Unique feature of chain reactions of thiols with quinone imines: thiols as reactants and simultaneously catalysts at the rate-determining step of chain propagation

The isomerization of radical adducts, formed due to the addition of thiyl radicals to the cyclohexadiene ring of quinone imine, to phenoxyl and aromatic aminyl radicals is considered by quantum chemical methods (DFT/PBE and CCSD). Isomerization via the intramolecular transfer of the highly mobile H atom of the C—H bond to the O or N atoms from the position of PhS? radical addition to the cyclohexadiene ring of quinone imine cannot virtually occur because of the high activation energy comparable or even exceeding the C—H bond dissociation energy. An alternative bimolecular mechanism involving the thiol molecule, which is inserted into the transition state thus extending it to be favorable for the reaction to occur, was proposed. After the reaction, the thiol is regenerated, i.e., acts as both the reactant and catalyst of the chain reaction of quinone imine with thiol. The reasons for the high rate of the H atom transfer via this mechanism are considered.     

Potentiometric detection of thiols: a mechanistic evaluation of quinone–thiol interactions

The potentiometric response resulting from the sequential addition of reduced thiols to a benzoquinone/hydroquinone couple has been investigated. The system has been shown to offer a simple route through which thiol could be quantified but a discrepancy in the reaction stoichiometry between glutathione and cysteine was observed. A mechanistic appraisal of the quinone–thiol interactions has been conducted and the proposed reaction pathways corroborated by spectroscopic analysis.     

Doubly Caged Linker for AND‐Type Fluorogenic Construction of Protein/Antibody Bioconjugates and In Situ Quantification

In situ quantification of the conjugation efficiency of azide‐terminated synthetic polymers/imaging probes and thiol‐functionalized antibodies/proteins/peptides was enabled by a doubly caged profluorescent and heterodifunctional core molecule C1 as a self‐sorting bridging unit. Orthogonal dual “click” coupling of C1 with azide‐ and thiol‐functionalized precursors led to highly fluorescent bioconjugates, whereas single‐click products remained essentially nonfluorescent. Integration with FRET processes was also possible. For the construction of antibody–probe conjugates from an anti‐carcinoembryonic antigen and a quinone‐caged profluorescent naphthalimide derivative, the dual “click” coupling process with C1 was monitored on the basis of the emission turn‐on of C1 , whereas prominent changes in FRET ratios occurred for antibody–imaging‐probe conjugates when specifically triggered by quinone oxidoreductase (NQO1), which is overexpressed in various types of cancer cells.     

Site-specific binding of quinones to proteins through thiol addition and addition-elimination reactions

Ubiquinone-0, menaquinone-0, and 2,3,5-trimethyl-1,4-benzoquinone were site-specifically bound to free cysteine of proteins (yeast iso-1 cytochrome c as a model protein) through thioether bond formation. Model thioether quinone conjugates showed unexpected reactivity to cysteine of proteins as their parent quinones by thiol addition-elimination reaction. Cyclic voltammetry studies of the model compounds showed only minor differences in their redox potentials as compared to their parent quinones. Thioether ligation provides a general, simple, and fast method to construct model quinone protein systems. In addition, these studies also contribute to the understanding of biological activities, toxicity, and anti-cancer mechanism of quinones and thioether quinone adducts.     

Characterization of covalent addition products of chlorogenic acid quinone with amino acid derivatives in model systems and apple juice by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry

High-performance liquid chromatography (HPLC) coupled to electrospray ionization tandem mass spectrometry (ESI-MS(n)) was used to study the covalent interactions between chlorogenic acid (CQA) quinone and two amino acid derivatives, tert-butyloxycarbonyl-L-lysine and N-acetyl-L-cysteine. In a model system at pH 7.0, the formation of covalent addition products was demonstrated for both derivatives. The addition product of CQA dimer and tert-butyloxycarbonyl-L-lysine was characterized by LC/MS(n) as a benzacridine structure. For N-acetyl-L-cysteine, mono- and diaddition products at the thiol group with CQA quinone were found. In apple juice at pH 3.6, covalent interactions of CQA quinone were observed only with N-acetyl-L-cysteine. Taking together these results and those reported by other groups it can be concluded that covalent interactions of amino side chains with phenolic compounds could contribute to the reduction of the allergenic potential of certain food proteins.     

紫草萘醌类化合物对乳酸脱氢酶和乙醇脱氢酶的共价修 饰作用

为研究紫草萘醌类化合物的细胞毒性作用机制,从新疆软紫草根中分离出四种紫草萘醌类化合物β,β-二甲基丙烯酰阿卡宁(1),乙酰阿卡宁(2),β-乙酰氧基异戊酰阿卡宁(3)和阿卡宁(4)。研究了四种天然紫草萘醌类化合物对乳酸脱氢酶和乙醇脱氢酶的共价修饰作用。酶活力测定结果表明,这四种紫草萘醌类化合物对两种脱氢酶都具有不同程度的抑制作用;酶分子中游离氨基和巯基修饰率的测定结果表明,紫草萘醌类化合物对两种酶的抑制作用主要是通过与酶分子中的巯基共价结合产生的。     

Chain mechanism of the reactions of <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>′-diphenyl-1,4-benzoquinone diimine with thiophenol and 1-decanethiol

The kinetics of the reactions of N,N′-diphenyl-1,4-benzoquinone diimine with thiophenol and 1-decanethiol in chlorobenzene at 343 K has been investigated spectrophotometrically in an argon atmosphere with monitoring of the disappearance of quinone diimine as its absorbance in the visible range. The acceleration of the reactions in the presence of initiators—tetraphenylhydrazine and azobisisobutyronitrile—indicates that the reactions proceed via a chain mechanism under the chosen experimental conditions. The chain length of the reactions in the absence of an initiator is estimated: ν ≈ 10 units in the reaction of quinone diimine with thiophenol and ν ≈ 100 units in the reaction with 1-decanethiol at a quinone diimine concentration of about 10⁻⁴ mol/L and thiol concentrations of about 10⁻³ mol/L. The dependence of the kinetic parameters of the initiated reaction on the thiophenol concentration suggests that the reaction of the thiyl radical with quinone diimine is the rate-determining step of chain propagation. The rate constant of this reaction is estimated at k _pr = 3.2 × 10⁵ L mol⁻¹ s⁻¹. The rates of chain initiation due to the direct interaction of the initial reactants are estimated. In these reactions, the homolytic cleavage of the S-H bond occurs in the thiol, due to which, other conditions being equal, the radical formation rate in the quinone diiminethiophenol system is at least two orders of magnitude higher than that in the quinone diimine-1-decanethiol, in which the strength of S-H bond is higher. A radical chain mechanism is proposed for the reaction of quinone diimine with the thiols on the basis of the data obtained.     

Mechanisms of photochemical reactions of <Emphasis Type="Italic">para</Emphasis>-benzoquinones with thiols

The mechanisms of quinone reduction by thiols containing α-hydrogen atoms were established using chemically induced dynamic nuclear polarization effects. It was found that substituents in the quinone nucleus change the nature of the primary radical pair. In the photolysis of 2,6-dimethyl-1,4-benzoquinone (1), the radical pair consists of semiquinone and thioalkyl radicals, whereas in the case of 2,6-diphenyl-1,4-benzoquinone (2), the radical pair is composed of semiquinone and thiyl radicals. Quinone 2 is readily photolyzed with any thiol to give dibenzofuran derivative as the final products.     

A mechanistic evaluation of the amperometric response of reduced thiols in quinone mediated systems

The specificity of the reaction of benzoquinone with reduced thiol species has been investigated and the nature of the amperometric electrode response elucidated. The analytical applicability of the methodology has been assessed and it has been shown that through appropriate selection of the redox properties of the indicating quinone, interference from other electroactive species can be minimised. A discrepancy in the reaction stoichiometry has however been found between the glutathione and cysteine quinone adducts and the implications in interpreting the resulting sensor response are rationalised. The adaptation of the approach to disposable, screen printed electrode assemblies has been investigated.     

Electroanalytical exploitation of quinone-thiol interactions: application to the selective determination of cysteine

Square wave voltammetry was applied to the detection of cysteine through the use of an indirect assay that exploits the reaction of the thiol with a quinone indicator. Voltammetric discrimination between unreacted quinone and the corresponding quinone-cysteine adduct is possible with clear resolution of the latter peak providing a linear response from 5 to 47 microM. The selectivity of the approach was assessed with no interference from cystine, lysine, paracetamol or 4-aminophenol. The response recorded in the presence of a massive excess of ascorbic acid was also investigated and the integrity of the approach confirmed. The effects of other sulfhydryl thiols, homocysteine and glutathione, were also assessed and found to present no appreciable change in the voltammetric profile. The practical utility of the approach was investigated through examining the response to cysteine in urine.     

Design and Synthesis of Aromatic Polyesters Bearing Pendant Clickable Maleimide Groups

A bisphenol bearing pendant maleimide group, namely, N‐maleimidoethyl‐3, 3‐bis(4‐hydroxyphenyl)‐1‐isobenzopyrrolidone (PPH‐MA) was synthesized starting from phenolphthalein. Aromatic (co)polyesters bearing pendant maleimide groups were synthesized from PPH‐MA and aromatic diacid chlorides, namely, isophthaloyl chloride (IPC), terephthaloyl chloride (TPC), and 50:50 mol % mixture of IPC and TPC by low temperature solution polycondensation technique. Copolyesters were also synthesized by polycondensation of different molar proportions of PPH‐MA and bisphenol A with IPC. Inherent viscosities and number‐average molecular weights of aromatic (co)polyesters were in the range of 0.52–0.97 dL/g and 20,200–32,800 g/mol, respectively indicating formation of medium to reasonably high‐molecular‐weight polymers. ¹³C NMR spectral analysis of copolyesters revealed the formation of random copolymers. The 10% weight loss temperature of (co)polyesters was found in the range 470–484 °C, indicating their good thermal stability. A selected aromatic polyester bearing pendant maleimide groups was chemically modified via thiol‐maleimide Michael addition reaction with two representative thiol compounds, namely, 4‐chlorothiophenol and 1‐adamantanethiol to yield post‐modified polymers in a quantitative manner. Additionally, it was demonstrated that polyester containing pendant maleimide groups could be used to form insoluble crosslinked gel in the presence of a multifunctional thiol crosslinker. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 630–640     

Smart photoluminescent nanohybrids based on CdSe quantum dots capped with multidentate thiolated pH‐responsive and thermoresponsive polymers for nanosensing

Well‐defined thermoresponsive polymers obtained by the atom transfer radical polymerization (ATRP) of short oligo(ethylene glycol) methyl ether methacrylates (MEO_nMA, n = 2, 3, or 8) with small ratios of a thiolated comonomer, 2‐(acetylthio)ethylmethacrytale, can replace the hydrophobic trioctylphosphine oxide (TOPO) capping of CdSe quantum dots (QDs). After this facile ligand exchange, the mild hydrolysis of the acetylthiol group into thiol is the key to enhance the QD luminescence. However, the length of the ethylene glycol side chain is critical for the success of the functionalization; it is established that the shortest MEO₂MA‐based copolymers result in a compact coating and a highest quantum yield (up to a factor of 6) when compared with that of CdSe@TOPO in dichloromethane. In addition, the amphiphilic character of the copolymer allows the CdSe@P(MEO_nMA‐co‐SEMA) nanohybrids to disperse in water. On the other hand, the residual ionizable thiol groups do not get attached to the QD surface, cause that the lower critical solubility temperature of the polymer depends on pH as well. Thus, at acidic pH, an abrupt increase in the luminescence emission accompanies the polymer collapse, which establishes the promise of these hybrids as temperature/pH nanosensors and targeted drug delivery. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3087–3095     

Bioinspired Functional Catechol Derivatives through Simple Thiol Conjugate Addition

The combination of the surface-adhesive properties of catechol rings and functional moieties conveying specific properties is very appealing to materials chemistry, but the preparation of catechol derivatives often requires elaborate synthetic routes to circumvent the intrinsic reactivity of the catechol ring. In this work, functional catechols are synthesized straightforwardly by using the bioinspired reaction of several functional thiols with o-benzoquinone. With one exception, the conjugated addition of the thiol takes place regioselectively at the 3-position of the quinone, and is rationalized by DFT calculations. Overall, this synthetic methodology provides a general and straightforward access to functional and chain-extended catechol derivatives, which are later tested with regard to their hydro-/oleophobicity, colloidal stability, fluorescence, and metal-coordinating capabilities in proof-of-concept applications.     

A selective colorimetric chemosensor for thiols based on intramolecular charge transfer mechanism

Compound 1 as an electron donor-acceptor compound with N,N-dimethylaniline and quinone units was designed for a highly selective colorimetric determination of thiol-containing amino acids and peptides, by making use of the unique reactivity of thiol towards quinone. Compound 1 shows a strong intramolecular charge transfer (ICT) band around 582 nm; but, it decreased after addition of either cysteine (Cys) or glutathione (GSH). Moreover, the ICT band intensity at 582 nm decreased linearly with the increasing concentrations of Cys or GSH. The interference from other amino acids can be neglected. Therefore, compound 1 can be employed as a selective colorimetric visual chemosensor for thiol-containing amino acids and peptides.     

New Bis(thio)-, Tris(thio)-, and Tetrakis(thio)-substituted Quinones from the Reactions of p-Chloranil with Some Thiols and Dithiols

p-Chloranil (1), when reacted with different thiol compounds, namely 1-buthylthiol, cyclohexylthiol, and 2,2-oxydiethanethiol in ethanol in the presence of Na ₂ CO ₃ , yielded the corresponding S-substituted quinone derivatives.     

A Reactive,Rigid GdIII Labeling Tag for In‐Cell EPR Distance Measurements in Proteins

The cellular environment of proteins differs considerably from in vitro conditions under which most studies of protein structures are carried out. Therefore, there is a growing interest in determining dynamics and structures of proteins in the cell. A key factor for in‐cell distance measurements by the double electron–electron resonance (DEER) method in proteins is the nature of the used spin label. Here we present a newly designed Gd^III spin label, a thiol‐specific DOTA‐derivative (DO3MA‐3BrPy), which features chemical stability and kinetic inertness, high efficiency in protein labelling, a short rigid tether, as well as favorable spectroscopic properties, all are particularly suitable for in‐cell distance measurements by the DEER method carried out at W‐band frequencies. The high performance of DO3MA‐3BrPy‐Gd^III is demonstrated on doubly labelled ubiquitin D39C/E64C, both in vitro and in HeLa cells. High‐quality DEER data could be obtained in HeLa cells up to 12 h after protein delivery at in‐cell protein concentrations as low as 5–10 μm .     

A Reactive,Rigid GdIII Labeling Tag for In‐Cell EPR Distance Measurements in Proteins

The cellular environment of proteins differs considerably from in vitro conditions under which most studies of protein structures are carried out. Therefore, there is a growing interest in determining dynamics and structures of proteins in the cell. A key factor for in‐cell distance measurements by the double electron–electron resonance (DEER) method in proteins is the nature of the used spin label. Here we present a newly designed Gd^III spin label, a thiol‐specific DOTA‐derivative (DO3MA‐3BrPy), which features chemical stability and kinetic inertness, high efficiency in protein labelling, a short rigid tether, as well as favorable spectroscopic properties, all are particularly suitable for in‐cell distance measurements by the DEER method carried out at W‐band frequencies. The high performance of DO3MA‐3BrPy‐Gd^III is demonstrated on doubly labelled ubiquitin D39C/E64C, both in vitro and in HeLa cells. High‐quality DEER data could be obtained in HeLa cells up to 12 h after protein delivery at in‐cell protein concentrations as low as 5–10 μm .     

Ormosil Encapsulated Pyrroloquinoline Quinone‐Modified Electrochemical Sensor for Thiols

An organically modified sol‐gel glass (ORMOSIL) encapsulating pyrroloquinoline quinone (PQQ)‐modified electrode for the rapid, sensitive and simple determination of thiol‐containing compounds such as cysteine and glutathione is reported. The effect of applied potential, nature of thiol compound and pH on the response of the sensor was examined and optimum conditions were determined. The electrochemical responses and detection limits were found to be sensitive to the nature of thiols and pH. The electrochemical responses for cysteine and glutathione at an applied potential of ?0.2 V (vs. Ag/AgCl) were found to be linear with detection limits of 18 nM for cysteine and 36 nM for glutathione at pH 3.5, whereas the detection limits at pH 8.5 were 0.5 μM for cysteine and 1 μM for glutathione. The electrode retained 95% of the original response for 7 days when stored at 4 °C. The ORMOSIL‐encapsulated PQQ was also characterized by spectrophotometry. The absorbance measurement using 5,5′‐dithiobis(2‐nitrobenzoic acid) at 412 nm justify the PQQ‐mediated oxidation of glutathione whereas fluorescence measurements (excitation wavelength=380 nm; emission wavelength=480 nm) justify the successful encapsulation of PQQ in ORMOSIL matrix.     

Amperometric L‐cysteine Sensor Using a Gold Electrode Modified with Thiolated Catechol

A thiolated catechol (CA) consisting of 1,6‐Hexanedithiol (HDT) and CA was modified on a gold (Au) electrode to obtain an amperometric L‐cysteine sensor with detection limit of 60.6 nM. The preparation of thiolated CA was conducted via a thiol addition between HDT and electro‐oxidized CA (EOCA). Briefly, the thiol addition reaction was accomplished by potential cycling of HDT/Au electrode in 0.1 M phosphate buffer (PB, pH 7.2) containing CA, and an EOCA‐HDT/Au electrode was produced. The obtained EOCA‐HDT/Au electrode exhibits a pair of well‐defined redox peaks (at 0.22/0.10 V) of o‐quinone moiety, which effectively mediates the oxidation of L‐cysteine in a 0.1 M PB (pH 7.2), with an over‐potential decrease by ca. 0.12 V (versus bare Au electrode). Electrochemical quartz crystal microbalance, cyclic voltammetry and surface‐enhanced Raman spectra were used to study relevant processes and/or film properties. The amperometric L‐cysteine sensor has good anti‐interferent ability and reproducibility. It also has acceptable recovery for detection of L‐cysteine in urine samples.