Thiolate ligation

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

Thiolate ligated zinc complexes have been characterized with a co-crystallized methanol molecule that is hydrogen bonded to a zinc-bound sulfur, and the possibility the alcohols may be activated without coordination to the metal center was discussed by Shoner et al.904... 

Dawson, J. H. and Sono, M. (1987) Cytochrome P450 chloroperoxidase thiolate-ligand heme enzymes. Spectroscopic determination of their active site structure and mechanistic implication of thiolate ligation. Chem. Rev. 87, 1255-1276. 

Figure 3. Roles of thiolate ligation and proton network on the heterolytic 0—0 bond cleavage of hydroperoxide bound to heme.

Dawson JF1, Sono M (1987) Cytochrome P-450 and chloroperoxidase - thiolate-ligated heme enzymes - spectroscopic determination of their active-site structures and mechanistic implications of thiolate ligation. Chem Rev 87 1255-1276... 

Figure 17 A schematic view of the involvement of axial hgands and distal residues in the push-puU mechanism for 0-0 bond cleavage of an iron-boimd peroxide in thiolate-ligated (left) and histidine-hgated (right) systems such as cyUochrome P450 and horseradish peroxidase, respectively ...

The [Fe2S2] + core, (2) (Figure 2), appears in plant-type (chloroplast) ferredoxins (Fd) and in Rieske proteins. In ferredoxins, the [Fe2S2] + core is ligated with thiolate from Cys residues, while in Rieske proteins, the thiolates ligating one of the iron atoms are substituted with imidazole ligands from His residues. 

Unlike hnman Mb, the His-to-Cys mntation alone in horse heart Mb did not result in heme-thiolate ligation. However, an additional mntation of the distal histidine to either a valine or isolencine resulted in a P450-like protein in its... 

Perera R, Sono M, Sigman JA, Pfister TD, Lu Y, Dawson JH. Neutral thiol as a proximal ligand to ferrous heme iron Implications for heme proteins that lose cysteine thiolate ligation on reduction. Proc. Natl. Acad. Sci. U.S.A. 2003 100 3641-3646. 

Kinetic studies of the reaction of a mononuclear N2S(thiolate)-ligated zinc hydroxide complex (PATH)Zn-OH with tris(4-nitrophenyl) phosphate in 33% ethanol-water and 7=0.10 (NaN03) also point to a hybrid-type mechanism (Fig. 43).228 Overall, this reaction is second order and a pH-rate profile indicates that the zinc hydroxide species (PATH)Zn-OH is involved in the reaction. The maximum rate constant for this reaction (16.1(7) M-1 s-1) is higher than that reported for free hydroxide ion (10.7 +0.2 M 1 s-1).225 This implies that a simple mechanism involving nucleophilic attack is not operative, as free OH- is a better nucleophile. Studies of the temperature dependence of the second-order rate constants for this reaction yielded activation parameters of A77 = 36.9(1) kJ mol-1 and AS = —106.7(4) JmolK-1. The negative entropy is consistent with considerable order in the transition state and a hybrid-type mechanism (Fig. 43, bottom). 

Lancaster KM, Zaballa M-E, Sproules S, Sundararajan M, DeBeer S, Neese F, Vila AJ, Richards JH, Gray HB (In Preparation) The type zero copper site Type 1 copper without thiolate ligation... 

Dawson JH, Holm RH, Trudell JR, Barth G, Linder RE, Bunnenberg E, Djerassi C, Tang SC (1976) Oxidized cytochrome-P450 - magnetic circular dichroism evidence for thiolate ligation in substrate-hound form - implications for catalytic mechanism. J Am Chem Soc 98 3707-3709... 

R. E. Linder, E. Bunnenberg etal. (1976). Oxidized cytochrome P-450. Magnetic circular dichroism evidence for thiolate ligation in the substrate-bound form. Implications for the catalytic mechanism. J. Am. Chem. Soc. 98, 3707-3709. 

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