Reactions of diheteroaryl sulfoxides with heteroaryllithiums—ligand coupling and exchange reactions

The reactions of heteroaryl sulfoxides with heteroaryllithiums gave various biheteroaryls and diheteroaryl sulfoxides as ligand coupling and ligand exchange products through sulfurane intermediates. Especially, the reaction of di-2-furyl sulfoxide with 2-pyridyllithium proceeded first by a ligand exchange reaction to form 2-furyllithium and 2-furyl 2-pyridyl sulfoxide, which then underwent ligand coupling with 2-pyridyllithium to afford 2,2′-bipyridyl. Furthermore, it was found that the occurrence of the ligand coupling reaction depends on the stereoisomerism of the sulfurane intermediate formed. © 1996 John Wiley & Sons, Inc.     

Reduction mechanism in class A methionine sulfoxide reductases: a theoretical chemistry investigation

Ab initio calculations at the B3LYP/6–311 ++G(2df,2p) and B3LYP/6–31G(d) level have been carried out to investigate the reaction mechanism of methionine sulfoxide reductases of class A. These enzymes reduce oxidized methionine in vivo and therefore play an important role in repairing protein damage caused by the oxidative stress. Our calculations have been carried out for a model reaction in a model active site. Several reaction mechanisms have been explored that can roughly be described as (2H⁺ + 2e⁻) or (H⁺ + e⁻). The results suggest that the actual reaction mechanism is of the (2H⁺ + 2e⁻) type corresponding to a more or less asynchronous-concerted double-proton transfer reaction leading to the formation of methionine (dimethylthioether in our model) and a sulfenic acid Cys-SOH. The Michaelis complex would involve one deprotonated Cys and one protonated Glu residues in the active site, this protonation state being mandatory to stabilize the sulfoxide substrate. Then, proton transfer from Glu to the substrate takes place, followed by proton transfer from one Tyr residue and fast reorganization of the system. The overall activation energy barrier is estimated to fall in the range 7–9 kcal/mol, much lower than the predicted barrier in DMSO solution (29.6 kcal/mol) reported before.     

Theoretical Analysis of Structural Characteristics of Morin Rearrangement for Cephalosporins Prepared from Penicillin Sulfoxide

The structure of penicillin sulfoxide is rearranged to cephalosporins by the Morin rearrangement. It is a unit reaction for the preparation of various types of cephalosporins. In order to make better use of the reaction and in view of the shortage of the reaction theory, this study used m062x/6-311++G(d, p) to explore the possible ring-opening reaction of the penicillin sulfoxide. It is found that the isomer of(S)-sulfoxide is a necessary structure. At the same time, the intramolecular hydrogen bonding effect between the side-chain amide proton(-CONH-) and the sulfinyl oxygen(-SO) is the decisive structure factor for the formation of alkenyl in ring-opening reaction, and the best reaction path is S0- TS2- IN1 channel. The main effect of acid catalysis is to catalyze the dehydration reaction of sulfenic acid to form sulfur cations for subsequently ring closing reaction.     

Synthesis of Peptide Cysteine Dimedone Using Fmoc-Cys(Dmd)-OH: Glutathione Cysteine Dimedone as a Probe in Investigating the Sulfenic Acid Mediated Oxidation of Glutathione

Dimedone is the most widely used chemical probe for detection of cysteine sulfenic acid in peptides and proteins. The reaction of dimedone with cysteine sulfenic acid results in the formation of unique cysteine dimedone motif containing thioether bridge. Based on the structure of cysteine dimedone residue in polypeptide, a new building block of Fmoc-Cys(Dmd)-OH was developed for solid phase synthesis of peptide cysteine dimedone. Mass spectrometric sequencing of synthetic peptides have confirmed successful incorporation of cysteine dimedone in peptide chain using HBTU/HOBt as a coupling agent. The new method permits synthesis of peptides containing both cysteine thiol and cysteine dimedone in the same sequence which was difficult to achieve by conventional methods. The synthetic peptide of glutathione cysteine dimedone was used as a standard in probing the air-mediated oxidation of thiol to disulfide form of glutathione. The co-elution of standard peptide and reaction mixture of oxidation of glutathione in presence of dimedone using RP-HPLC have confirmed the formation of glutathione cysteine sulfenic as an intermediate in the air-mediated oxidation of glutathione. The synthetic peptides of cysteine dimedone may find application in the field of redox proteomics and generation of antibodies against modified cysteine residue.     

Thermolysis of alkyl sulfoxides and derivatives: a comparison of experiment and theory.

Gas-phase activation data were obtained for model sulfoxide elimination reactions. The activation enthalpy for methyl 3-phenylpropyl sulfoxide is 32.9 +/- 0.9 kcal/mol. Elimination by methyl vinyl sulfoxide to form acetylene has an enthalpic barrier of 41.6 +/- 0.8 kcal/mol and that of 3-phenylpropyl methanesulfinate to form hydrocinnamaldehyde is 34.6 +/- 0.6 kcal/mol. Calculations at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level for simplified models of these reactions provide barriers of 32.3, 40.3, and 32.7 kcal/mol, respectively. A series of other compounds are examined computationally, and it is shown that the substituent effects on the sulfoxide elimination reaction are much more straightforward to interpret if DeltaH data are available in addition to the usually determined DeltaH++. The activation enthalpy of the reverse addition reaction is also subject to structural variation and can usually be rationalized on the basis of nucleophilicity of the sulfur or polarity matching between the sulfenic acid and olefin derivative.     

Mechanism of the oxidation of sulfides by dioxiranes: conformational mobility and transannular interaction in the oxidation of thianthrene 5-oxide

The detailed study of the oxidation of thianthrene 5-oxide (1) with methyl(trifluoromethyl)dioxirane (5b) in different solvents and in the presence of (18)O isotopic tracers is reported. Thianthrene 5-oxide (1) is a flexible molecule in solution, and this property allows for transannular interaction of the sulfoxide group with the expected zwitterionic 7 and hypervalent 10-S-4 sulfurane 9 intermediates formed in the oxidation and biases the course of the reaction toward the monooxygenation pathway.     

Kinetics of omeprazole degradation in the presence of 2-mercaptoethanol

The reactions of omeprazole, a potent proton pump inhibitor, were investigated in the presence of 2-mercapotoethanol. Reactions were monitored in solutions buffered to pH values ranging from 2.0 to 5.0 using differential pulse polarography at the static mercury drop electrode. First-order reaction network was proposed for all conversions. It is demonstrated that acid degradation predominates at pH values ∼2–3, whereas the reaction of sulfenic acid with the more nucleophilic thiol becomes prominent at pH values around 4–5. The acidic medium is necessary for the formation of sulfenic acid, the key intermediate believed to be the active inhibitor, but acid also converts this sulfenic acid into other degradation products that are unreactive toward thiol. The present work suggests that acid inhibition may occur at pH values higher than those previously thought. Thus, “highly acidic medium is required to achieve both accumulation and activation of omeprazole in the parietal cell, but less acidic medium is necessary for the reaction of sulfenic acid intermediate with the sulfhydryl groups of cysteine residues of H⁺/K⁺-ATPase proton pump in vivo.” © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 352–358, 2008     

Boron doped diamond and glassy carbon electrodes comparative study of the oxidation behaviour of cysteine and methionine

The electrochemical oxidation behaviour at boron doped diamond and glassy carbon electrodes of the sulphur-containing amino acids cysteine and methionine, using cyclic and differential pulse voltammetry over a wide pH range, was compared. The oxidation reactions of these amino acids are irreversible, diffusion-controlled pH dependent processes, and occur in a complex cascade mechanism. The amino acid cysteine undergoes similar three consecutive oxidation reactions at both electrodes. The first step involves the oxidation of the sulfhydryl group with radical formation, that undergoes nucleophilic attack by water to give an intermediate species that is oxidized in the second step to cysteic acid. The oxidation of the sulfhydryl group leads to a disulfide bridge between two similar cysteine moieties forming cysteine. The subsequent oxidation of cystine occurs at a higher potential, due to the strong disulfide bridge covalent bond. The electro-oxidation of methionine at a glassy carbon electrode occurs in two steps, corresponding to the formation of sulfoxide and sulfone, involving the adsorption and protonation/deprotonation of the thiol group, followed by electrochemical oxidation. Methionine undergoes a one-step oxidation reaction at boron doped diamond electrodes due to the negligible adsorption, and the oxidation also leads to the formation of methionine sulfone.     

Characterization by tandem mass spectrometry of stable cysteine sulfenic acid in a cysteine switch peptide of matrix metalloproteinases

Cysteine sulfenic acid (Cys-SOH) is an elusive intermediate in reactive oxygen species-induced oxidation reactions of many proteins such as peroxiredoxins and tyrosine phosphatases. Cys-SOH is proposed to play a vital role in catalytic and signaling functions. The formation of cysteine sulfinic acid (Cys-SO(2)H) and cysteine sulfonic acid (Cys-SO(3)H) has been implicated in the activation of matrix metalloproteinase-7 (MMP-7) and oxidation of thiol to cysteine sulfinic acid has been associated with the autolytic cleavage of MMP-7. We have examined the formation of cysteine sulfenic acid in a synthetic peptide PRCGVPDVA, which is a cysteine switch domain of MMP-7 and other matrix metalloproteases. We have prepared the cysteine sulfenic acid containing peptide, PRC(SOH)GVPDVA, by reaction with hydroxyl radicals generated by the Fenton reaction (Fe(+2)/H(2)O(2)). We characterized this modified peptide by tandem mass spectrometry and accurate mass measurement experiments. In addition, we used 7-chloro-4-nitrobenzo-2-oxa-1,3-diazol (NBD-Cl) reagent to form an adduct with PRC(SOH)GVPDVA to provide additional evidence for the viability of PRC(SOH)GVPDVA in solution. We also characterized an intramolecular cysteine sulfinamide cross-link product PRC[S(O)N]GVPDVA based on tandem mass spectrometry and accurate mass measurement experiments. These results contribute to the understanding of a proteolytic cleavage mechanism that is traditionally associated with MMP activation.     

Effects of aromatic thiols on thiol-disulfide interchange reactions that occur during protein folding

The folding of disulfide containing proteins from denatured protein to native protein involves numerous thiol-disulfide interchange reactions. Many of these reactions include a redox buffer, which is a mixture of a thiol (RSH) and the corresponding disulfide (RSSR). The relationship between the structure of RSH and its efficacy in folding proteins in vitro has been investigated only to a limited extent. Reported herein are the effects of aliphatic and especially aromatic thiols on reactions that occur during protein folding. Aromatic thiols may be particularly efficacious as their thiol pK(a) values and reactivities match those of the in vivo catalyst, protein disulfide isomerase (PDI). This investigation correlates the thiol pK(a) values of aromatic thiols with their reactivities toward small molecule disulfides and the protein insulin. The thiol pK(a) values of nine para-substituted aromatic thiols were measured; a Hammett plot constructed using sigma(p-) values yielded rho = -1.6 +/- 0.1. The reactivities of aromatic and aliphatic thiols with 2-pyridyldithioethanol (2-PDE), a small molecule disulfide, were determined. A plot of reactivity versus pK(a) of the aromatic thiols had a slope (beta) of 0.9. The ability of these thiols to reduce (unfold) the protein insulin correlates strongly with their ability to reduce 2-PDE. Since the reduction of protein disulfides occurs during protein folding to remove mismatched disulfides, aromatic thiols with high pK(a) values are expected to increase the rate not only of protein unfolding but protein folding as well.     

Redox regulation of protein tyrosine phosphatase 1B (PTP1B): a biomimetic study on the unexpected formation of a sulfenyl amide intermediate

The effect of steric and electronic environments around the sulfur and nitrogen atoms and the role of nonbonded S...O/N interactions on the cyclization reactions of amide substituted benzene sulfenic acids are described. The reaction profiles and the role of different substituents on the cyclization are investigated in detail by theoretical calculations. It is shown that the synthetic thiols having ortho-amide substituents may serve as good models for the enforced proximity of the amide and cysteine thiol groups at the active site of protein tyrosine phosphatase 1B (PTP1B). However, some of the sulfenic acids derived from such models do not effectively mimic the cyclization of protein sulfenic acids. This is mainly due to the requirement of very high energy for breaking the S-O bond to form a planar five-membered ring of isothiazolidinone. It is shown that the sulfenic acid having two substituents-an amide moiety and a heterocyclic group-in the ortho-positions undergoes a rapid cyclization reaction to produce the corresponding sulfenyl amide species. These studies reveal that the introduction of a substituent at the 6-position of the benzene ring enhances the cyclization process not only by facilitating a closer approach of the -OH group and the backbone -NH moiety but also by increasing the electrophilicity of the sulfur atom in the sulfenic acid.     

Activation of leinamycin by thiols: a theoretical study

Reaction of thiols with the 1,2-dithiolan-3-one 1-oxide heterocycle found in leinamycin (1) results in the conversion of this antitumor antibiotic to a DNA-alkylating episulfonium ion (5). While the products formed in this reaction have been rationalized by a mechanism involving initial attack of thiol on the central sulfenyl sulfur (S2') of the 1,2-dithiolan-3-one 1-oxide ring, the carbonyl carbon (C3') and the sulfinyl sulfur (S1') of this heterocycle are also expected to be electrophilic. Therefore, it is important to consider whether nucleophilic attack of thiol at these sites might contribute either to destruction of the antibiotic or conversion to its episulfonium ion form. To address this question, we have used computational methods to examine the attack of methyl thiolate on each of the three electrophilic centers in a simple analogue of the 1,2-dithiolan-3-one 1-oxide heterocycle found in leinamycin. Calculations were performed at the MP2/6-311+G(3df,p)//B3LYP/6-31G level of theory with inclusion of solvent effects. The results indicate that the most reasonable mechanism for thiol-mediated activation of leinamycin involves initial attack of thiolate at the S2'-position of the antibiotic's 1,2-dithiolan-3-one 1-oxide heterocycle, followed by conversion to the 1,2-oxathiolan-5-one intermediate (3).     

Infrared spectra of the sulfenic ester CH3SOCH3 and its photodissociation products in solid argon

UV light irradiation of dimethyl sulfoxide (DMSO) in low temperature solid argon matrix produces sulfenic ester, CH 3SOCH 3, a high energy structural isomer of DMSO. The sulfenic ester molecule further dissociates to the CH 2O-CH 3SH complex under 266 nm laser irradiation. The CH 2S-CH 3OH complex is also formed upon UV light irradiation. The infrared spectra of the aforementioned species are assigned on the basis of isotopic substitutions ( (13)C and deuterium) as well as density functional frequency calculations.     

Thiol metabolomics of endothelial cells using capillary liquid chromatography mass spectrometry with isotope coded affinity tags

Thiol and disulfide levels are critical to maintaining the redox potential of a cell. Perturbations of these levels are important in disease pathogenesis. To improve endogenous mammalian metabolome quantitation, thiol specific tagging, extraction and relative quantitation were undertaken. Reduced and oxidized thiol (disulfide) metabolites from endothelial cells were tagged and extracted using cleavable isotope coded affinity tags (cICAT). Extracted cICAT labeled thiols were analyzed using capillary reverse phase liquid chromatography coupled to mass spectrometry (capLC-MS) with positive mode electrospray ionization. Reactions between thiol metabolite standards and the reactive group of cICAT indicate completion by 8h at pH 9 with no apparent disulfide formation. cICAT labeled reduced thiols from endothelial cells showed 1-5% RSD using ratiometric quantitation of isotopes and 6-17% RSD based on signal intensity alone. Sample injection was optimized to 16 pmol. Using high mass accuracy MS, 75 putative thiol metabolites were detected in all experimental samples. Treatment of endothelial cells with 2,3-dimethoxy-5-methyl-1,4-benzoquinone (BQ) shows decreased levels in 28 putative reduced thiols and increased levels of 27 putative disulfides. Treatment of endothelial cells with 30 mM glucose resulted in 22 putative reduced thiols with decreased levels and 7 putative disulfides with increased concentration. Thiols were identified based on accurate mass within 3 ppm and analysis of fragmentation patterns. Using higher collision induced dissociation (HCD), shared product ions between different thiols led to the analysis of thiols from the cysteine-glutathione (Cys-GSH) pathway. Specific reduced thiols and disulfides in this pathway revealed changes different from the overall trends of thiols/disulfides. This suggests varying regulation of the Cys-GSH pathway distinct from other thiol-containing pathways and dependence on the type of environmental stimulus. These results indicate the utility of analyzing reduced thiols and disulfides in eukaryotic samples.     

Quantitative determination of SH groups using 19F NMR spectroscopy and disulfide of 2,3,5,6-tetrafluoro-4-mercaptobenzoic acid

A new method of measurement of thiol concentration by 19F NMR spectroscopy is developed. The method is based on the detection of products of the exchange reaction of thiols with a newly synthesized fluorinated disulfide, 2,3,5,6-tetrafluoro-4-mercaptobenzoic acid (BSSB). A significant broadening of the 19F NMR signal of BSSB in the presence of thiols was observed and attributed to the exchange reaction between the parent disulfide and 2,3,5,6-tetrafluoro-4-mercaptobenzoic acid. The rate constant for this reaction was found to be equal to (63 +/- 11) x 10(3) M(-1) s(-1) at pH 7.0. The method was applied for the measurement of concentration of glutathione and albumin in rat blood.     

CS与NO分子反应势能面的理论研究

应用量子化学从头计算和密度泛函理论(DFT)对CS分子和NO分子的反应机理进行了研究. 在B3LYP/6- 311G**和CCSD(T)/6-311G**水平上计算了CS分子与NO分子反应的二重态和四重态反应势能面. 计算结果表明, 二重态反应势能面中, CS分子的C端和NO的N端连接是主要的反应方式. 反应物先经过过渡态TS1, 形成具有直线结构的中间体1 (CSNO). 中间体1经过一系列异构化得到主要产物P₁ (CO＋SN). 此反应是放热反应, 反应热为－183.75 kJ/mol . 而四重态由于反应入口势垒过高, 是不重要的.     

Classical versus redox tautomerism: substituent effects on the keto/enol and sulfoxide/sulfenic acid equilibria

MP2/6-311+G** ab initio calculations have been carried out for a classical example of tautomerism, the keto/enol [RCOCH₃/RC(OH)CH₂] and an example of redox tautomerism, the sulfoxide/sulfenic acid [RS(O)H/RSOH]. Eleven R substituents have been examined. Both equilibria show proportional energies and similar dependence on the Swain-Lupton Fand Rparameters.     

Thermal rearrangement of 1,3-thiazolidine sulfoxides: Thiolsulfinate and thioaldehyde intermediates

Stereospecific ring opening of the sulfoxides cis- 13 and trans- 14 in refluxing toluene gave the corre sponding sulfenic acids 9 , 10 intermediates respectively. The sulfenic acid 9 dimerized to the thiolsulfinate 17 by dual function of the sulfenic acid as S-nucleophile/S-electrophile with loss of water while the sulfenic acid 10 was unchanged. The stereospecific recyclization of 10 to the parent sulfoxide 14 increases the higher pi-electron density of the double. The thermolysis of the thiolsulfinate 17 gave the transient sulfenic acid 9 , which dimerized again to repeat the process and unisolable thioaldehyde 21 . The thioaldehyde 21 was con verted to either pyrrole 15 by the action of a sulfinic acid 20 catalyst formed inevitably by hydrolysis of 17 under the reaction conditions, or thiazole 18 under neutral conditions. In these rearrangements, the amide carbonyl group facilitated the elimination of a neighboring hydrogen.     

二氟硅烯与甲醛环加成反应机理的理论研究

The mechanism of the cycloadditohn reaction of singlet difluorosilylene with formaldehyde have been studied by RHF/6-311G* gradient method. The electron correlation energy corrections of energies for all the structures were computed using second-order Moller-Plesset perturbation theory(MP2). The results show that this reaction proceeds via two steps:1)Difluorosilylene and formaldehyde form an intermediate complex, it is an exothermal reaction with no barrier.2) The intermediate complex isomerizes to form the product, after being corrected by zero-point energies, the barrier is 127.28 kJ•mol-1 (MP2/6-311G* 6-311G*).     

Rapid and quantitative disulfide bond formation for a polypeptide chain using a cyclic selenoxide reagent in an aqueous medium

To elucidate the reaction mechanism of the disulfide (SS) bond formation reaction of a polypeptide molecule with a water‐soluble selenoxide reagent, trans‐3,4‐dihydroxyselenolane oxide (DHS^ox), short‐term oxidation experiments were carried out for the reduced state (R) of a recombinant hirudin CX‐397 variant at pH 7.0 and 25 °C. In the reaction, R was oxidized sequentially to one‐SS, two‐SS, and three‐SS intermediate ensembles within 1 min. The kinetic analysis revealed that the three second‐order rate constants for the SS formation are proportional to the number of thiol groups existing in the reactant SS intermediates, indicating the stochastic nature of the SS formation. Ab initio calculation at the HF/6‐31++G(d,p) level in water by using the polarizable continuum model suggested that the SS formation reaction is highly exothermic and proceeds via a reactive thioselenurane intermediate with a distorted linear O‐Se‐S linkage. The results clearly demonstrated that the rate‐determining step of the SS formation reaction is the first bimolecular process between a thiol substrate and DHS^ox rather than the subsequent process to release a SS product.