Synthetic modeling of zinc thiolates: Quantitative assessment of hydrogen bonding in modulating sulfur alkylation rates

Abstract

A series of mononuclear zinc thiolate complexes have been prepared and fully characterized. The reactions of the complexes with alkyl halides, leading to zinc halides and the corresponding thioethers, have been examined by kinetic methods. In toluene, the reactions obey a second-order rate law displaying activation parameters consistent with a S N2 attack of the zinc-bound thiolate on the carbon electrophile. Intramolecular hydrogen bonding of an amide NOH to the thiolate sulfur reduces the nucleophilicity and consequently, the rate of alkylation more than 30-fold at 25°C. The H-bonding shows an inverse H͞D isotope effect of 0.33 (60°C) ascribed to differential H-bonding for the two isotopomers due to zero point energy differences. These model studies provide quantitative evaluation of H-bonding on reaction rates relevant to zinc thiol-activating proteins.T he biological functions of zinc are varied, with seminal roles including the structure and function of a wide range of metalloproteins (1). Structural zinc sites are typically characterized by tetrahedral coordination, in which the four ligands are protein residues. Prominent examples of structural zinc sites are the transcriptional activator zinc finger proteins (2), and the structural zinc in liver alcohol dehydrogenase (1), the latter defined by four Cys ligands coordinated to zinc. The lack of catalytic activity has been ascribed to the ligation by four protein-derived ligands yielding a coordinatively saturated ion (1). Nonetheless, higher coordinate complexes are well established for the d 10 metal ion. Alternatively, functional, i.e., catalytic, zinc sites possess a single labile coordination site, generally occupied by water or hydroxide in the resting state of enzymes. The Lewis acidity of the metal reduces the pKa of coordinated water molecules, rendering zinc enzymes particularly potent in effecting hydrolysis reactions. Hydrolases (and lyases) are the most thoroughly understood class of zinc proteins, with leading examples including the mononuclear, carbonic anhydrase, carboxypeptidase A, thermolysin, matrix metalloproteinases, and the polynuclear zinc proteins, aminopeptidases, and alkaline phosphatase (1, 3). Despite the diverse array of substrates consumed by these proteins, the consensus mechanistic feature for all is nucleophilic attack of a zinc hydroxide on the electrophilic position of the substrate.An emerging group of mononuclear zinc enzymes fashions carbon-sulfur bonds via the transalkylation of carbon electrophiles and thiol substrates (4-6). Members of this class include methyl reductase (7), the Ada DNA repair protein (8), farnesyl transferase (5), betaine homocysteine methyl transferase (9), epoxyalkane coenzyme M transferase (10), and the methionine synthases (11,12). Whereas the physiological activity and active site ligation vary, aspects of their catalytic mechanisms seem common. Specifically, thiol ligation to zinc rendering an active site bound thiolate poised for nucleophilic attack of the electrophilic substrate has been pr...

Identification of Essential Catalytic Residues of the Cyclase NisC Involved in the Biosynthesis of Nisin

“…The zinc site in other enzymes that alkylate thiols (38) is thought to have a finely balanced coordination environment that on the one end facilitates a role as a Lewis acid to lower the pK a of the thiol substrates (39 -41) and at the same time activates the thiolate for nucleophilic attack. The strong rate acceleration of thiolate alkylation by increasing the number of sulfur donors to the catalytic zinc has been established in several model studies on small molecule metal complexes with varying sets of donor ligands (42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52). Indeed, upon substrate coordination, the zinc in NisC is liganded by three thiolates in the model (Fig.…”

Nucleophilicity of zinc-bound thiolates

“…We and others have shown in previous studies [21][22][23][24]30] that substituting the phenylthiolate by the o-NHC(O)H-phenylthiolate, inducing a H bond with the sulfur atom of the arylthiolate, reduces the reactivity of the corresponding zinc-bound arylthiolate. The same aryl-substitution on complexes 6 and 13 leads to two new complexes, noted 6' and 13', respectively, for which it is possible to evaluate the influence of the H bond toward the arylthiolate on the nucleophilicity of both the aryl-and the alkyl-thiolate.…”

“…In the case of zinc-bound thiolates, several studies provide the rate constants of their reaction against electrophile but it is difficult from these data to have a general view of their relative nucleophilicities as the experimental conditions are not the same. [19,21,23,24,29] Such a determination would however be of great significance in order to evaluate the relative reactivity of enzymes and to determine which thiolate is reactive in zinc active sites including more than one cysteinate.…”

“…[15] The influence of the nature of the metal cation on the nucleophilicity of p-toluenethiolate has been studied both experimentally and by DFT computation, revealing that nickel-and zinc-thiolate complexes are more reactive than ironand cobalt-thiolate complexes. [16] In the case of zinc complexes, both the composition of the ligand set, [17][18][19][20] in terms of donating capability and steric hindrance, and the presence of hydrogen bonding towards the reactive thiolate [21][22][23][24] have been shown to modulate the reactivity against electrophiles.…”

Biological inorganic chemistry at the beginning of the 21st century

“…Charlie Riordan has shown that intramolecular NHOS hydrogen bonding affects the rates of alkylation reactions of zinc thiolates, suggesting that such interactions could modulate the activities of functionally related zinc enzymes (99), and Peter Sadler has identified binding sites in albumin that could function in the transport and delivery of zinc by blood (100). Indeed, the dipositive zinc ion, although redox-inactive, is still a hot item in biological inorganic chemistry (101)(102)(103)(104).…”