope of rational metalloprotein design that allows imparting functions into protein scaffolds that have natively not evolved to possess the same function of the target metalloprotein constructs.