Oxomolybdenum Tetrathiolates with Sterically Encumbering Ligands: Modeling the Effect of a Protein Matrix on Electronic Structure and Reduction Potentials
Abstract
The effect of sterically encumbering ligands on the electronic structure of oxomolybdenum tetrathiolate complexes was determined using a combination of electronic absorption and magnetic circular dichroism spectroscopies, complimented by DFT bonding calculations, to understand geometric and electronic structure contributions to reduction potentials. These complexes are rudimentary models for a redox-active metalloenzyme active site in a protein matrix and allow for detailed spectroscopic probing of specific oxomolybdenum-thiolate interactions that are directly relevant to Mo-S(cysteine) bonding in pyranopterin molybdenum enzymes. Data are presented for three para-substituted oxomolybdenum tetrathiolate complexes ([PPh4][MoO(p-SPhCONHCH3)4], [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2CN)3)4], and [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2COOCH2CH3)3)4]). The Mo(V/IV) reduction potentials of the complexes in DMF are -1213, -1251, and -1247 mV, respectively. The remarkably similar electronic absorption and magnetic circular dichroism spectra of these complexes establish that the observed reduction potential differences are not a result of significant changes in the electronic structure of the [MoOS4]- cores as a function of the larger ligand size. We provide evidence that these reduction potential differences result from the driving force for a substantial reorganization of the O-Mo-S-C dihedral angle upon reduction, which decreases electron donation from the thiolate sulfurs to the reduced molybdenum center. The energy barrier to favorable O-Mo-S-C geometries results in a reorganizational energy increase, relative to [MoO(SPh)4](-/2-), that correlates with ligand size. The inherent flexible nature of oxomolybdenum-thiolate bonds indicate that thiolate ligand geometry, which controls Mo-S covalency, could affect the redox processes of monooxomolybdenum centers in pyranopterin molybdenum enzymes.
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The alteration in covalency along with a change in dielectric properties alters the reduction potential of the molybdenum center, providing the driving force of the reaction.
“This alteration in covalency along with a change in the dielectric properties alters the reduction potential of the molybdenum center, thus providing the driving force of the reaction.142,143”
From Nitrate and periplasmic nitrate reductases (2014) · cites this paper as [143]
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Nitrate and periplasmic nitrate reductases
Sparacino-Watkins, C., Stolz, J. F., & Basu, P. (2014) · Chem. Soc. Rev. · doi:10.1039/c3cs60249d · Cited by 426 · see citing works
Self-citation
- Authors
- McNaughton, R. L., Mondal, S., Nemykin, V. N., Basu, P., & Kirk, M. L.
- Affiliations
- Duquesne University (US)
- Corresponding author
- Kirk, Martin L.
- Venue
- Inorganic Chemistry
- Publisher
- American Chemical Society (ACS)
- Published
- 2005-10-12
- Volume
- 44
- Issue
- 23
- Pages
- 8216-8222
- ISSN
- 0020-1669 (ISSN-L)
- 1520-510X
- OpenAlex
- W2093496711
- Cited by
- 18
- References
- 40
- Citations by year
- 2024: 1 · 2017: 1 · 2016: 1 · 2015: 2 · 2014: 2 · 2013: 1 · 2012: 1
- FWCI
- 1.31
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Inorganic Chemistry (publishedVersion)
- PubMed (publishedVersion)
- Funders
- National Institute of General Medical Sciences (R15GM061555, GM057378, GM61555, R01GM057378)
- Topics
- Physical Sciences › Energy › Renewable Energy, Sustainability and the Environment › Metalloenzymes and iron-sulfur proteins
- Physical Sciences › Chemistry › Inorganic Chemistry › Metal-Catalyzed Oxygenation Mechanisms
- Physical Sciences › Chemistry › Organic Chemistry › Inorganic and Organometallic Chemistry
- Keywords
- redox potential, reorganization energy, electronic structure, magnetic circular dichroism, electronic absorption spectroscopy, ligand size effects
- MeSH terms
- Circular Dichroism, Iron-Sulfur Proteins, Models, Molecular, Molecular Conformation, Molybdenum, Organometallic Compounds, Oxidation-Reduction, Oxidoreductases, Quantum Theory, Spectrophotometry, Infrared, Spectrophotometry, Ultraviolet, Sulfhydryl Compounds, Xanthine Oxidase, Sulfite Oxidase
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