How Do Oxidized Thiolate Ligands Affect the Electronic and Reactivity Properties of a Nitrile Hydratase Model Compound?

Abstract

Nitrile hydratases (NHase) are non-heme Fe(III)-containing, or noncorrinoid Co(III)-containing, microbial enzymes that catalyze nitrile hydration. 1 The iron form has been studied most extensively. The Fe(III) active site is low spin (S ) 1 / 2 ), and ligated by three cysteinates, two peptide amide nitrogens and either a hydroxide or an NO. [2][3][4] Given the high amount of sequence homology in the active site region, it is likely that the Co-NHase active site is virtually identical to Fe-NHase. 1,5 In one of two recent Fe NHase crystal structures 2,4 two of the metal-bound sulfurs appear to be oxidized, one to a sulfenate ( 114 S cys dO) and the other to a sulfinate ( 112 S cys (dO) 2 ). 4 The sulfenate is not observed by mass spectrometry. 6 Sulfenic acids are usually unstable, 7 and metal-sulfenates are readily oxidized to metalsulfinates. 8,9 A few synthetic NHase models containing oxidized sulfurs have been reported; 10-12 however, none of these incorporate a sulfenate, and only one 12 has an open coordination site.Our group has shown that the spin-state and spectroscopic properties of Fe-NHase can be nicely reproduced by six-coordinate Fe(III) model complexes containing two cis-thiolates and imines. 3,13-15 These models lack oxidized sulfurs, yet their spectroscopic properties are remarkably similar to the enzyme, suggesting that two, of the three, cysteinate NHase sulfurs remain unmodified. To understand how the sulfinate and, possibly, the sulfenate sulfur influences the electronic and reactivity properties of NHase, we have synthesized a series of sulfur-ligated, fivecoordinate Co(III) model complexes containing progressively more oxidized sulfurs.Five-coordinate [Co(III)(S 2 Me2 N 3 (Pr,Pr))] + (1) was synthesized in the same manner as its iron analogue. 15 Complex 1 is intermediate spin (S ) 1) over the temperature range 50-300 K (supplemental Figure S-1), and is reversibly reduced at E 1/2 ) -460 mV vs SCE. The average Co-S distance (2.16(2) Å) in 1 (Figure 1) 16 is shorter than most Co(III) thiolates (average ) 2.24 Å). [17][18][19] Azide and SCN -bind quantitatively to 1 at room temperature trans to one of the thiolate sulfurs. 20 Trigonal bipyramidal 1 (τ ) 0.87) 21 is converted to a more square pyramidal (τ ) 0.48) sulfinate/thiolate-ligated complex, [Co(III)(S Me2 (S O2 )N 3 (Pr,Pr))] + (2; Figure 2), 22 upon stirring in air for 3 days. Only one of the two thiolate sulfurs (S(2)) is oxidized, even upon prolonged stirring. Oxidation of S(2) causes the spinstate to change, from S ) 1 (in 1) to 0 (in 2), and the reduction potential to shift cathodically to E 1/2 ) -380 mV vs SCE. The mean S(2)-O(1,2) distance (1.453(2) Å) in 2 falls in the usual range (1.42-1.48 Å). 17,23,24 The Co-S(2) distance in 2 is indistinguishable from Co-S(1) (Figure 2). Both of the Co-S bonds in 2 are slightly shorter than the Co-S bonds in 1, because (1) Kobayashi, M.; Shimizu, S. Nature Biotechnol. 1998, 16, 733-36. (2) Huang, W.; Jia, J.; Cummings, J.; Nelson, M.; Schneider, G.; Lindqvist, Y. Structure 1997, 5, 691-6...

Sulfoxygenation of Active Site Models of [NiFe] and [FeFe] Hydrogenases – A Commentary on Possible Chemical Models of Hydrogenase Enzyme Oxygen Sensitivity

“…[36,37,38] In these cases a tripeptide motif, Cys-Ser-Cys, assumes an N 2 S 2 tetradentate binding array within the protein, and post-translational S-modification renders the cis-dithiolates into a sulfoxide and sulfenate, presumably enhancing the electrophilicity of the Fe or Co metal centers and their ability to bind the substrates, nitrile or thiocyanate. [39] In addition, the sulfoxy units are proposed to orient, via H-bonding, the reactive water molecules so as to efficiently perform the required hydrolyses ( Figure 6). Model studies have found stable S-oxygenates of both Fe and Co analogues as well as those of ruthenium.…”

How Does Single Oxygen Atom Addition Affect the Properties of an Fe−Nitrile Hydratase Analogue? The Compensatory Role of the Unmodified Thiolate

“…[38][39][40] Only one model has been reported containing both a singly oxygenated and a doubly oxygenated thiolate sulfur in the same molecule. 40 The selective incorporation of an odd number of oxygens is nontrivial, since sulfenates (RS-O(H)) are more reactive than thiolates and readily disproportionate to Herein, the synthesis and characterization of a series of structurally related Fe-NHase model complexes is described, in which the oxidation and protonation states of one of the thiolates are systematically altered. Comparison of the electronic, magnetic, redox, bonding, and reactivity properties of this series reveals how oxo atom addition and protonation affects the metal ion Lewis acidity.…”

“…37 Very few models containing singly oxygenated sulfurs are available, and none of these contain iron. [38][39][40] Only one model has been reported containing both a singly oxygenated and a doubly oxygenated thiolate sulfur in the same molecule. 40 The selective incorporation of an odd number of oxygens is nontrivial, since sulfenates (RS-O(H)) are more reactive than thiolates and readily disproportionate to sulfinates (RSO 2 − ) and thiolates.…”

“…[38][39][40] Only one model has been reported containing both a singly oxygenated and a doubly oxygenated thiolate sulfur in the same molecule. 40 The selective incorporation of an odd number of oxygens is nontrivial, since sulfenates (RS-O(H)) are more reactive than thiolates and readily disproportionate to sulfinates (RSO 2 − ) and thiolates. Very little is known about how the electronic structure and, more importantly, Fe-S bonding within NHase (or model compounds thereof) respond to oxidation and/or protonation of the coordinated sulfur ligands, and how this might affect reactivity.…”

Incorporation of thiolate donation using 2,2′-dithiodibenzaldehyde: Complexes of a pentadentate N2S3 ligand with relevance to the active site of Co nitrile hydratase

“…They have also been able to isolate monomeric and dimeric compounds in which the thiolates have been oxidized, although a crystal structure of the monomeric version has not been published. Kovacs' compounds utilize a ligand with an N 3 S 2 donor set consisting of one amino and two imino N atoms and two aliphatic thiolate S atoms [43][44][45]. Structures of both 5-and 6-coordinate (with azide or thiocyanate coligands) have been reported.…”