Thioester Hydrolysis Reactivity of an Fe(III)Zn(II) Complex

Transition Metal Mediated Hydrolysis of C-S Bonds: An Overview of a New Reaction Strategy

Desulfurization of organosulfur substrates is highly important due to its relation with the industrial hydrodesulfurization (HDS) process of fossil fuels, which helps to eliminate the sulfur-containing impurities such as thiols, sulfide, thiophenes, etc. from crude oil for the production of easily processed and more cleanly combusted fuel with very low sulfur content. While the HDS process involves a hydrogenolysis reaction under a high pressure of hydrogen gas at high temperature, the hydrolysis of C-S bonds of organosulfur substrates at ambient conditions may very well be considered as a potential alternative for model desulfurization reactions. However, unlike the availability of an appreciable number of reports on base, acid, and metal ion mediated hydrolysis of thioesters in the literature, reports on the hydrolysis of more difficult substrates such as thiolates, sulfides, and other organosulfur substrates remained unavailable until 2017. The very recent discovery of a transition metal mediated hydrolysis reaction of C-S bonds at ambient conditions, however, has rapidly filled in this gap within the past few years. Development of this new stoichiometric reaction allowed the desulfurization of a large number of organosulfur substrates, including aliphatic and aromatic thiols, thiocarboxylic acids, sulfides, disulfides, thiophenes, and dibenzothiophene, at ambient conditions and was subsequently converted to a catalytic process for the hydrolysis of thiols. A brief overview of this new reaction strategy, a proposed reaction mechanism, a critical analysis of the efficiency, and future prospects are presented.

Catalytic Hydrolysis of Thiolates to Alcohols

A new and efficient catalytic hydrolysis of aliphatic and aromatic thiolates under ambient conditions is presented. Previously, we have demonstrated (Ganguly et al., Inorg. Chem. 2018, 57, 11306-11309) the Co(II) mediated stoichiometric hydrolysis of thiols to produce alcohols/phenols along with a binuclear dicobalt(II)-hydrosulfide complex, [Co₂(PhBIMP)(μ₂-SH)(DMF)]²⁺ (1) (PhBIMP is the anion of 2,6 bis[(bis((N-1-methyl-4,5- diphenylimidazoylmethyl) amino)methyl]- 4-methylphenol). In the present work, we have shown that the product of the stoichiometric reaction, 1, may act as an efficient catalyst for the catalytic hydrolysis of a broad range of aliphatic and aromatic thiolates in DMF at room temperature to produce alcohols/phenols. Complex 1 takes up a thiolate (RS^-) and a water molecule to generate an active intermediate complex, [Co₂(PhBIMP)(μ₂-SH)(RS)(H₂O)]¹⁺ (2), which, in turn, releases the alcohol/phenol (ROH), hydrosulfide (HS^-), and regenerates 1.

Rapid X-ray photoreduction of dimetal-oxygen cofactors in ribonucleotide reductase

Prototypic dinuclear metal cofactors with varying metallation constitute a class of O2-activating catalysts in numerous enzymes such as ribonucleotide reductase. Reliable structures are required to unravel the reaction mechanisms. However, protein crystallography data may be compromised by x-ray photoreduction (XRP). We studied XPR of Fe(III)Fe(III) and Mn(III)Fe(III) sites in the R2 subunit of Chlamydia trachomatis ribonucleotide reductase using x-ray absorption spectroscopy. Rapid and biphasic x-ray photoreduction kinetics at 20 and 80 K for both cofactor types suggested sequential formation of (III,II) and (II,II) species and similar redox potentials of iron and manganese sites. Comparing with typical x-ray doses in crystallography implies that (II,II) states are reached in <1 s in such studies. First-sphere metal coordination and metal-metal distances differed after chemical reduction at room temperature and after XPR at cryogenic temperatures, as corroborated by model structures from density functional theory calculations. The inter-metal distances in the XPR-induced (II,II) states, however, are similar to R2 crystal structures. Therefore, crystal data of initially oxidized R2-type proteins mostly contain photoreduced (II,II) cofactors, which deviate from the native structures functional in O2 activation, explaining observed variable metal ligation motifs. This situation may be remedied by novel femtosecond free electron-laser protein crystallography techniques.

High-valent [MnFe] and [FeFe] cofactors in ribonucleotide reductases

Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-Ia RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconstituted with Mn and Fe ions, structural and electronic properties of higher valence MnFe and FeFe sites were determined by X-ray absorption spectroscopy and complementary techniques, in combination with bond-valence-sum and density functional theory calculations. At least ten different cofactor species could be tentatively distinguished. In Ct R2, two different Mn(IV)Fe(III) site configurations were assigned either L(4)Mn(IV)(μO)(2)Fe(III)L(4) (metal-metal distance of ~2.75Å, L = ligand) prevailing in metal-grown R2, or L(4)Mn(IV)(μO)(μOH)Fe(III)L(4) (~2.90Å) dominating in metal-reconstituted R2. Specific spectroscopic features were attributed to an Fe(IV)Fe(III) site (~2.55Å) with a L(4)Fe(IV)(μO)(2)Fe(III)L(3) core structure. Several Mn,Fe(III)Fe(III) (~2.9-3.1Å) and Mn,Fe(III)Fe(II) species (~3.3-3.4Å) likely showed 5-coordinated Mn(III) or Fe(III). Rapid X-ray photoreduction of iron and shorter metal-metal distances in the high-valent states suggested radiation-induced modifications in most crystal structures of R2. The actual configuration of the MnFe and FeFe cofactors seems to depend on assembly sequences, bound metal type, valence state, and previous catalytic activity involving subunit R1. In Ct R2, the protonation of a bridging oxide in the Mn(IV)(μO)(μOH)Fe(III) core may be important for preventing premature site reduction and initiation of the radical chemistry in R1.

A Potentially Polymerizable Heterodinuclear FeIIIZnII Purple Acid Phosphatase Mimic. Synthesis, Characterization, and Phosphate Ester Hydrolysis Studies

An analogue of the purple acid phosphatase biomimetic 2-((bis(pyridin-2-ylmethyl)amino)methyl)-6-(((2-hydroxybenzyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol has been synthesized. The analogue, 2-((bis(pyridin-2-ylmethyl)amino)methyl)-6-(((2-hydroxy-4-(4-vinylbenzyloxy)benzyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (H2BPBPMPV) possesses a pendant olefin suitable for copolymerization. Complexation with FeIII/ZnII resulted in the complex [FeIIIZnII(BPBPMPV)(CH3COO)2](ClO4), characterized with mass spectrometry, microanalysis, UV/vis, and IR spectrometry. The catalytic activity of the complex toward bis-(2,4-dinitrophenyl) phosphate was determined, resulting in Km of 4.1 ± 0.6 mM, with kcat 3.8 ± 0.2 × 10–3 s–1 and a bell-shaped pH–rate profile with pKa values of 4.31, 5.66, 8.96, the profile exhibiting residual activity above pH 9.5.

Thioester hydrolysis promoted by a mononuclear zinc complex

The mononuclear zinc complex [(bpta)Zn](ClO(4))(2).0.5H(2)O promotes the hydrolysis of the thioester PhCH(OH)C(O)SCD(3) when dissolved in CH(3)CN:H(2)O (50:50 buffered at pH 9.0). This reaction results in the formation of a mixture of CD(3)SH and a zinc thiolate complex, the latter of which can be protonated to generate additional CD(3)SH. Kinetic studies revealed an overall second-order reaction with an activation energy that is similar to that found for aqueous OH(-) promoted thioester hydrolysis. These studies represent the first investigation of chemistry relevant to that occurring in the monozinc-containing form of human glyoxalase II.