Cobinamide production of hydrogen in a homogeneous aqueous photochemical system, and assembly and photoreduction in a (βα)_8 protein

摘要

Components of a protein-integrated, earth-abundant metal macrocycle catalyst, with the purpose of H_2 production from aqueous protons under green conditions, are characterized. The cobalt-corrin complex, cobinamide, is demonstrated to produce H_2 (4.4 ± 1.8 × 10~(-3) turnover number per hour) in a homogeneous, photosensitizer/ sacrificial electron donor system in pure water at neutral pH. Turnover is proposed to be limited by the relatively low population of the gateway cobalt(III) hydride species. A heterolytic mechanism for H_2 production from the cobalt(II) hydride is proposed. Two essential requirements for assembly of a functional protein-catalyst complex are demonstrated for interaction of cobinamide with the (βα)8 TIM barrel protein, EutB, from the adenosylcobalamin-dependent ethanolamine ammonia lyase from Salmonella typhimurium: (1) high-affinity equilibrium binding of the cobinamide (dissociation constant 2.1 × 10~(-7) M) and (2) in situ photoreduction of the cobinamide-protein complex to the Co(I) state. Molecular modeling of the cobinamide-EutB interaction shows that these features arise from specific hydrogen-bond and apolar interactions of the protein with the alkylamide substituents and the ring of the corrin, and accessibility of the binding site to the solution. The results establish cobinamide-EutB as a platform for design and engineering of a robust H_2 production metallocatalyst that operates under green conditions and uses the advantages of the protein as a tunable medium and material support. Graphical abstract: [Figure not available: see fulltext.]