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Electronic and Geometric Structures of Paramagnetic Diazadiene Complexes of Lithium and Sodium.

ABSTRACT: The electronic and molecular structures of the lithium and sodium complexes of 1,4-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diazabutadiene (^Me2DAD^Dipp) were fully characterized by using a multi-frequency electron paramagnetic resonance (EPR) spectroscopy approach and crystallography, together with density functional theory (DFT) calculations. EPR measurements, using T₁ relaxation-time-filtered pulse EPR spectroscopy, revealed the diagonal elements of the A and g tensors for the metal and ligand sites. It was found that the central metals in the lithium complexes had sizable contributions to the SOMO, whereas this contribution was less strongly observed for the sodium complex. Such strong contributions were attributed to structural specifications (e.g. geometrical data and atomic size) rather than electronic effects.

SUBMITTER: Haeri HH

PROVIDER: S-EPMC6123648 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature

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Electronic and Geometric Structures of Paramagnetic Diazadiene Complexes of Lithium and Sodium.

Haeri Haleh H HH Duraisamy Ramesh R Harmgarth Nicole N Liebing Phil P Lorenz Volker V Hinderberger Dariush D Edelmann Frank T FT

ChemistryOpen 20180905 9

The electronic and molecular structures of the lithium and sodium complexes of 1,4-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diazabutadiene (Me2DADDipp) were fully characterized by using a multi-frequency electron paramagnetic resonance (EPR) spectroscopy approach and crystallography, together with density functional theory (DFT) calculations. EPR measurements, using T1 relaxation-time-filtered pulse EPR spectroscopy, revealed the diagonal elements of th ...[more]

PMID: 30202705

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Project description:The active-site structures of the oxidized and reduced forms of manganese-substituted iron superoxide dismutase (Mn(Fe)SOD) are examined, for the first time, using a combination of spectroscopic and computational methods. On the basis of electronic absorption, circular dichroism (CD), magnetic CD (MCD), and variable-temperature variable-field MCD data obtained for oxidized Mn(Fe)SOD, we propose that the active site of this species is virtually identical to that of wild-type manganese SOD (MnSOD), with both containing a metal ion that resides in a trigonal bipyramidal ligand environment. This proposal is corroborated by quantum mechanical/molecular mechanical (QM/MM) computations performed on complete protein models of Mn(Fe)SOD in both its oxidized and reduced states and, for comparison, wild-type (WT) MnSOD. The major differences between the QM/MM optimized active sites of WT MnSOD and Mn(Fe)SOD are a smaller (His)N-Mn-N(His) equatorial angle and a longer (Gln146(69))NH···O(sol) H-bond distance in the metal-substituted protein. Importantly, these modest geometric differences are consistent with our spectroscopic data obtained for the oxidized proteins and high-field electron paramagnetic resonance spectra reported previously for reduced Mn(Fe)SOD and MnSOD. As Mn(Fe)SOD exhibits a reduction midpoint potential (E(m)) almost 700 mV higher than that of MnSOD, which has been shown to be sufficient for explaining the lack of SOD activity displayed by the metal-subtituted species (Vance, C. K.; Miller, A. F. Biochemistry 2001, 40, 13079-13087), E(m)'s were computed for our experimentally validated QM/MM optimized models of Mn(Fe)SOD and MnSOD. These computations properly reproduce the experimental trend and reveal that the drastically elevated E(m) of the metal substituted protein stems from a larger separation between the second-sphere Gln residue and the coordinated solvent in Mn(Fe)SOD relative to MnSOD, which causes a weakening of the corresponding H-bond interaction in the oxidized state and alleviates steric crowding in the reduced state.

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Electronic and Geometric Structures of Paramagnetic Diazadiene Complexes of Lithium and Sodium.

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Electronic and Geometric Structures of Paramagnetic Diazadiene Complexes of Lithium and Sodium.

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