The hydrogenase metalloenzyme and the artificial bio-inspired catalysts for hydrogen production
Discovering a new type of green energy resource of low cost is of contemporary importance to ease rapid growth of global demand for fossil fuels that produces excessive amount of CO2 and causes tremendous climate changes. Molecular hydrogen is considered as the next generation clean fuel, since its stored chemical energy releases with formation of water. In biological systems, hydrogenases are used to catalyze H2. Understanding the catalytic mechanism thus facilitates chemists to design artificial biomimitic catalysts to hydrogen production.
The research in Dr. Chiang’s group is focused on the rational synthesis of novel transition metal catalysts for efficient hydrogen production. Design of the catalyst is inspired by the active site of [FeFe] hydrogenase in virtue of its superior ability for hydrogen production. The active site consists of a {Fe2S2} core attached by a [4Fe4S] cluster. Two major issues essential to catalysis of the active site of [FeFe] hydrogenase are addressed: the substrate accessible site and the attached [4Fe4S] unit.
Molecular manipulation techniques are deployed to construct structural models with variations to simulate structural confinement by the protein pocket. The products under reducing and acidic conditions are isolated for the purpose of mechanistic elucidation. Aside from the influence of the peptide chains in the vicinity of the active site, the rotated geometry of the distal Fe center within the active site is also induced by changes of electronic structures of the Fe2 core. The [4Fe4S] unit within the H cluster plays a key role on the electronic changes. Several Fe-S model complexes with redox-active units are characterized to gain insights of the electronic influence by the presence of the redox-active fragments.
Mimicking the active site of [FeFe] hydrogenase
The influeneces of the primary (a) and secondary (b) ligand fields
Electronic influence of the redox partners
The dioxygenase metalloenzyme and the artificial bio-inspired catalysts for oxidation of aromatic carbons
The catabolism of catechol species including halogenated derivatives in bacterial degradative pathways is achieved by ring-cleaving dioxygenases. The specific metalloenzymes utilize molecular oxygen to generate the aliphatic products, which allows recycling of the carbons sequestered in aromatic organics. Two categories for oxygen insertion to the aromatic carbons are classified on the basis of the position of the C-C bond: the intradiol and extradiol cleavage. The former type breaks the 1,2-carbon-carbon bond, resulting in
cis,cis-muconic acids. The latter cleavages the 2,3-carbon-carbon bond to produce muconic semialdehydes. The Fe ion is contained in the active site of both types of dioxygenases. For the extradiol dioxygenase, Mn2+ (Mn-MndD,
Arthrobacter globiformis) is also identified. Here, Dr. Chiang’s group is interested in activation of molecular oxygen via dioxygenases to generate oxidation products of the aromatic compounds. The research focus is on the mechanistic study to obtain advanced knowledge related to how bacteria utilize aromatic carbons in term of energy and decontamination of halogenated molecules.
(a) Ring cleavage by catechol dioxygenase (b) Proposed catalytic cycle of homoprotocatechuate 2,3-dioxygenase
Synthesis and characterization of the peroxomanganese(IV) complex
Nano-sized porous catalysts
In search of potential applications of artificial catalysts for pilot runs, Dr. Chiang is developing nano-sized porous catalysts. Nanomaterials are of several advantages: They can serve as a support to catalytically active transition metals. Catalytic selectivity and efficiency are tunable by pore orifices and effective surface areas. Dr. Chiang has teamed up with several research groups to synthesize metal-organic coordination polymers from a variety of transition metal ions and organic ligands based on a combinatorial strategy. Of particular interest is the fact that when catalytic metal sites are immobilized on the backbones of three-dimensional structures, magnetic behaviors are observed due to interactions among the paramagnetic metal centers. The magnetic property in turn may influence the catalysis. Such networks furnished with paramagnetic ions are attractive in terms of potential magneto applications.
Nano-porous structures