title: Substrate binding preferences and pka determinations of a nitrile hydratase model complex: variable solvent coordination to [(bmmp-TASN)Fe]OTf.
abstract: The five-coordinate iron-dithiolate complex (N,N'-4,7-bis-(2'-methyl-2'-mercatopropyl)-1-thia-4,7-diazacyclononane)iron(III), [LFe]+, has been isolated as the triflate salt from reaction of the previously reported LFeCl with thallium triflate. Spectroscopic characterization confirms an S = 1/2 ground state in non-coordinating solvents with room temperature microeff = 1.78 microB and electron paramagnetic resonance (EPR) derived g-values of g1 = 2.04, g2 = 2.02 and g3 = 2.01. [LFe]+ binds a variety of coordinating solvents resulting in six-coordinate complexes [LFe-solvent]+. In acetonitrile the low-spin [LFe-NCMe]+ (g1 = 2.27, g2 = 2.18, and g3 = 1.98) is in equilibrium with [LFe]+ with a binding constant of Keq = 4.7 at room temperature. Binding of H2O, DMF, methanol, DMSO, and pyridine to [LFe]+ yields high-spin six-coordinate complexes with EPR spectra that display significant strain in the rhombic zero-field splitting term E/D. Addition of 1 equiv of triflic acid to the previously reported diiron species (LFe)2O results in the formation of [(LFe)2OH]OTf, which has been characterized by X-ray crystallography. The aqueous chemistry of [LFe]+ reveals three distinct species as a function of pH: [LFe-OH2]+, [(LFe)2OH]OTf, and (LFe)2O. The pKa values for [LFe-OH2]+ and [(LFe)2OH]OTf are 5.4 +/- 0.1 and 6.52 +/- 0.05, respectively.
authors.affiliation: Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA.
authors.forename: Martin G
authors.initials: MG
authors.lastname: O'Toole
authors.forename: Brian
authors.initials: B
authors.lastname: Bennett
authors.forename: Mark S
authors.initials: MS
authors.lastname: Mashuta
authors.forename: Craig A
authors.initials: CA
authors.lastname: Grapperhaus
mesh_terms: D000577:Amides / Q000737:chemistry / Q000378:metabolism; D020134:Catalytic Domain; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry*; D006836:Hydro-Lyases / Q000737:chemistry* / Q000378:metabolism*; D006878:Hydroxides / Q000737:chemistry; D007097:Imines / Q000737:chemistry; D008958:Models, Molecular; D009570:Nitriles / Q000737:chemistry / Q000378:metabolism; D009584:Nitrogen / Q000737:chemistry; D012997:Solvents / Q000737:chemistry*; D013379:Substrate Specificity; D013696:Temperature; D014867:Water / Q000737:chemistry / Q000378:metabolism
chemical_list: C539732:(N,N'-4,7-bis-(2'-methyl-2'-mercatopropyl)-1-thia-4,7-diazacyclononane)iron(III); D000577:Amides; D005290:Ferric Compounds; D006878:Hydroxides; D007097:Imines; D009570:Nitriles; D012997:Solvents; D014867:Water; C031356:hydroxide ion; D006836:Hydro-Lyases; C048831:nitrile hydratase; D009584:Nitrogen