Spectroscopic and computational insights into nickel biocatalysis

In order to obtain insight into nickel-catalyzed biochemistry, we have employed various spectroscopic techniques (electronic absorption, magnetic circular dichroism (MCD), variable-temperature variable-field MCD, electron paramagnetic resonance, and resonance Raman) in conjunction with computational methods (semi-empirical INDO/S-CI, density functional theory (DFT), and time-dependent DFT calculations) to investigate the nickel-containing active sites of carbon monoxide dehydrogenase (Ni-CODH), acetyl-coenzyme A synthase (ACS), and methyl-coenzyme M reductase (MCR) and its cofactor Ni-F₄₃₀. Highlights of these studies are outlined below.; On the basis of spectroscopic data, the FeS component of the Ni-CODH active site was predicted to be a 3Fe site coupled to another Fe atom, not a typical [Fe₄S₄) cluster as previously proposed. Following publication of these studies, two high-resolution crystal structures showed the active site to be a unique [NiFe₄S₄₋₅] cluster (a NiFe₃S₄ cubane bridged to an outlying Fe center via a S atom), concurrent with our spectroscopic analysis.; Spectroscopic and computational studies of the Ni(I)-CO model complex [PhTt′-Bu]NiCO indicated that such an intermediate is catalytically viable in the proposed “paramagnetic” mechanism of ACS. Coincidentally, recent computational studies involving the Ni-Ni-[Fe₄S₄] ACS active site, demonstrate that a Ni(I)-CO moiety at the distal nickel site is indeed a good description of a putative reaction intermediate.; Detailed bonding descriptions of the Ni(II) and Ni(I) states of isolated cofactor Ni-F₄₃₀ were developed using a combination of spectroscopy and computations. These results served to discount a recently proposed hydrocorphin-reduced model of Ni(I)F₄₃₀, also ruling out the possibility for hydrocorphin reduction in the catalytically-active form of the enzyme, MCR_red1. Additionally, the in vitro precursor to MCR_red1, termed MCR_ox1, was found to contain Ni(II) coupled to a sulfur-based radical, not a Ni(I) site as previously asserted. The implications of such Ni-S chemistry may be important to the MCR catalytic mechanism, perhaps indicating that a thiyl radical intermediate is involved.; Together, these studies have elucidated several aspects of nickel biocatalysis, particularly with respect to Ni-CO bonding and Ni-S redox chemistry, as well as provided the framework for future structure/function investigations.