Craft Jennifer L, Horng Yih-Chern, Ragsdale Stephen W, Brunold Thomas C
Department of Chemistry, University of Wisconsin, Madison, Wisconsin, USA.
J Am Chem Soc. 2004 Apr 7;126(13):4068-9. doi: 10.1021/ja038082p.
Magnetic circular dichroism (MCD) spectroscopy and variable-temperature variable-field MCD are used in combination with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to characterize the so-called ox1-silent, red1, and ox1 forms of the Ni-containing cofactor F430 in methyl-coenzyme M reductase (MCR). Previous studies concluded that the ox1 state, which is the precursor of the key reactive red1 state of MCR, is a Ni(I) species that derives from one-electron reduction of the Ni(II)-containing ox1-silent state. However, our absorption and MCD data provide compelling evidence that ox1 is actually a Ni(II) species. In support of this proposal, our DFT and TD-DFT calculations indicate that addition of an electron to the ox1-silent state leads to formation of a hydrocorphin anion radical rather than a Ni(I) center. These results and biochemical evidence suggest that ox1 is more oxidized than red1, which prompted us to test a new model for ox1 in which the ox1-silent species is oxidized by one electron to form a thiyl radical derived from coenzyme M that couples antiferromagnetically to the Ni(II) ion. This alternative ox1 model, formally corresponding to a Ni(III)/thiolate resonance form but with predicted S = 1/2 EPR parameters reminiscent of a Ni(I) (3dx2-y2)1 species, rationalizes the requirement for reduction of ox1 to yield the red1 species and the seemingly incongruent EPR and electronic spectra of the ox1 state.
磁圆二色性（MCD）光谱和变温变场MCD与密度泛函理论（DFT）和含时密度泛函理论（TD-DFT）计算相结合，用于表征甲基辅酶M还原酶（MCR）中含镍辅因子F430的所谓ox1沉默态、red1态和ox1态。先前的研究得出结论，ox1态是MCR关键反应性red1态的前体，是一种Ni(I)物种，源自含Ni(II)的ox1沉默态的单电子还原。然而，我们的吸收和MCD数据提供了令人信服的证据，表明ox1实际上是一种Ni(II)物种。为支持这一观点，我们的DFT和TD-DFT计算表明，向ox1沉默态添加一个电子会导致形成氢化卟啉阴离子自由基，而不是Ni(I)中心。这些结果和生化证据表明，ox1比red1氧化程度更高，这促使我们测试一种新的ox1模型，其中ox1沉默物种被单电子氧化，形成一种源自辅酶M的硫自由基，该自由基与Ni(II)离子反铁磁耦合。这种替代的ox1模型，形式上对应于Ni(III)/硫醇盐共振形式，但预测的S = 1/2 EPR参数让人联想到Ni(I) (3dx2-y2)1物种，解释了将ox1还原以产生red1物种的要求以及ox1态看似不一致的EPR和电子光谱。
