Nucleophile Concentration Research Articles

Influence of acid-base equilibrium on interactions of some monofunctional coumarin Pd(II) complexes with biologically relevant nucleophiles-comprehensive kinetic study.

This aimed to develop a comprehensive theoretical protocol for examining substitution reaction processes. The researchers used a theoretical quantum-mechanical protocol based on the QM-ORSA approach, which estimates the kinetic parameters of thermodynamically favourable reaction pathways. This theoretical protocol was validated by experimentally investigating substitution mechanisms in two previously synthesised Pd(II) complexes: chlorido-[(3-(1-(2-hydroxypropylamino)ethylidene)chroman-2,4-dione)]palladium(II) (C1) and chlorido-[(3-(1-(2-mercaptoethylamino)-ethylidene)-chroman-2,4dione)]palladium(II) (C2), along with biologically relevant nucleophiles, namely L-cysteine (l-Cys), L-methionine (l-Met), and guanosine-5'-monophosphate (5'-GMP). Reactions were investigated under pseudo-first-order conditions, monitoring nucleophile concentration and temperature changes using stopped-flow UV-vis spectrophotometry. All reactions were conducted under physiological conditions (pH = 7.2) at 37 °C. The reactivity of the studied nucleophiles follows the order: l-Cys > l-Met > 5'-GMP, and the reaction mechanism is associative based on the activation parameters. The experimental and theoretical data showed that C2 is more reactive than C1, confirming that the complexes' structural and electronic properties greatly affect their reactivity with selected nucleophiles. The study's findings have confirmed that the primary interaction occurs with the acid-base species L-Cys, mostly through the involvement of the partially negative sulfur atom (87.2%). On the other hand, C2 has a higher propensity for reacting with L-Cys-, primarily through the partially negative oxygen atom (92.6%). The implementation of this theoretical framework will significantly restrict the utilization of chemical substances, hence facilitating cost reduction and environmental protection.

Substitution reactions of cis-platinum(II) complexes containing bidentate N,N-donor pyridinecarboxamide ligands with different substituents

Substitution reactions of [2-(pyridinecarboxamide)dichloride Pt(II)] [PtCl2 ], [N-phenyl-(2-pyridinecarboxamide)dichloride Pt(II)] [PhPtCl2 ], [N-(4-methylphenyl)-2-pyridinecarboxamide)dichloride Pt(II)] [CH3PhPtCl2 ], [N-(4-methoxyphenyl)-(2-pyridine-carboxamide)dichloride Pt(II)] [CH3OPhPtCl2 ] and [N-(4-fluorophenyl)-(2-pyridinecarboxamide)dichloride Pt(II)] [FPhPtCl2 ], with nucleophiles; thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) were studied under pseudo first-order conditions. The rates of substitution were investigated as a function of nucleophile concentration and temperature using stopped-flow and UV-visible absorption spectrophotometers. Substitutions of the two coordinated chloride ligands of the Pt(II) complexes occur consecutively, with the first substitution occurring opposite the coordinated pyridyl. The observed pseudo first-order rate constants regressed linearly with concentration of the incoming nucleophiles according to the equation k obs = k 2[Nu]. The highest substitution rates were measured for PtCl2, which is attributed to the unsubstituted non-leaving carboxamide ligand. The Pt(II) center of this complex is the most electrophilic as a result of the strong withdrawal of electron density through π-resonance by the carboxamide group. The introduction of a 4′-substituted phenyl group on the amido N of the carboxamide reduces the reactivity of the complexes due to proportional electronic effects from the ancillary substituents on the phenyl ring as well as its conformational disposition with respect to the plane of the complex. The order of reactivity of studied nucleophiles is TU > DMTU > TMTU. The substitution is associatively activated as supported by the negative entropy of activation values for the reactions.

Stable, Bright, and Long-Fluorescence-Lifetime Dyes for Deep-Near-Infrared Bioimaging.

Near-infrared (NIR) fluorophores absorbing maximally in the region beyond 800 nm, i.e., deep-NIR spectral region, are actively sought for biomedical applications. Ideal dyes are bright, nontoxic, photostable, biocompatible, and easily derivatized to introduce functionalities (e.g., for bioconjugation or aqueous solubility). The rational design of such fluorophores remains a major challenge. Silicon-substituted rhodamines have been successful for bioimaging applications in the red spectral region. The longer-wavelength silicon-substituted congeners for the deep-NIR spectral region are unknown to date. We successfully prepared four silicon-substituted bis-benzannulated rhodamine dyes (ESi5a-ESi5d), with an efficient five-step cascade on a gram-scale. Because of the extensive overlapping of their HOMO-LUMO orbitals, ESi5a-ESi5d are highly absorbing (λabs ≈ 865 nm and ε > 105 cm-1 M-1). By restraining both the rotational freedom via annulation and the vibrational freedom via silicon-imparted strain, the fluorochromic scaffold of ESi5 is highly rigid, resulting in an unusually long fluorescence lifetime (τ > 700 ps in CH2Cl2) and a high fluorescence quantum yield (ϕ = 0.14 in CH2Cl2). Their half-lives toward photobleaching are 2 orders of magnitude longer than the current standard (ICG in serum). They are stable in the presence of biorelevant concentration of nucleophiles or reactive oxygen species. They are minimally toxic and readily metabolized. Upon tail vein injection of ESi5a (as an example), the vasculature of a nude mouse was imaged with a high signal-to-background ratio. ESi5 dyes have broad potentials for bioimaging in the deep-NIR spectral region.

Investigation of Substitution Reactions Between Zinc(II) Complexes with Different Geometries and N-Bonding Nucleophiles

Aims: The study aimed at investigating interactions between zinc(II) complexes with different geometrical structures and relevant nitrogen donor nucleophiles at physiological pH. Background: Lack of clear distinction between the therapeutic and toxic doses of platinum drugs is a major challenge for the design of novel non-platinum DNA and protein targeting metal-based anticancer agents. The non-platinum antitumor complexes could be alternatives to platinum-based drugs due to their better characteristics and different mechanisms of action. Objective: This study could provide more information regarding the design of future zinc-based anticancer drugs, providing a better understanding of the mechanism of interactions between Zn(II) complexes and nitrogen-donor nucleophiles (important from the medical point of view), and clarifying the changes in geometrical structures of zinc(II) that are referred to structure-reactivity correlations. Methods: Mole-ratio method and UV-V is spectroscopic kinetic method have been used. Results: The results indicated additional coordination of chlorides in the first coordination sphere with changes in coordination geometry and formation of the octahedral complex anion [ZnCl4(en)]2- while the excess of chloride did not affect the square-pyramidal structure of [ZnCl2(terpy)]. The substitutions of studied complexes and relevant nucleophiles proceeded in two consecutive reaction steps that depended on the nucleophile concentration. Octahedral complex anion [ZnCl4(en)]2- forms rapidly and all substitution processes of this complex species should be considered. We assume that the first reaction step is accompanied by the dissociation of chloride ligands. Nucleophile 1,2,4- triazoles have shown the highest affinity toward [ZnCl2(en)], and rates of both steps were almost of the same value, indicating parallel reactions. Conclusion: The different order of reactivity of relevant N-donor ligands toward [ZnCl2(en)] and [ZnCl2(terpy)] complexes for the first reaction step occurred because of the influence of different geometrical structures of complexes, while low reaction rates for the second reactions of [ZnCl2(en)] complex with imidazole and pyrazine were a consequence of interconversion between octahedral and tetrahedral structure during substitution processes.

Determination of the Mechanism of Nucleophilic Reaction of Fenitrothion Using Substituted Phenoxide Nucleophiles in Aqueous Media

The mechanism of the nucleophilic displacement reaction at the phosphorus centre of organophosphates was determined. Phenoxide nucleophiles were reacted with fenitrothion (O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate) in water at 25oC and pseudo-first order rate constant measurements taken. Second-order rate constant (kNuc) was determined for the different concentrations of nucleophiles while the second-order rate constant (klg) for the investigation of 2,4-dichlorophenoxide ion with and series of aryl phosphorothioate esters was also determined. Linear free energy relationship was further determined using the Brϕnsted-type plot. The plots are linear over a range of pKaNuc of 7.15-11.10 that straddles the pKa of the leaving 3-methyl-4-nitrophenoxide ion (pKa = 7.20) with statistically acceptable linear correlations (R2 = 0.987) and (R2 = 0.980). The linearity in the traditional Brϕnsted-type plots shows the sensitivity of the nucleophilic displacement to the basicity of the nucleophiles and hence is consistent with a single transition-state mechanism whose barrier to decomposition is low hence concerted. Analysis of the values of βNuc, βLg and βeq (0.734) with the effective charge distribution in the transition state shows that it has no positive character. The Leffler index presents bond formation being slightly ahead of bond rupture.

Chemical Recycling of Post-Consumer PLA Waste for Sustainable Production of Ethyl Lactate

Though synthetic plastics are indispensable in our everyday life, the uncertainty surrounding the sustainability of fossil reserves has led to the development of a novel class of plastics, referred to as bio-based plastics. Poly(lactic) acid (PLA) is the most frequently used member of this family. However, due to the lack of a holistic recycling strategy, its large scale utilization can turn out to be an acute source of plastic pollution in the future. Unlike other attempts directed towards chemical recycling of PLA which violate the basic principles of green chemistry, the following research establishes an eco-friendly recycling concept aimed at the production of a valuable lactate ester through solvent assisted transesterification of PLA waste. The scope of this research is not only limited to the selection of an appropriate system (solvent, nucleophile and catalyst) but also extends to analysing the selectivity of the solvent towards the PLA fraction in a commingled stream and the effect of the concentration of nucleophile and different PLA substrates on the yield of the lactate ester. It was observed that, irrespective of the source of PLA, a high yield of ethyl lactate (approx. 80%) with complete retention of stereochemistry was obtained for a molar ratio of nucleophile per mole repeat unit of PLA (nnuc:nrpu) equivalent to 3. Thus, this work represents an attempt towards instituting circular bio-economy by overcoming the engineering and environmental challenges associated with PLA-waste management and production of ethyl lactate; while strictly adhering to the principles of green chemistry and sustainable chemical engineering.

Controlling the reactivity of [Pd(II)(N^N^N)Cl]+ complexes using 2,6-bis(pyrazol-2-yl)pyridine ligands for biological application: Substitution reactivity, CT-DNA interactions and in vitro cytotoxicity study

Four [(N^N^N)Pd(II)Cl]+ complexes [chloride-(2,2′:6′,2′′-terpyridine)Pd(II)]Cl (PdL1), [chlorido(2,6-bis(N-pyrazol-2-yl)pyridine)Pd(II)]Cl (PdL2), [chlorido(2,6-bis(3,5-dimethyl-N-pyrazol-2-yl)pyridine)Pd(II)]Cl (PdL3) and [chlorido(2,6-bis(3,5-dimethyl-N-pyrazol-2-ylmethyl)pyridine)Pd(II)]BF4 (PdL4) were synthesized and characterized. The rates of substitution of these Pd(II) complexes with thiourea nucleophiles viz; thiourea (Tu), N,N′-dimethylthiourea (Dmtu) and N,N,N′,N′-tetramethylthiourea (Tmtu) was investigated under pseudo first-order conditions as a function of nucleophile concentration [Nu] and temperature using the stopped-flow technique. The observed rate constants vary linearly with [Nu]; kobs = k2[Nu] and decreased in the order: PdL1 > PdL2 > PdL3 ≫ PdL4. The lower π-acceptability of the cis-coordinated N-pyrazol-2-yl groups (which coordinates via pyrazollic-N π-donor atoms) of the PdL2–4 significantly decelerates the reactivity relative to PdL1. Furthermore, the six-membered chelates having methylene bridge in PdL4 do not allow π-extension in the ligand and introduces steric hindrance further lowering the reactivity. Trends in DFT calculated data supported the observed reactivity trend. Spectrophotometric titration data of complexes with calf thymus DNA (CT-DNA) and viscosity measurements of the resultant mixtures suggested that associative interactions occur between the complexes and CT-DNA, likely through groove binding with high binding constants (Kb = 104 M−1). In vitro MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cytotoxic activity data showed that PdL1 was the most potent complex against MCF7 breast cancer cells; its IC50 value is lower than that of cisplatin. The results demonstrate how modification of a spectator ligand can be used to slow down the reactivity of Pd(II) complexes. This is of special importance in controlling drug toxicity in both pharmaceutical and biomedical applications.

Proteomic analysis of the cellular response to a potent sensitiser unveils the dynamics of haptenation in living cells

Haptenation of model nucleophiles, representing the key MIE in skin sensitisation, is routinely measured in chemico to provide data for skin allergy risk assessment. Better understanding of the dynamics of haptenation in human skin could provide the metrics required to improve determination of sensitiser potency for risk assessment of chemicals. We have previously demonstrated the applicability and sensitivity of the dual stable isotope labelling approach to detect low level haptenation in complex mixtures of proteins. In the present study, we investigated haptenation in a relevant living cell model over time at a subtoxic concentration. DNCB, an extremely potent sensitiser, caused minimal changes in overall protein differential expression in HaCaT cells and haptenated approximately 0.25 % of all available nucleophiles when applied at a subtoxic concentration (10μM) for 4 h. The data shows that the maximum level of haptenation occurs at 2 h and that DNCB, whilst being a promiscuous hapten, shows a preference for Cys residues, despite the considerably higher concentration of amine-based nucleophiles. Although a proportion of highly abundant proteins were haptenated, numerous haptenated sites were also detected on low abundant proteins. Certain proteins were modified at residues buried deep inside the protein structure which are less accessible to haptenation compared with surface exposed nucleophiles. The microenvironment of the buried residues may be a result of several factors influencing the reactivity of both the target nucleophile and the hapten.

The influence of α,α′-diimine bridging ligand on the reactivity of binuclear para-cymene ruthenium(II) complexes. Kinetic, mechanistic and computational study

Substitution kinetics of the aqua ligands in four binuclear ruthenium(II) para-cymene complexes with different α,α′-diimine bridging ligands [2-pyridylaldazine (Ru-1), p-phenylenebis(picoline)aldimine (Ru-2), p-biphenylenebis(picoline)aldimine (Ru-3) and p-xylenebis(picoline)aldimine (Ru-4)] was investigated as a function of nucleophile concentration and temperature under pseudo-first order conditions using thiourea nucleophiles. The rates of the simultaneous substitution of the aqua ligands decreased in the order: Ru-1 > Ru-4 > Ru-3 > Ru-2. The reactivity of the complexes is controlled by the inherent electronic and steric contributions of the bridging ligand. The strong π-acceptor bridging ligand is responsible for the high reactivity observed in Ru-1 compared to the rest of the complexes. From Ru-2 to Ru-4, the reactivity increases with decrease in steric congestion around the metal centres. The cage effect plays a role in the enhanced reactivity of Ru-4 compared to Ru-3 and Ru-2. Reactivity trends are excellently supported by computational results. All the complexes showed a stepwise deprotonation of the coordinated aqua ligands except Ru-4 and the pKa values increased from Ru-1 to Ru-4 due to progressive increase in σ-donicity of the spacers. The activation parameters (ΔH≠>0, ΔS≠<0) obtained for all the complexes support an associative mechanism of activation.

Substitution kinetics of platinum N,N,N’,N’-Tetrakis (2-pyridyl methyl) octane diamine aqua complex with thiourea, glutathione and L-cysteine nucleophiles

The ligand substitution kinetics of the platinum(II) complex, [Pt2(L)(OH2)2](ClO4)4 (L = N, N,N’,N’-tetrakis (2-pyridylmethyl) octane diamine) by a series of nucleophiles, viz. thiourea (TU), L-cystein (L-CYST), and glutathione (GLT) were investigated in 0.2 M NaClO4 ionic strength. The reactions were studied under pseudo-first order conditions at different temperatures and nucleophile concentrations. Reactions involving glutathione and L-cysteine were investigated using UV-Vis spectrophotomer while the faster reactions of thiourea were monitored on the stopped-flow spectrofluorimeter. Second order rate constants (M-1s-1) at 298 K are 0.194, 0.567, and 1.505 for GLT, L-CYST, and TU, respectively. The activation parameters obtained for the three nucleophiles in the same order of GLT, L-CYST, and TU, are ΔH≠ (kJ mol-1): 58.0, 13.0, 41.6 and ΔS≠ (J K-1 mol-1): -63.6, -206.1 and -101.8, respectively.

A kinetic investigation of mononuclear trans-platinum(II) complexes with mixed amine ligands

This study was aimed at investigating the substitution behaviour of mononuclear trans-platinum(II) complexes with mixed amine ligands. The rate of substitution of the chloride moieties from the complexes trans-Pt(NH3)(NH2C2H5)Cl2 (tPt2), trans-Pt(NH3)(NH2C3H7)Cl2 (tPt3), trans-Pt(NH3)(NH2C4H9)Cl2 (tPt4) and trans-Pt(NH3)(NH2C5H11)Cl2 (tPt5), by three nucleophiles, viz. thiourea (TU), 1-methyl-2-thiourea (MTU) and 1,3-dimethyl-2-thiourea (DMTU), was studied by stopped-flow spectrophotometry using a large excess of nucleophile. Pseudo-first-order rate constants (kobs) were measured as a function of nucleophile concentration and temperature. Reactions were first order in both [complex] and [nucleophile] and therefore second-order overall (rate = kobs[complex] where kobs = k2[nucleophile]). The kinetics are consistent with a stepwise mechanism involving rate determining substitution of the first chloride followed by a fast second substitution step, with no intermediates being detected. The reactivity of the complexes was largely dependent on the length of the alkyl chain of the alkylamine moiety of the complexes. Computational modelling using density functional theory calculations showed that an increase in chain length by a methylene unit has no direct electronic consequence on the metal centre but did, however, pose significant steric hindrance on the substitution sites due to the flexibility of the alkyl chains and thus governed the overall reaction pattern. 195Pt NMR kinetic studies established that the mixed amine ligands remain coordinated to the metal centre in the final kinetic product. This implies that mononuclear trans-platinum(II) complexes are resistant to complete substitution of ligands by the incoming thiourea nucleophiles at the reaction sites. The reactions follow an associative mechanism of substitution.

The rate of substitution from η6-arene ruthenium(II) complexes

The rate and mechanism of substitution in the Ru(II) complexes (C1–C6) by thiourea nucleophiles was studied at pH 2 and rate constants measured as a function of nucleophile concentrations and temperature using spectrometric methods. There is increased electron density at the Ru metal atom of C2 as a result of inductive donation by substituents on the arene ligand, making it less positive and therefore less reactive than C1. For the complexes C3–C6 bearing the 2,2′-bipyridyl ligand, the aqua ligands are located trans to the arene ligands, and hence, their reactivity increases in accordance to the number and type of alkyl substituents on the η6-arene ligands which donate inductively into the π-molecular orbitals, causing increased trans labialisation of the coordinated aquo co-ligand. Compared to the reactivity of triaquo complex (C1), the auxiliary bipyridyl ligand of (C3) complex lowers the rate of substitution for the later complex by a factor of about 100, possibly due to its steric hindrance at the Ru(II) metal centre. The significantly negative activation entropies and positive activation enthalpies suggest an associative mode of substitution. The reactivity of the nucleophiles follow the order DMTU > TU > TMTU.

Steric and electronic tuning of the reactivity of [RuII(terpy)(N^N)Cl]Cl complexes

The substitution behavior of [RuII(terpy)(phen)Cl]Cl and [RuII(terpy)(tmen)Cl]ClO4 (where terpy = 2,2′:6′,2″-terpyridine, phen = 1,10-phennatroline, tmen = N,N,N′,N′-tetramethylethylenediamine) have been investigated. The crystal structures of the complexes were determined by single crystal X-ray diffraction. The structures of the complexes in solution were confirmed by ESI-MS and multinuclear NMR spectroscopy. The stability of [RuII(terpy)(phen)Cl]+ and [RuII(terpy)(tmen)Cl]+ in aqueous solution, acidity and substitution behavior of the corresponding aqua derivatives with chloride, thiourea and N,N′-dimethylthiourea, were studied. The kinetics of the substitution reactions was followed under pseudo-first order conditions using UV–visible spectroscopy as a function of nucleophile concentration and temperature. The reactivity and pKa values of the tmen and phen complexes were compared with the earlier studied Ru(II) polypyridine complexes. The aqueous behavior of all the examined Ru(II) complexes depends on the electronic and steric properties provided by the spectator bidentate ligands.

Quantitative analysis of micellar effect on the reaction rate of cationic triphenylmethine dyes with water according to Berezin’s model

Several approaches quantitatively describe the effect of surfactant micellar solution on the reaction rate. The most used among them are Piszkiewicz’s, Berezin’s, and Pseudophase Ion-Exchange (PIE) models. The last-named was developed by Bunton and Romsted. Piszkiewicz’s model is based on representations of the micellization according to the mass action law with the formation of a catalytic micelle, which consists of some surfactant molecules and a substrate. In our previously paper, this model was used to explain the kinetic micellar effect on the reaction of cationic triphenylmethine dyes with water once again showed the main disadvantages of this approach. Berezin’s model is based on another model of micelle formation viz. the pseudophase model, and the binding of reagents by micelles is considered as the distribution of a substance between two phases. In this work, we aim to consider the applicability of Berezin’s approach for the interaction of malachite green and brilliant green cations with water molecule as a nucleophile in aqueous systems of nonionic, anionic, cationic, and zwitterionic surfactants. On the whole, Berezin's model performed well when applied to the description of the micellar effect on the reaction of similar dye with the hydroxide ion. However, it was revealed that this model does not take into account the change in the local concentration of the HO– ions due to a compression of the double electric layer upon addition of reacting ions to the system, as well as the constant of association of the HO– ions with cationic head groups of surfactant. In this case, when water is used as a nucleophile, the question of the degree of nucleophile binding can be solved differently. The PIE model is also based on a pseudophase model of micellization, but a substrate binding by micelles is considered as an association in a stoichiometric ratio of 1:1, and a nucleophile concentration is expressed in a local concentration based on the neutralization degree of micelles. Given the latter, its approach cannot be applied to the kinetic micellar influence on the reaction of cationic triphenylmethine dyes with water.

The role of N,N-chelate ligand on the reactivity of (η6-p-cymene)Ru(II) complexes: kinetics, DNA and protein interaction studies

This study reports the kinetic studies of aqua complexes (Ru1–Ru6) as well as the calf-thymus DNA (CT-DNA) and bovine serum albumin binding studies of their chloro derivatives (Ru7–Ru11). The rate of substitution of the aqua ligand(s) in Ru1–Ru6 by thiourea nucleophiles (thiourea, N,N-dimethylthiourea and N,N,N′,N′-tetramethylthiourea) in 0.1 M HClO4/NaClO4 aqueous medium was investigated as a function of nucleophile concentration and temperature under pseudo-first order conditions. The reactivity of the binuclear and mononuclear complexes decreased in the order Ru3 > Ru4 > Ru5 > Ru6 and Ru1 > Ru2, respectively. The trend in reactivity of the binuclear complexes showed a dependence on the electronic and static factors of the rigid N,N-chelate bridging ligand in the complexes. The reactivity trends are well supported by the DFT-calculated data. The activation parameters (ΔH≠ > 0, ΔS≠ < 0) for the substitution process in all complexes support an associative mechanism of activation. The complexes effectively bind to CT-DNA via intercalation and this was corroborated by the molecular docking results. Likewise, the complexes bind favorably with bovine serum albumin. The order of CT-DNA and bovine serum albumin interactions with these complexes are in line with the trends in aqua ligand substitution.

A Self-Assembled Cage with Endohedral Acid Groups both Catalyzes Substitution Reactions and Controls Their Molecularity.

A self-assembled Fe4 L6 cage complex internally decorated with acid functions is capable of accelerating the thioetherification of activated alcohols, ethers and amines by up to 1000-fold. No product inhibition is seen, and effective supramolecular catalysis can occur with as little as 5 % cage. The substrates are bound in the host with up to micromolar affinities, whereas the products show binding that is an order of magnitude weaker. Most importantly, the cage host alters the molecularity of the reaction: whereas the reaction catalyzed by simple acids is a unimolecular, SN 1-type substitution process, the rate of the host-mediated process is dependent on the concentration of nucleophile. The molecularity of the cage-catalyzed reaction is substrate-dependent, and can be up to bimolecular. In addition, the catalysis can be prevented by a large excess of nucleophile, where substrate inhibition dominates, and the use of tritylated anilines as substrates causes a negative feedback loop, whereby the liberated product destroys the catalyst and stops the reaction.

Electrochemical Oxidation of Catechol in the Presence of L-Lysine at Different pH

The reaction of electrochemically produced o-benzoquinone from oxidation of catechol as Michael acceptor with L-lysine as nucleophiles has been studied in aqueous solution with various pH values, different electrodes and different concentration of L-lysine using cyclic voltammetry, controlled potential coulometry and differential pulse voltammetry. The participation of reaction of o-benzoquinone with L-lysine at higher concentration of nucleophiles in the second scan of potential was observed. The products generated from the reaction are assumed to be 2-amino-6-((3,4-dihydroxyphenyl)amino)hexanoic acid that undergo electron transfer at more negative potentials than the catechol. The effect of pH of catechol in presence of L-lysine was studied by varying pH from 5 to 11. The reaction was strongly influenced by the pH as well as concentration of L-lysine. The reaction was mostly favorable in 2 mM of catechol and 70 mM of L-lysine at pH 7. The behavior of the reaction mechanism was of electron transfer, chemical reaction and electron transfer (ECE) type.

Study on the reactions between dichlorido[2,2′:6′,2″-terpyridine] zinc(II) and biologically relevant nucleophiles in aqueous solution

Substitution reactions of square-pyramidal [ZnCl2(terpy)] complex (terpy = 2,2′:6′,2″-terpyridine) with biologically relevant nucleophiles such as imidazole, glutathione, 1,2,4-triazole, and pyrazine were investigated at pH 7.0 as a function of nucleophile concentration. The reactions were followed under pseudo first-order conditions by UV-Vis spectrophotometry. The substitution reactions comprised two steps of consecutive displacement of chlorido ligands. Different reaction pathways for the first reaction step of nucleophilic substitution were defined. The order of reactivity of the investigated nucleophiles for the first reaction was imidazole &gt; glutathione &gt; pyrazine &gt; 1,2,4-triazole, while for the second reaction step it was pyrazine &gt; 1,2,4-triazole &gt; imidazole &gt; glutathione.

Fast living cationic polymerization of isobutyl vinyl ether tailored by single nucleophile in microflow system

The application of living cationic polymerization suffers from complex and costly additives as well as low productivity. In this work, we attempted to develop fast living cationic polymerization of isobutyl vinyl ether (IBVE) tailored by reduced single nucleophile in microflow system. Living polymers were successful obtained for the systems of IBVE-HCl/FeCl3·DO/DO and IBVE-HCl/FeCl3·DOL/DOL. The kinetic behavior of IBVE polymerizations at various concentration of nucleophiles revealed that a microflow system could afford living characteristics as the time to reach 50% conversion (t1/2) is as short as 0.22 s. Using single nucleophile can avoid the negative cooperativity of multi-nucleophiles in stabilizing carbocations, allow the decrement of nucleophile, and enhance the controllability of polymerization process. DFT simulations indicate that only the nucleophile with relatively higher binding energy could endow the system with living characteristic. From this point of view, the addition of other nucleophile with relatively lower binding energy may have no positive effect. These recognitions are meaningful for the development of living cationic polymerization towards commercial application.

Impact of Chloride Concentration on Ligand Substitution Reactions of Zinc(II) Complexes with Biologically Relevant Nitrogen Nucleophiles

The mole-ratio method was used to determine the metal–ligand stoichiometry between [ZnCl2(en)] and [ZnCl2(terpy)] (where en = 1,2-diaminoethane or ethylenediamine and terpy = 2,2′:6′,2″-terpyridine) and imidazole at pH 7.2 in the presence of different chloride concentrations. The results indicated step-wise formation of 1:1 and 1:2 complexes in the presence of 0.010 M NaCl and 1:1 complexes in the presence of 0.001 M NaCl for the [ZnCl2(en)] complex. These results are correlated with additional coordination of chlorides in the first coordination sphere and with changes in coordination geometry. In the presence of 0.001 M NaCl the five-coordinate complex anion [ZnCl3(en)]- is formed initially and then a substitution reaction with imidazole occurs. In the presence of 0.010 M NaCl the octahedral complex anion [ZnCl4(en)]2- is formed. Additional coordination of chloride in the [ZnCl2(terpy)] complex is not found and the metal–ligand stoichiometry is 1:2. The kinetics of ligand substitution reactions of zinc(II) complexes and biologically relevant nitrogen nucleophiles such as imidazole, 1,2,3-triazole and L-histidine were investigated at pH 7.2 as a function of nucleophile concentration in the presence of 0.001 M and 0.010 M NaCl. The reactions were followed under pseudo first-order conditions by UV-Vis spectrophotometry. The substitution reactions included two steps of consecutive displacement of chlorido ligands with changes only in the coordination geometry of the [ZnCl2(en)] complex. The order of reactivity of the investigated nucleophiles for the first reaction step towards both complexes was L-histidine &gt; 1,2,3-triazole &gt; imidazole, while in the presence of 0.010 M NaCl the most reactive ligand was 1,2,3-triazole towards the [ZnCl2(en)] complex.