Magnetic Circular Dichroism Study of a Dicobalt(II) Complex with Mixed 5- and 6-Coordination: A Spectroscopic Model for Dicobalt(II) Hydrolases

详细信息    查看全文

• 作者：James A. Larrabee ; W. Rainey Johnson ; Adam S. Volwiler
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：September 21, 2009
• 年：2009
• 卷：48
• 期：18
• 页码：8822-8829
• 全文大小：869K
• 年卷期：v.48,no.18(September 21, 2009)
• ISSN：1520-510X

文摘

The magnetic circular dichroism (MCD) study of [Co₂(μ-OH)(μ-Ph₄DBA)(TMEDA)₂(OTf)], in which Ph₄DBA is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis(diphenylacetate) and TMEDA is N,N,N′,N′-tetramethylethylenediamine, is presented. This complex serves as an excellent spectroscopic model for a number of dicobalt(II) enzymes and proteins that have both the μ-hydroxo, μ-carboxylato bridging and asymmetric 6- and 5-coordination. The low-temperature MCD spectrum of the model complex shows bands at 490, 504, and 934 nm arising from d-d transitions on the 6-coordinate Co^II and bands at 471, 522, 572, 594, and 638 nm arising from d-d transitions on the 5-coordinate Co^II. The most intense MCD bands are at 504 and 572 nm for 6- and 5-coordinate Co^II, respectively, and these two bands are found in the MCD spectra of dicobalt(II)-substituted methionine aminopeptidase from Escherichia coli (CoCoMetAP), glycerophosphodiesterase from Enterobacter aerogenes (CoCoGpdQ), aminopeptidase from Aeromonas proteolytica (CoCoAAP), and myohemerythrin from Themiste zostericola (CoCoMyoHry). These dicobalt(II)-substituted proteins are known to have one 5- and one 6-coordinate Co^II bridged by one or two carboxylates and either a water or a hydroxide. The uncertainty of the bridging water’s state of protonation is problematic, as this is a likely candidate for the attacking nucleophile in the dimetallohydrolases. Analysis of the variable-temperature variable-field (VTVH) MCD data determined that the Co^II ions in the model complex are ferromagnetically coupled with a J of 3.0 cm⁻¹. A comparison of all dicobalt(II) complexes and dicobalt(II)-substituted protein active sites with the μ-hydroxo/aqua, μ-carboxylato bridging motif reveals that J is either zero or negative (antiferromagnetic) in the μ-aqua systems and positive (ferromagnetic) in the μ-hydroxo systems. It was also determined that the Co^II ions in CoCoAAP and CoCoMyoHry are ferromagnetically coupled, each with a J of 3.4 cm⁻¹, which suggests that these ions have a μ-hydroxo bridging ligand.