Ligand-Bound S = 1/2 FeMo-Cofactor of Nitrogenase: Hyperfine Interaction Analysis and Implication for the Central Ligand X Identity

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• 作者：Vladimir Pelmenschikov ; David A. Case ; Louis Noodleman
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：July 21, 2008
• 年：2008
• 卷：47
• 期：14
• 页码：6162 - 6172
• 全文大小：3640K
• 年卷期：v.47,no.14(July 21, 2008)
• ISSN：1520-510X

文摘

Broken symmetry density functional theory (BS-DFT) has been used to address the hyperfine parameters of the single atom ligand X, proposed to be coordinated by six iron ions in the center of the paramagnetic FeMo-cofactor (FeMoco) of nitrogenase. Using the X = N alternative, we recently found that any hyperfine signal from X would be small (calculated A_iso(X = ¹⁴N) = 0.3 MHz) due to both structural and electronic symmetry properties of the [Mo−7Fe−9S−X] FeMoco core in its resting S = ³/₂ state. Here, we extend our BS-DFT approach to the 2e⁻ reduced S = ¹/₂ FeMoco state. Alternative substrates coordinated to this FeMoco state effectively perturb the electronic and/or structural symmetry properties of the cofactor. Using an example of an allyl alcohol (H₂C<img src="http://pubs.acs.org/images/entities/dbd_2.gif">CH−CH₂−OH) product ligand, we consider three different binding modes at single iron site and three different BS-DFT spin state structures and show that this binding would enhance the key hyperfine signal A_iso(X) by at least 1 order of magnitude (3.8 ≤ A_iso(X = ¹⁴N) ≤ 14.7 MHz), and this result should not depend strongly on the exact identity of X (nitrogen, carbon, or oxygen). The interstitial atom, when the nucleus has a nonzero magnetic moment, should therefore be observable by ESR methods for some ligand-bound FeMoco states. In addition, our results illustrate structural details and likely spin-coupling patterns for models for early intermediates in the catalytic cycle.