- Photoactivatable Odorants for Chemosensory Research
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The chemosensory system of any animal relies on a vast array of detectors tuned to distinct chemical cues. Odorant receptors and the ion channels of the TRP family are all uniquely expressed in olfactory tissues in a species-specific manner. Great effort has been made to characterize the molecular and pharmacological properties of these proteins. Nevertheless, most of the natural ligands are highly hydrophobic molecules that are not amenable to controlled delivery. We sought to develop photoreleasable, biologically inactive odorants that could be delivered to the target receptor or ion channel and effectively activated by a short light pulse. Chemically distinct ligands eugenol, benzaldehyde, 2-phenethylamine, ethanethiol, butane-1-thiol, and 2,2-dimethylethane-1-thiol were modified by covalently attaching the photoremovable protecting group (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ). The CyHQ derivatives were shown to release the active odorant upon illumination with 365 and 405 nm light. We characterized their bioactivity by measuring activation of recombinant TRPV1 and TRPA1 ion channels expressed in HEK 293 cells and the electroolfactogram (EOG) response from intact mouse olfactory epithelium (OE). Illumination with 405 nm light was sufficient to robustly activate TRP channels within milliseconds of the light pulse. Photoactivation of channels was superior to activation by conventional bath application of the ligands. Photolysis of the CyHQ-protected odorants efficiently activated an EOG response in a dose-dependent manner with kinetics similar to that evoked by the vaporized odorant amyl acetate (AAc). We conclude that CyHQ-based, photoreleasable odorants can be successfully implemented in chemosensory research.
- Gore, Sangram,Ukhanov, Kirill,Herbivo, Cyril,Asad, Naeem,Bobkov, Yuriy V.,Martens, Jeffrey R.,Dore, Timothy M.
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p. 2516 - 2528
(2020/10/02)
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- COPPER PROTECTIVE AGENT
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A copper protective agent is provided. The copper protective agent is represented by a general formula (GI): HS—R (GI); and R is a linear or branched alkyl group having 1 to 20 carbon atoms.
- -
-
Paragraph 0042-0043
(2019/05/24)
-
- Quantitative Reactivity Scales for Dynamic Covalent and Systems Chemistry
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Dynamic covalent chemistry (DCC) has become a powerful tool for the creation of molecular assemblies and complex systems in chemistry and materials science. Herein we developed for the first time quantitative reactivity scales capable of correlation and prediction of the equilibrium of dynamic covalent reactions (DCRs). The reference reactions are based upon universal DCRs between imines, one of the most utilized structural motifs in DCC, and a series of O-, N-, and S- mononucleophiles. Aromatic imines derived from pyridine-2-carboxyaldehyde exhibit capability for controlling the equilibrium through distinct substituent effects. Electron-donating groups (EDGs) stabilize the imine through quinoidal resonance, while electron-withdrawing groups (EWGs) stabilize the adduct by enhancing intramolecular hydrogen bonding, resulting in curvature in Hammett analysis. Notably, unique nonlinearity induced by both EDGs and EWGs emerged in Hammett plot when cyclic secondary amines were used. This is the first time such a behavior is observed in a thermodynamically controlled system, to the best of our knowledge. Unified quantitative reactivity scales were proposed for DCC and defined by the correlation log K = SN (RN + RE). Nucleophilicity parameters (RN and SN) and electrophilicity parameters (RE) were then developed from DCRs discovered. Furthermore, the predictive power of those parameters was verified by successful correlation of other DCRs, validating our reactivity scales as a general and useful tool for the evaluation and modeling of DCRs. The reactivity parameters proposed here should be complementary to well-established kinetics based parameters and find applications in many aspects, such as DCR discovery, bioconjugation, and catalysis.
- Zhou, Yuntao,Li, Lijie,Ye, Hebo,Zhang, Ling,You, Lei
-
supporting information
p. 381 - 389
(2016/01/26)
-
- A di-tert-butyl-terminated chain multi-sulfide synthesis method
-
The invention provides a synthesis method for di-tert-butyl terminated chain polythiaether. The method is characterized by comprising the steps of: putting elemental sulfur and a catalyst (amino based metal-organic framework microporous material) into a reaction kettle, replacing the air in the reaction kettle with nitrogen, then injecting hydrogen sulfide, conducting stirring heating to 120-160DEG C, slowly injecting isobutene into the reaction kettle, then carrying out reaction at a pressure of 4-7MPa for 2-6 h, then cooling the reaction product to 100DEG C, performing purging with nitrogen, condensing the low-boiling point by-product tert-butyl mercaptan and di-tert-butyl sulfide, then performing recovery for reuse as a raw material, conducting filtering when the product is cooled to room temperature to obtain a di-tert-butyl terminated chain polythiaether product, and using the catalyst obtained by filtering repeatedly. The synthesis method provided by the invention has the characteristics of high yield, reusable catalyst, and reaction atom economy near 100%, realizes zero pollution and zero emission, and belongs to an environment-friendly synthesis method.
- -
-
Paragraph 0017; 0041; 0041
(2017/03/24)
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- C-S bond cleavage by a polyketide synthase domain
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Leinamycin (LNM) is a sulfur-containing antitumor antibiotic featuring an unusual 1,3-dioxo-1,2-dithiolane moiety that is spiro-fused to a thiazole-containing 18-membered lactam ring. The 1,3-dioxo-1,2- dithiolane moiety is essential for LNM's antitumor activity, by virtue of its ability to generate an episulfonium ion intermediate capable of alkylating DNA. We have previously cloned and sequenced the lnm gene cluster from Streptomyces atroolivaceus S-140. In vivo and in vitro characterizations of the LNM biosynthetic machinery have since established that: (i) the 18-membered macrolactam backbone is synthesized by LnmP, LnmQ, LnmJ, LnmI, and LnmG, (ii) the alkyl branch at C-3 of LNM is installed by LnmK, LnmL, LnmM, and LnmF, and (iii) leinamycin E1 (LNM E1), bearing a thiol moiety at C-3, is the nascent product of the LNM hybrid nonribosomal peptide synthetase (NRPS)-acyltransferase (AT)-less type I polyketide synthase (PKS). Sulfur incorporation at C-3 of LNM E1, however, has not been addressed. Here we report that: (i) the bioinformatics analysis reveals a pyridoxal phosphate (PLP)-dependent domain, we termed cysteine lyase (SH) domain (LnmJ-SH), within PKS module-8 of LnmJ; (ii) the LnmJ-SH domain catalyzes C-S bond cleavage by using L-cysteine and L-cysteine S-modified analogs as substrates through a PLP-dependent β-elimination reaction, establishing L-cysteine as the origin of sulfur at C-3 of LNM; and (iii) the LnmJ-SH domain, sharing no sequence homology with any other enzymes catalyzing C-S bond cleavage, represents a new family of PKS domains that expands the chemistry and enzymology of PKSs and might be exploited to incorporate sulfur into polyketide natural products by PKS engineering.
- Ma, Ming,Lohman, Jeremy R.,Liu, Tao,Shen, Ben
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p. 10359 - 10364
(2015/09/01)
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- Kinetics and mechanism of the nucleophilic substitution of tellurium(II) dialkanethiolates, Te(SR1)2 with thiols, HSR2
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The equilibrium reaction between tellurium(II) dithiolates and thiols, Te(SR1)2 + 2 HSR2 ? Te(SR 2)2 + 2 HSR1 was studied by means of 1H- and 125Te NMR spectroscopy and ab initio quantum chemical methods. It was found that the reaction is catalyzed by Bronsted acids and bases, the catalytic activity corresponding to the strength of the respective acid or base. Investigation of the initial step of the reaction, Te(SR1)2 + HSR2 ? Te(SR 1)(SR2) + HSR1, showed it to proceed according to first order kinetics for Te(SR1)2, HSR2 and for the catalyst. Ab initio geometry optimizations and frequency calculations suggest [Te(SR1)(HSR1)(HSR2)]+ and [Te(SR1)2(SR2)]- to be stable intermediates and not transition states in the acid and base catalyzed reactions, respectively. The reaction hence proceeds via an additional elimination rather than an SN2 mechanism. The catalytic activity displayed by acids and bases can be applied to reduce the temperature in synthesis of thermally labile tellurium(II) dithiolates. Copyright Taylor & Francis Inc.
- Fleischer, Holger
-
p. 815 - 825
(2007/10/03)
-
- Remarkably fast direct synthesis of thiols from alcohols under mild conditions
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One pot rapid synthesis of thiols from alcohols via trifluoroacetates using polymer supported hydrosulfide in acetonitrile under mild conditions has been described.
- Bandgar,Sadavarte,Uppalla
-
p. 1304 - 1305
(2007/10/03)
-
- Solid Supported Reagents and Reactions. Part 21.1 Rapid and Clean Synthesis of Thiols from Halides Using Polymer-supported Hydrosulfide
-
A variety of thiols are prepared from corresponding halides using polymer-supported hydrosulfide in excellent yields. Isolation of pure products by simple filtration and evaporation is an important feature of this method.
- Bandgar, Babasaheb P.,Pawar, Sanjay B.
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p. 212 - 213
(2007/10/03)
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- Mechanism of the Solution-Phase Reaction of Alkyl Sulfides Atomic Hydrogen. Reduction via a 9-S-3 Radical Intermediate
-
The low selectivity of benzyl alkyl sulfide fragmentation subsequent to its reaction with atomic hydrogen is indicative of a reaction that proceeds via an early transition state. The competitive reduction of a series of substituted-benzyl alkyl sulfides was insensitive to the substituent on the aromatic ring (ρ = -0.13, r = 0.99). The relative rates of fragmentation of a series of the substituted-benzyl alkyl sulfides gave a V-shaped Hammett plot. Both electron-donating and electron-withdrawing groups destabilized the transition state (ρ = +0.99, r = 0.999; ρ = -0.82, r = 0.992). Since the relative rates of disappearance of the alkyl benzyl sulfides are not substituent dependent, but the relative rates of fragmentation are, a 9-S-3 intermediate is preferred as the structure leading to products.
- Tanner, Dennis D.,Koppula, Sudha,Kandanarachchi, Pramod
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p. 4210 - 4215
(2007/10/03)
-
- Investigation of the direct and indirect reduction processes of some disulfides by electrochemical means
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The free energy relationship for the reaction between aromatic radical anions A.- having about the same intrinsic barrier and three disulfides RSSR (diphenyl disulfide, dimethyl disulfide and di-tert-butyl disulfide) in N,N-dimethylformamide has been obtained. For each RSSR second-order rate constants were measured electrochemically in the interval 10-3-105 M-1 s-1 by changing the nature of A.- and thereby the driving force. In the case of diphenyl disulfide the standard potential ERSSR/RSSR.-? and the self-exchange reorganization energy λ(0) of the RSSR/RSSR.- couple could be extracted from the free energy relationship to values of -1.4 (±0.1) V vs. SCE and 65 kcal mol-1, respectively. This knowledge may be combined with the kinetic features of the heterogeneous reduction process of diphenyl disulfide to obtain, among other parameters, the standard heterogeneous rate constant. The cleavage rate constant kc of the corresponding radical anion was estimated from kinetic measurements to be about 5 × 108 s-1. No such information was accessible for the aliphatic disulfides due to a high self-exchange energy of the RSSR/RSSR.- couple but still a maximum value of kc could be determined at 2x108 s-1. The minimum values of ERSSR/RSSR.-? and λ(0) were estimated to be -1.9 V vs. SCE and 75 kcal mol-1 and -2.2 V vs. SCE and 75 kcal mol-1 for dimethyl disulfide and di-tert-butyl disulfide, respectively. ? Acta Chemica Scandinavica 1997.
- Bo Christensen, Torben,Daasbjerg, Kim
-
p. 307 - 317
(2007/10/03)
-
- Process for making tertiary-thiols
-
A process for making t-butylthiol by reacting hydrogen sulfide and an olefin having a carbon number of 3 to 15 in the presence of a HZSM-5, HZSM-12, or MCM-22 zeolite catalyst. The reaction takes place under autogenous pressure conditions and a temperature of about 70° C. to about 160° C.
- -
-
-
- NO-Group transfer (transnitrosation) between S-nitrosothiols and thiols. Part 2
-
The kinetics of NO-group transfer have been measured for the reaction between a nitrosothiol (HOCH2CH2SNO) and nine thiols, mostly based on the cysteine structure.The reaction is second-order and there is evidence for a steric effect for thiols containing 1,1-dimethyl substituents (penicillamine derivatives).Reaction occurs via the thiolate anion as shown by the pH-rate constant profile, and a full kinetic analysis for the reactions of two thiols (N-acetylcysteine and glutathione) is quantitatively in agreement with this mechanism.Variation of the nitrosothiol structure for reaction with N-acetylcysteine shows that electron-withdrawing substituents in the nitrosothiol promote reaction; there is a similarity with the corresponding reactions of alkyl nitrites.
- Barnett, D.Jonathan,Rios, Ana,Williams, D. Lyn H.
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p. 1279 - 1282
(2007/10/03)
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- The Regioselective Reaction of Atomic Hydrogen with Unsymmetric Disulfides and Sulfides
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Unsymmetrical disulfides undergo solution phase reduction with atomic hydrogen regioselectively by displacement at the least hindered sulfur atom.The cleavage of the sulfur-sulfur bond forms mixtures of two thiol and two thiyl radicals.At the temperature at which the reactions are carried out, the thiyl radicals form symmetric disulfides by thiyl-thiyl radical coupling and not by thiyl radical displacement on the starting material.The reaction of atomic hydrogen with an unsymmetric sulfide is a cleavage that favors the formation of the most stable radical.The reaction of phenyl cyclohexyl sulfide produces benzene, cyclohexane, cyclohexyl thiol, and thiophenol.Benzene and cyclohexyl thiol produced from the cleavage of the phenyl-sulfur bond are proposed to arise from the α-scission of an intermediate formed by ipso-addition of atomic hydrogen to the benzene ring.
- Tanner, Dennis D.,Zhang, Liying,Vigneswaran, Markandu,Kandanarachchi, Pramod
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p. 4481 - 4487
(2007/10/02)
-
- NEW DERIVATIVES OF DITHIOCARBAMIC ACID. II.* N-(2-VINYLOXYETHYL)DITHIOCARBAMATES AND THEIR TRANSFORMATIONS
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Previously unknown N-(2-vinyloxyethyl)dithiocarbamate esters were obtained with high yields by the reaction of potassium N-(-2-vinyloxyethyl)dithiocarbamate with equimolar amounts of organic halides in DMSO or ethanol.When heated above 100 deg C without a catalyst or in the presence of trifluoroacetic acid as catalyst at 20-60 deg C, the obtained esters undergo cyclization to 2-methyl-3--1,3-oxazolidines; the reaction is accompanied by the formation of 1,1-diethanes.The reaction of potassium N-(2-vinyloxyethyl)dithiocarbamate with acid chlorides in DMSO leads to 2-vinyloxyethyl isothiocyanate, which is obtained with a better yield by the cleavage of methyl N-(2-vinyloxyethyl)dithiocarbamate in toluene in the presence of catalytic amounts of potassium hydroxide.
- Amosova, S. V.,Tarasova, O. A.,Ivanova, N. I.,Perzhabinskaya, L. I.,Sigalov, M. V.,et al.
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p. 1478 - 1484
(2007/10/02)
-
- Kinetics of Deuterium Isotope Exchange between 2-Methylpropane-2-thiol and Propane-2-thiol in Aprotic Solvents
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The rate constants for bomolecular deuterium isotope exchange between 2-methylpropane-2-thiol and propane-2-thiol in C6D12, CCl4, (CD3)2CO, HCON(CH3)2, (CD2)4O, and (CH3)2SO solutions were determined by use of 1H n.m.r. analysis.The rate constants, k, and activation enthalpy, ΔH, decrease, and entropy, -ΔS, increases, with an increase in the ability of the solvents to form hydrogen bonds with alkanethiols.The rate constant of concerted hydrogen transfer inside the cyclic dimer of alkanethiols with two H-bonds, k3, increases with increasing dielectric constant of the solvent.
- Wawer, Andrzej
-
p. 1295 - 1298
(2007/10/02)
-
- HIGH-TEMPERATURE ORGANIC SYNTHESIS. INVESTIGATION OF THE MECHANISM OF PYROLYSIS OF DIORGANIC SULFIDES BY THE CHLOROBENZENE TRAP METHOD
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The copyrolysis of various diorganic sulfides with chlorobenzene in the gas phase at 600-650 deg C was investigated.The degree of conversion of the chlorobenzene, which is a trap for thiyl radicals, increases with increase in the reaction temperature and depends significantly on the nature of the sulfide.The copyrolysis of chlorobenzene with branched dialkyl sulfides (secondary and tertiary) takes place more slowly than the reaction with the corresponding di(n-alkyl) sulfides.Alkyl vinyl sulfides are similar to the latter in reactivity.Ethyl β,β-dichlorovinyl sulfide and bis(ethylthio)acetylene have low reactivity.The main products from copyrolysis of all the diorganic sulfides with chlorobenzene are benzene, thiophenol, diphenyl sulfide, thiophene, and benzothiophene.The ratio between the products depends on the nature of the initial sulfide and an the temperature.The ratio between the thiophene and benzothiophene makes it possible to form an opinion about the mechanism of the formation of thiophene during the pyrolysis of the sulfides.Several paths were determined for the formation of the thiophene molecule through the thiyl radicals, and they supplemented the known mechanism of the pyrolysis of dialkyl sulfides, i.e., intermolecular cyclization of ethenethiol, intramolecular cyclization of butenethiol, cyclization of divinyl sulfide, and thermal dissociation of bis(ethylthio)acetylene.Intramolecular heterocyclization of the thiyl radicals is realized at a higher rate than intermolecular cyclization.
- Voronkov, M. G.,Deryagina, E. N.,Sukhomazova, E. N.,Mirskova, A. N.,Seredkina, S. G.
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p. 1453 - 1458
(2007/10/02)
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- Rate and Equilibrium Constants for the Reaction of Thiolate Ions with Dibenzo-1,2-dithiin and Naphtho-1,2-dithiole 1,1-Dioxides
-
In aqueous dioxane the cyclic thiosulfonate dibenzo-1,2-dithiin 1,1-dioxide (1) reacts rapidly with thiolate ions and undergoes opening of the thiosulfonate ring (eq 2), forming disulfide 3a.Acidification of solutions of 3a with carboxylic acid buffers of appropiate pH leads to facile reversal of ring-opening reaction and the quantitative regeneration of 1.Since this reversal of ring opening is not acid-catalyzed, it must take place via a simple intramolecular displacement of RS- by the sulfinate (SO2-) group present in 3a and is therefore the microscopic reverse of the ring-opening reaction.Rate constants have been determined for both ring opening (kRS) and reversal of ring opening (k-RS) for a series of alkanethiolates of varying pKa.From these data one may also calculate the equilibrium constant, Keq(=kRS/k-RS), for reaction of each thiolate with 1.From comparison of the log Keq's with previously determined equilibrium constants for reaction of cyanide and sulfite ions with 1 one obtains quantitative information on the thermodynamics of reactions of the type ArSSR + CN- = ArSCN + RS- and ArSSR + SO32- = ArSSO3- + RS- that should be of considerable value for predicting the magnitude of equilibrium constants for cyanide-disulfide and sulfite-disulfide equilibria.Plots of log Keq, log KRS, and log k-RS vs. the pKa of RSH reveal that βeq=1.25, βRS=0.26, and β-RS=-0.99.These β values show that the transition state for eq 2 is quite unsymmetrical, with a structure Δ-...S-SO2δ-> where the RS-S bond is only ca. 20percent formed.The βRS and β-RS values are compared with the β values for several other previously studied displacements involving disulfides.The reaction of naphthol-1,2-dithiole 1,1-dioxide (2) with thiolates behaves in a fashion analogous to that of the reaction of RS- with 1.Comparison of Keq, kRS, and k-RS for an equilibrium involving 2 and a thiolate with those for the corresponding thiolate reacting with 1 allows one to assess how a change from a six- to a five-membered thiosulfonate ring influences Keq, kRS, and k-RS.The major effects are that k-RS is much larger and Keq is considerably smaller.
- Boduszek, Bogdan,Kice, John L.
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p. 2055 - 2060
(2007/10/02)
-
- A new structural type in iron-sulfide-thiolate Chemistry: Preparation, properties, and structure of the hexanuclear cluster [Fe6S9(S-t-C4H9)2] 4-
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A reaction system containing FeCl3/Li2S/Li(S-t-Bu)/LiOMe in methanol solution in the initial mole ratios 6:9:(4-8):12 affords the hexanuclear cluster [Fe6S9(S-t-Bu)2]4-, which has been obtained in 50-70% purified yield as its (Me3NCH2Ph)+ salt. The compound (Me3NCH2Ph)4[Fe6S 9(S-t-Bu)2]·MeOH crystallizes in the triclinic space group Ci1-P1 with a = 17.617 (10) A?, b = 19.958 (10) A?, c = 10.162 (7) A?, α = 98.73 (4)°, β = 101.97 (3)°, γ = 72.54 (3)°, and Z = 2 (T = 125 K). On the basis of 4864 unique data (Fo2 > 3σ(Fo2)) the structure has been refined to R = 5.9%. The [Fe6S9]2- core of the cluster is formed of eight nonplanar Fe2S2 rhombs that are fused by edge sharing to give four Fe(μ2-S)(μ3-S)Fe, two Fe(μ2-S)(μ4-S)Fe, and two Fe(μ3-S)(μ4-S)Fe subunits. Their arrangement produces a structure of idealized C2v symmetry in which the twofold axis contains the μ4-S atom and is normal to the Fe4 plane of a pyramidal Fe4(μ4-S) fragment, the central portion of the structure. All Fe atoms are present in tetrahedral FeS4 sites; the thiolates are terminally coordinated to two Fe atoms at the exterior of the structure. The cluster is mixed valence but does not contain localized Fe(II,III) sites. The [Fe6S9]2- core represents a new structural type in iron-sulfide-thiolate chemistry, and [Fe6S9(S-t-Bu)2]4- is the only discrete cluster known to contain three types of bridging sulfur atoms. Other significant properties of [Fe6S9(S-t-Bu)2]4- include chemically reversible redox reactions affording a 3-/4-/5- electron-transfer series and ligand substitution with 2 equiv of benzenethiol to give [Fe6S9(SPh)2]4-, which has been characterized in Me2SO solution. Large excesses of the thiol (?28 equiv) degrade the initial cluster to [Fe4S4(SPh)4]2- and [Fe2S2(SPh)4]2-, which are formed in a ～1.8:1 ratio. Properties common to the three known types of Fe/S/SR clusters, containing Fe2S2, Fe4S4, and Fe6S9 core units, are briefly summarized.
- Christou,Sabat,Ibers, James A.,Holm
-
p. 3518 - 3526
(2008/10/08)
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- Reactivity of Nucleophiles toward and the Site of Nucleophilic Attack on Bis(alkylthio) Selenides
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Bis(alkylthio)selenides, RSSeSR (1), a class of compounds that occupy a key role as intermediates in the incorporation of inorganic selenium into biological systems, react quite readily with nucleophilic reagents.In principle, such reactions could take place at either sulfur (eq 3a) or at selenium (eq 3b).In the present work the reaction of a series of 1 having R groups of varying steric dimensions (n-Bu, i-Pr, t-Bu) with a group of common nucleophiles (thiolate, cyanide, and sulfite ions, phenyllithium, piperidine) has been examined.The results show that when R = t-Bu, reaction with the nucleophile at selenium occurs considerably faster than reaction at sulfur, but that when R is an n-alkyl group (n-Bu), reaction at sulfur is considerably faster than reaction at selenium.With R = i-Pr, the rates of reaction at the two sites are comparable.For reaction at sulfur, the retardation in rate accompanying a change in R from n-Bu to i-Pr to t-Bu parallels closely the decrease in rate with the same change in R that has been observed in other nucleophilic substitution at dicoordinate sulfur.As might be expected, an increase in steric dimensions of R has a considerably smaller effect on the rate of mechanism at selenium.Comparison of the rate of reaction of n-BuS- with n-BuSSeSBu-n and n-BuSSBu-n shows that bis(n-alkylthio) selenides can be expected generally to be more than several orders of magnitude more reactive toward nucleophiles than the corresponding disulfide.
- Kice, John L.,Slebocka-Tilk, Henryka
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p. 7123 - 7130
(2007/10/02)
-
- NEW SYNTHESES OF α-OXIMINO-α-HALOKETONES : REACTIONS OF α-HALOKETONES WITH ALKYL THIONITRITES
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α-Oximo-α-haloketones were readily synthesized in good yields by treating α-haloketone with alkyl thionitrites under mild conditions.
- Kim, Yong Hae,Jung, Sun Ho
-
p. 2371 - 2372
(2007/10/02)
-
- Leaving Group Effects in Thiolester Alkaline Hydrolysis. Part 1. A Keten-mediated (E1cB) Pathway for Basic Hydrolysis of S-Acetoacetylcoenzyme A and Analogues
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The basic hydrolysis of a series of leaving-group substituted acetothiolacetates (CH3COCH2COSR) has been studied in aqueous media.Hydrolysis of N-acetyl-S-acetoacetylcysteinamine follows a kinetic ionisation curve with an inflexion corresponding to the pK of this ester as determined by spectrophotometric and electrometric titrations.The rate constant at high pH was shown to follow a Broensted relationship with βL.G. -1.13, where βL.G. is the slope of a plot of the logarithm of the rate constant versus the pKa of the conjugate acid of the leaving group.This, and other evidence from rate comparisons, activation parameters, and kinetic solvent isotope effects, indicated an E1cB hydrolytic mechanism involving unimolecular collapse of the ester enolate ions via a ketenoid transition-state.S-Acetoacetylcoenzyme-A was also hydrolysed in base by this mechanism.Direct comparison of rates of leaving group expulsion for ArS and ArO was possible by means of this unimolecular process.For a leaving group with pKL.G. 10, the oxyanion departs ca. 1 or 2 orders of magnitude faster than the thiolate anion; for pKL.G. 6.0, the advantage of oxygen over sulphur is 103-104 fold.In a direct structural comparison, PhS departs 32 times as rapidly as PhO.The contribution of steric release in the E1cB transition-state for S-t-butyl acetothiolacetate hydrolysis is discussed.The pKa values of some acetothiolacetates were measured.
- Douglas, Kenneth T.,Yaggi, Norbert F.
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p. 1037 - 1044
(2007/10/02)
-
- Rate Constants and Equilibrium Constants for Thiol-Disulphide Interchange Reactions Involving Oxidized Glutathione
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The rate of reduction of oxidized glutathione (GSSG) to glutathione (GSH) by thiolate (RS-) follows a Broensted relation in pKas of the conjugate thiols (RSH): βnuc ca. 0.5.This value is similar to that for reduction of Ellman's reagent: βnuc ca. 0.4 - 0.5.Analysis of a number of rate and equilibrium data, taken both from this work and from the literature, indicates that rate constants, k, for a range of thiolate-disulphide interchange reactions are correlated well by equations of the form log k = C + βnucpKanuc + βcpKac + βlgpKalg ( nuc = nucleophile, c = central, and lg = leaving group sulfur): eq 36 - 38 give representative values of the Broensted coefficients.The values of these Bronsted coefficients are not sharply defined by the available experimental data, although eq 36 - 38 provide useful kinetic models for rates of thiolate-disulfide interchange reactions.The uncertainty in these parameters is such that their detailed mechanistic interpretation is not worthwhile, but their qualitative interpretation - that all three sulphur atoms experience a significant effective negative charge in the transition state, but that the charge is concentrated on the terminal sulfurs - is justified.Equilibrium constants for reduction of GSSG using α,ω-dithiols have been measured.The reducing potential of the dithiol is strongly influenced by the size of the cyclic disulfide formed on its oxidation: the most strongly reducing dithiols are those which can form six-membered cyclic disulfides.Separate equilibrium constants for thiolate anion-disulphide interchange (KS-) and for thiol-disufide interchange (KSH) have been estimated from literature data: KS- is roughly proportional to 2ΔpKa is the difference between the pKas of the two thiols involved in the interchange.The contributions of thiol pKa values to the observed equilibrium constants for reduction of GSSG with α,ω-dithiols appear to be much smaller than those ascribable to the influence of structure on intramolecular ring formation.These equilibrium and rate constants are helpful in choosing dithiols for use as antioxidants in solutions containing proteines: dithiothreitol (DTT), 1,3-dimercapto-2-propanol (DMP), and 2-mercaptoethanol have especially useful properties.
- Szajewski, Richard P.,Whitesides, George M.
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p. 2011 - 2026
(2007/10/02)
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- Hydrodesulphurization mechanism of thiophene and tetrahydrothiophene on a cobalt molybdenum catalyst
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The hydrodesulphurization mechanism of thiophene and tetrahydrothiophene has been developed at high pressures and over a broad range of temperature and contact time on a commercial CoMo-Al2O3 catalyst.The influence of the pretreatment on the catalyst activity and stability was also studied.The pretreatment with a mixture of H2 and H2S was found to be the most convenient.It was found that the sulphur uptake of the fresh catalyst increases with temperature and that an excess of sulphur in the catalyst leads to an initial higher activity.The thiophene reaction seems to occur simultaneously by two pathways: one consists of ring opening, and the second is yielding tetrahydrothiophene.The latter is the slowest step.The tetrahydrothiophene reacts faster than the thiophene and its reaction mechanism involves mainly the rupture of the C-S bond.However, thiophene was detected in small concentrations, showing the contribution of a second route for the tetrahydrothiophene hydrodesulphurization.Experiments carried out with benzene seem to indicate the existence of three different kinds of active sites in the catalyst: desulphurization, aromatics hydrogenation, and olefin saturation sites.
- Pazos, Jose M.,Andreu, Paulino
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p. 479 - 484
(2007/10/02)
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- Antimicrobial composition and method containing N-(3,5-dihalophenyl)-imide compounds
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Novel N-(3,5-dihalophenyl)imide compounds, which exhibit a strong antimicrobial activity against microorganisms including phytopathogenic fungi, parasites of industrial products and pathogenic microorganisms, represented by the formula, STR1 wherein X and X' each represent halogens and A represents a substituted ethylene such as chloroethylene, C1 - C4 alkylthioethylene, C1 - C2 alkyl-ethylene or 1,2-di-C1 - C2 -alkyl-ethylene, a cyclopropylene such as 1,3-dimethylcyclopropylene, trimethylene, a cyclohexylene-1,2-, cyclohexenylene-1,2-, cyclohexadienylene-1,2- or o-phenylene. The N-(3,5-dihalophenyl)imide compounds can be obtained by any of methods which produce imide compounds or reaction of an N-(3,5-dihalophenyl)maleimide compound with a mercaptan, a hydrogen halide, phosphorus chloride or thionylchloride.
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