Kinetics and mechanism of adenosine 5′-triphosphate hydrolysis catalyzed by the Cu2+ ion: The role of conformation and the catalytic effect of the OH− ion

The kinetics of 5′-ATP hydrolysis catalyzed by the Cu²⁺ ion has been investigated by HPLC in the pH range 5.6–7.8 at 25°C. Two series of experiments differing in the initial [Cu · ATP]₀ (1: 1) concentration have been carried out. The reaction was being conducted up to ≈40% ATP conversion. The (CuATP^2?)₂OH^??ub;DOH^??ub; complex, which consists of two monomeric Cy(CuATP^2?) molecules (in which the N7 atom and the γ-phosphate group are coordinated to Cu²⁺), is responsible for the formation of CuADP^? + P_i (P_i is an inorganic phosphate). The highest possible DOH^? concentration at a given pH is reached at the initial stage of hydrolysis. The pH value at which the highest initial rate of ADP formation is reached (pH_max (w _{0, ADP})) decreases as the D concentration increases. At pH > pH_max, the decrease in the ADP formation rate in the course of the processes is pH-independent and, once an ATP conversion of 20–26% is reached, hydrolysis proceeds in a steady-state regime such that ADP and AMP form from ATP by parallel reactions. The participation of the OH^? ion in the catalysis of the formation of hydrolysis intermediates is considered.     

Binding of AMP,ADP, and ATP Nucleotides by Polyammonium Macrocycles

The macrocyclic polyamines 4 – 6 , when protonated, bind strongly and selectively nucleotides (AMP, ADP, ATP) and pyrophosphate in aqueous solution. The stoichiometry of the complexes formed was determined by titration experiments followed by ³¹ P-NMR spectroscopy. Compounds 4 and 5 form 1:1 complexes with ATP, ADP, and pyrophosphate, whereas 6 forms complexes with ATP and ADP involving 2 nucleotides and 1 receptor molecule. The stability constants of these complexes have been determined by pH-metric measurements. At pH 7, both 5 and 6 give complexes of mainly the fully protonated species 5 . 6H ⁺ and 6 . 8H ⁺, whereas 4 yields predominantly complexes of 4 . 5H ⁺ and 4 . 4H ⁺.     

Thermodynamics of protonation of AMP,ADP, and ATP from 50 to 125°C

The interaction of adenosine 5-monophosphate (AMP), adenosine 5-diphosphate (ADP), and adenosine 5-triphosphate (ATP) ions with protons in aqueous solution has been studied calorimetrically from 50 to 125°C and 1.52 MPa. At each temperature, the reaction of acidic AMP with tetramethylammonium hydroxide was combined with the heat of ionization for water to obtain the enthalpy of protonation of AMP, while the reactions of HCl with deprotonated tetramethylammonium salts of ADP and ATP were used to obtain the enthalpies of protonation of ADP and ATP. Equilibrium constant K, enthalpy change H^o, entropy change S^o, and heat capacity change C _p ^o values were calculated for the stepwise protonation reactions as a function of temperature. The reactions involving the first protonation of AMP, ADP, and ATP and the third protonation of ADP and ATP were endothermic over the temperature range studied, while that involving the second protonation is exothermic for AMP and ADP, but is exothermic below 100°C and endothermic at 125°C in the case of ATP. Consequently, log K values for the first and third protonation reactions (phosphate groups) increase while those for the second protonation reaction (N₁-adenine) decrease in the cases of AMP and ADP and go through a minimum in the case of ATP as temperature increases. The H^o values for all protonation reactions increase with temperature. The magnitude and the trend for the H^o, S^o, and C _p ^o values with temperature are discussed in terms of solvent-solute interactions. The magnitude of the C _p ^o values for the second protonation is consistent with little interaction between the phosphate ion and the protonated N₁ site of the adenine moiety in AMP, but indicates moderate interaction between these groups in ADP, and strong interaction in ATP.     

Mathematical modeling of kinetics of adenosine-5’-triphosphate hydrolysis catalyzed by the Zn2+ ion in the pH range 8.5–9.0

The kinetics of adenosine-5’-triphosphate (ATP) hydrolysis catalyzed by Zn²⁺ at pH 8.5–9.0 is analyzed by numerical simulation. The rates of product formation (adenosine diphosphate (ADP) and adenosine monophosphate (AMP)) are determined by a conformational transformation. In the sequence of steps of mutual transformations of cyclic (Cy) pH-dependent species, which are active in ATP hydrolysis to ADP, and open (Op) species, the rate-limiting step is the slow isomerization of ZnATP²-complexes. This slow step is determined by the abstraction of the OH^- group from a pentacovalent intermediate catalyzed by H₃O⁺. In the Op species,Zn ²⁺ is bound to the phosphate chain. In the Cy species, which can be hydrolized to ADP, Zn²⁺ coordinates a nitrogen atom in position 7 and γ-phosphate. The mutual transformations of conformers occur via pentacovalent intermediates with the participation of γ-phosphorus and include pseudotransformations. In the direct transformation CyOH^-⦚r OpOH^-, pseudotransformation is a rate-controlling step. The deprotonated open monomeric form OpOH^- is inactive in hydrolysis. Within the framework of the dimeric model and a more complex model that accounts for the role of trimeric associates ZnATP^2-, the general scheme of intermediate transformations is considered that accounts for the existence of a pH-independent pathway of hydrolysis. The rate and equilibrium constants are estimated. Concentration profiles for intermediate products during hydrolysis are described.     

Larval Tenebrio molitor (Coleoptera: Tenebrionidae) Fat Body Extracts Catalyze Firefly D-Luciferin- and ATP-Dependent Chemiluminescence: A Luciferase-like Enzyme*

Ultraweak light emission was detected upon injection of firefly luciferin into live Tenebrio larvae. A chemilumi-nescent enzymatic activity dependent on molecular oxygen, D-luciferin and MgATP was then isolated from larval fat body extracts by precipitation with 70% ammonium sulfate. D-Luciferin and ATP can be replaced by luciferyl-adenylate. Pyrophosphate is a main product from the chemiluminescent reaction. The in vitro chemiluminescence intensity was not affected by peroxidase inhibitors such as N₃^?_- (0.5 mM) and CN^? (1 mM), attesting to its nonperoxidatic nature but was strongly inhibited by AMP (1 mM), luciferin 6′-ethyl ether (1 mM) and sodium pyrophosphate (2 mM), well-known firefly lucifer-ase inhibitors. Some physical-chemical properties of this enzymatic activity were similar to those of firefly lucif-erase (K_MATP = 195 μM; K_0.5 luciferin - 0.8 mM; optimum pH 8.5; δ_max= 610 nm at pH 8.5; firefly lucifer-ase: δ_max= 565 nm at pH 8.0 and 619 mm at pH 6.0), but the chemiluminescence was not affected by addition of polyclonal antibodies raised against Photinus pyralis luciferase. These data suggest that this chemiluminescence results from a ligase with luciferase activity.     

Mathematical Modeling of Kinetics of Adenosine 5"-Triphosphate Hydrolysis Catalyzed by the Zn2+ Ion in the pH Range 7.4–8.3

Kinetic data on adenosine triphosphate (ATP) hydrolysis catalyzed by the Zn²⁺ ion in the pH range 7.4–8.3 are analyzed by the method of numerical simulation. The rates of forward and reverse reactions of isomeric conversion of the open conformation of ZnATP^2– (Op), which is inactive in hydrolysis to ADP, to the active cyclic conformation ZnATP^2– (Cy) in the specified range of pH are proportional to the concentration of H₃O⁺ and characterized by the same rate constants as in the range of pH above 8.5. The mechanism of the isomeric conversion Op Cy involves the formation of a pentacovalent state at -P, pseudorotation, and the abstraction of OH^– from -P of the pentacovalent intermediate with the participation of H₃O⁺ in a slow step. The sequence of steps for the formation and transformation of intermediates, which was established earlier for the ZnATP^2– associates in the pH range 7.1–7.4, is applicable to this range of pH as well. In the analyzed range of pH, the contributions from the pH-independent channel of hydrolysis of the ZnATP^2– associates and the pH-dependent channel of CyOH^– and Op(OH^–)₂ species, which determine the formation of ADP and AMP at pH > 8.5, are comparable. Changes in the concentrations of intermediate products (monomeric and associates) in the course of hydrolysis are described. General base catalysis by a nitrogen base in the steps of formation of active centers for hydrolysis, the general acid catalysis of a coordinated water molecule, the exchange of medium OH^– with OH of -phosphate, the catalysis of conversion of the inactive conformation ZnATP^2– to the active one by a proton, and a change of the rate-limiting stage of hydrolysis with a change in pH indicate the enzyme-like mechanism of the reaction.     

Metal complexes with macrocyclic ligands. Part XXXIV. Geometrical and conformational changes during the on/off reaction of the side chain in the CuII complex of 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane

Solution studies of the Cu²⁺ complex with 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane(L) indicate that, depending on the pH and on the age of the solution, different species are present. Dissolving the solid [CuL](ClO₄)₂ in slightly acidic solution gives the protonated complex AH , characterized by an absorption maximum at 574 nm, by a relatively fast proton-induced dissociation kinetics and by the typical colour change in basic solution to give the deprolonated form A with coordinated side chain. AH slowly interconverts in acidic solution to a new species BH , which has an absorption maximum at 547 nm, and which is kineticaily more stable against acid dissociation and shows no coordination of the amino group of the side chain. In alkaline solution, however, the deprotonated form B deliver A in a base induced reaction. The X-ray diffraction studies of A and BH allow to determine the geometry of the metal ion and the configuration of the macrocycle. In A , the Cu²⁺ is pentacoordinated by the five N-atoms of the ligand and the macrocycle is in the RRSR configuration, whereas in BH the Cu²⁺ is octahedrally coordinated by the four N-atoms of the macrocycle and two axial perchlorate O-atoms with the macrocycle in the RRRS configuration. The amino group of the side chain is protonated and not coordinated. Thus, the on/off equilibrium of the side chain not only changes the geometry of the metal ion, as is generally found, but also alters the macrocyclic moiety.     

Coordination Centres of Purine Nucleotides: Adenosine‐5′‐diphosphate and Adenosine‐5′‐triphosphate in their Reactions with Nickel(II), Cobalt(II) and Tetramines

Interactions between the nucleotides: adenosine‐5′‐diphosphate (ADP) and adenosine‐5′‐triphosphate (ATP) with Ni^II and Co^II ions, as well as with spermine (Spm) and 1,11‐diamine‐4,8‐diazaundecane (3,3,3‐tet) are the subject of this study. Composition and stability constants of mixed complexes thus formed have been determined on the basis of the potentiometric measurements, whereas interaction centres in ligands have been identified by VIS and NMR spectral parameter analysis. Mixed tetraprotonated complexes with Ni^II, i.e. Ni(ADP)H₄(Spm), Ni(ATP)H₄(Spm), Ni(ADP)H₄(3,3,3‐tet) and Ni(ATP)H₄(333‐tet), are identified as ML·······L′ type adducts, in which the main coordination centre is the nucleotide nitrogen N(1) or N(7) donor atom, and the fully protonated polyamine is engaged in noncovalent interactions with nucleotide phosphate group oxygen atoms. Ni(ADP)H₂(Spm), Ni(ATP)H₂(Spm), Ni(ADP)H₂(3,3,3‐tet) and Ni(ATP)H₂(3,3,3‐tet) complexes represent the {N3} coordination type In diprotonated mixed complexes of Ni^II with spermine are weak noncovalent interligand interactions, providing an additional stabilising effect. Formation of ML·······L′ type molecular complexes has been observed in systems with Co^II: Co(ADP)H₄(Spm), Co(ATP)H₄(Spm), Co(ADP)H₄(3,3,3‐tet) and Co(ATP)H₄(3,3,3‐tet), in which the N(7) atom and oxygen atoms of the phosphate group are involved in coordination and the fully protonated polyamine is engaged in noncovalent interactions with the nucleotide N(1).     

Simulation of the kinetics of adenosine 5′-triphosphate hydrolysis catalyzed by the Cu2+ ion: The role of conformation and the catalytic effect of the OH− ion

The hydrolysis kinetics of the dimeric complex (CuATP^2? · OH₂)₂ {D} up to ≈40% ATP conversion at 25°C, pH 5.7–7.8, and [Cu · ATP]₀ = (2.07 ± 0.03) × 10^?3 mol/l is analyzed by numerical simulation. CuADP^? + P_i (P_i is an inorganic phosphate) form from DOH^?, and the latter forms rapidly from D. The abstraction of H⁺ from the coordinated H₂O molecule is an irreversible reaction involving an OH^? ion from the medium. The maximum possible DOH^? concentration at a given pH is reached at the initial stage of hydrolysis (0.3–6.0 min after the initiation of hydrolysis). CuADP^? + P_i form from D via two consecutive irreversible steps. The ADP buildup rate in the process is determined by the reversible conformational transformation of DOH^? resulting in a pentacovalent intermediate (IntK). OH^? ions from the medium are involved both in IntK formation and in the reverse reaction and are a hydrolysis inhibitor. AMP forms from the intermediate IntK₃, which forms reversibly from DOH^?, OH^? ions from the medium being involved in the forward and reverse reactions. This is followed by irreversible (AMPH)^? formation involving H₃O⁺ ions from the medium. The rate and equilibrium constants are determined for the formation and decomposition of hydrolysis intermediates. The concentrations of the intermediates are plotted versus time for various pH values. The structures of the intermediates are suggested. The causes of a peak appearing in the initial ADP formation rate versus pH curve are analyzed.     

Fluorescence enhancement of terbium(III) by nucleotides and polyhomonucleotides in the presence of phenanthroline

Summary The fluorescence enhancement of terbium(III) by nucleotides (AMP, ADP, ATP, GMP, GDP, GTP) and polyhomonucleotides [poly(A), poly(G), poly(C), poly(U)] in the presence of phenanthroline (phen) was studied. Investigation of the composition of the terbium(III)/ANP(AMP, ADP, ATP)/phen complexes and conditions of optimization suggest a 1:2 molar ratio of terbium(III) and phen for the ternary complexes. The results showed that the presence of phen enhanced the net fluorescence of terbium(III)/ANP, poly(A), poly(C) or poly(U) from several fold to more than one-hundred fold, while it has little effect on the fluorescence of terbium(III)/GNP(GMP, GDP and GTP) or the poly(G) system. The possibility of spectrofluorimetric measurements of these compounds were studied under optimal conditions (pH 7.0 in tris-HCl buffer; E_x=298 nm, E_m=543.5 nm). The detection limits were 2.0×10^–7, 6.0×10^–7 and 1.0×10^–6 mol/l for AMP, ADP and ATP, respectively. The relative standard deviations (6 replicates) were within 2.0% in the middle of the linear range.     

Serum Albumin Targeted,pH‐Dependent Magnetic Resonance Relaxation Agents

The objective of this work was the synthesis of serum albumin targeted, Gd^III‐based magnetic resonance imaging (MRI) contrast agents exhibiting a strong pH‐dependent relaxivity. Two new complexes ( Gd‐glu and Gd‐bbu ) were synthesized based on the DO3A macrocycle modified with three carboxyalkyl substituents α to the three ring nitrogen atoms, and a biphenylsulfonamide arm. The sulfonamide nitrogen coordinates the Gd in a pH‐dependent fashion, resulting in a decrease in the hydration state, q, as pH is increased and a resultant decrease in relaxivity (r₁). In the absence of human serum albumin (HSA), r₁ increases from 2.0 to 6.0 mM ^?1 s^?1 for Gd‐glu and from 2.4 to 9.0 mM ^?1 s^?1 for Gd‐bbu from pH 5 to 8.5 at 37 °C, 0.47 T, respectively. These complexes (0.2 mM ) are bound (>98.9 %) to HSA (0.69 mM ) over the pH range 5–8.5. Binding to albumin increases the rotational correlation time and results in higher relaxivity. The r₁ increased 120 % (pH 5) and 550 % (pH 8.5) for Gd‐glu and 42 % (pH 5) and 260 % (pH 8.5) for Gd‐bbu . The increases in r₁ at pH 5 were unexpectedly low for a putative slow tumbling q=2 complex. The Gd‐bbu system was investigated further. At pH 5, it binds in a stepwise fashion to HSA with dissociation constants K_d1=0.65, K_d2=18, K_d3=1360 μM . The relaxivity at each binding site was constant. Luminescence lifetime titration experiments with the Eu^III analogue revealed that the inner‐sphere water ligands are displaced when the complex binds to HSA resulting in lower than expected r₁ at pH 5. Variable pH and temperature nuclear magnetic relaxation dispersion (NMRD) studies showed that the increased r₁ of the albumin‐bound q=0 complexes is due to the presence of a nearby water molecule with a long residency time (1–2 ns). The distance between this water molecule and the Gd ion changes with pH resulting in albumin‐bound pH‐dependent relaxivity.     

Halogenation and amination dual properties of haloamines in Raschig environment: A chlorine transfer reaction between chloramine and 3-azabicyclo[3,3,0]octane

The chlorine transfer reaction between 3-azabicyclo[3,3,0]octane “AZA” and chloramine was studied over pH 8–13 in order to follow both the amination and halogenation properties of NH₂Cl. The results show the existence of two competitive reactions which lead to the simultaneous formation of N-amino- and N-chloro- 3-azabicyclo[3,3,0]octane by bimolecular kinetics. The halogenation reaction is reversible and the chlorine derivative obtained, which is thermolabile and unstable in the pure state, was identified by electrospray mass spectrometry. These phenomena were quantified by a reaction between neutral species according to an apparent S_N2-type mechanism for the amination process and a ionic mechanism involving a reaction between chloramine and protonated amine for the halogenation process. Amination occurs only in strongly basic solutions (pH ≥ 13) while chlorination occurs at lower pH's (pH ≤ 8). At intermediate pH's, a mixture of these two compounds is obtained. The relative proportions of the products are a function of intrinsic rate constants, pH and pK_a of the reactants. The rate constants and thermodynamic activation parameters are the following: k₁ = 45.5 × 10⁻³ M⁻¹ s⁻¹; ΔH₁^0# = 59.8 kJ mol⁻¹; ΔS₁^0# = − 86.5 J mol⁻¹ K⁻¹ for amination; k₂ = 114 × 10⁻³ M⁻¹ s⁻¹; ΔH₂^0# = 63.9 kJ mol⁻¹; and ΔS₂^0# = − 48.3 J mol⁻¹ K⁻¹ for chlorination. The ability of an interaction corresponding to a specific (NH₃Cl⁺/RR′NH) or general (NH₂Cl/RR′NH) acid catalysis has been also discussed. © 1997 John Wiley & Sons, Inc.     

Mechanistic Studies on the Oxidation of Glyoxylic and Pyruvic Acid by a [Mn4O6]4+ Core in Aqueous Media: Kinetics of Oxo‐Bridge Protonation

In aqueous media (pH 2.5–6.0), the Mn^IV tetramer [Mn₄(μ‐O)₆(bipy)₆]⁴⁺ ( 1 ⁴⁺; bipy = 2,2′‐bipyridine) oxidizes both glyoxylic and pyruvic acid to formic and acetic acid, respectively, under formation of CO₂. Kinetics studies suggest that the species 1 ⁴⁺, its oxo‐bridge protonated form [ 1 H]⁵⁺, i.e., [Mn₄(μ‐O)₅(μ‐OH)(bipy)₆]⁵⁺, the reducing acids (RH) and their conjugate bases (R^?) all take part in the reaction. The oxo‐bridge protonated oxidant [ 1 H]⁵⁺ was found to react much faster than 1 ⁴⁺. Thereby, the gem‐diol forms of the α‐oxo acids (especially in the case of glyoxylic acid) are the possible reductants. A one‐electron/one‐proton electroprotic mechanism operates in the rate‐determining step.     

Myosin-catalyzed ATP hydrolysis elucidated by 31P NMR kinetic studies and 1H PFG-diffusion measurements

We conducted ³¹P NMR kinetic studies and ¹H-diffusion measurements on myosin-catalyzed hydrolysis of adenosine triphosphate (ATP) under varied conditions. The data elucidate well the overall hydrolysis rate and various factors that significantly impact the reaction. We found that the enzymatic hydrolysis of ATP to adenosine diphosphate (ADP) was followed by ADP hydrolysis, and different nucleotides such as ADP and guanosine triphosphate acted as competitors of ATP. Increasing ATP or Mg²⁺ concentration resulted in decreased hydrolysis rate, and such effect can be related to the decrease of ATP diffusion constants. Below 50 °C, the hydrolysis was accelerated by increasing temperature following the Arrhenius’ law, but the hydrolysis rate was significantly lowered at higher temperature (~60 °C), due to the thermal–denaturation of myosin. The optimal pH range was around pH 6–8. These results are important for characterization of myosin-catalyzed ATP hydrolysis, and the method is also applicable to other enzymatic nucleotide reactions.     

ATP Sensing with Anthryl‐Functionalized Open‐Chain Polyaza‐alkanes

The anthryl‐functionalized open‐chain polyaza‐alkanes L ¹ , L ² , and L ³ have been synthesized, and their activity as fluorescent chemosensors has been studied in MeCN/H₂O 70 : 30 (v/v) and H₂O at 25° against the anions bromide, phosphate, sulfate, ATP, ADP, and GMP. The crystal structure of L ³ has been solved by single‐crystal X‐ray‐diffraction techniques. The emission intensity of L ¹ and L ² is selectively quenched in the presence of ATP at acidic pH in MeCN/H₂O 70 : 30 (v/v). In H₂O, the emission intensity of L ¹ and L ² is enhanced at neutral pH in the presence of ADP and ATP. The sensing behavior is discussed in terms of H‐bonding or electrostatic anion‐cation interactions. Receptor L ³ does not show any significant change in fluorescence emission upon addition of anions. Protonation constants of the three ligands and stability constants of L ² with phosphate and sulfate were determined by potentiometric titration in MeCN/H₂O. The stability constants obtained are compared with those obtained for the interaction of these anions with related open‐chain polyamines.     

An ammonium/bis-ammonium switchable molecular shuttle

A pH shuttle has been developed in which the mean position of the macrocycle can be switched between dialkylammonium stations of differing acidities. With only the most basic binding site protonated (dibenzylammonium), the macrocycle resides almost exclusively on the protonated station; when the second ammonium group (diethylammonium) is generated by further protonation, a mixture of translational diastereomers is observed in CD₂Cl₂ at 298 K, with the diethylammonium binding site preferred by the crown ether macrocycle in a ratio of ∼2:1.     

Influence of the Preparation Procedure on the Catalytic Activity of Gold Supported on Diamond Nanoparticles for Phenol Peroxidation

The catalytic activity of diamond‐supported gold nanoparticle (Au/D) samples prepared by the deposition/precipitation method have been correlated as a function of the pH and the reduction treatment. It was found that the most active material is the one prepared at pH 5 followed by subsequent thermal treatment at 300 °C under hydrogen. TEM images show that Au/D prepared under optimal conditions contain very small gold nanoparticles with sizes below 2 nm that are proposed to be responsible for the catalytic activity. Tests of productivity using large phenol (50 g L ^?1) and H₂O₂ excesses (100 g L ^?1) and reuse gives a minimum TON of 458,759 moles of phenol degraded per gold atom. Analysis of the organic compounds extracted from the deactivated solid catalyst indicates that the poisons are mostly hydroxylated dicarboxylic acids arising from the degradative oxidation of the phenyl ring. By determining the efficiency for phenol degradation and the amount of O₂ evolved two different reactions of H₂O₂ decomposition (the Fenton reaction at acidic pH values and spurious O₂ evolution at basic pH values) are proposed for Au/D catalysis. The activation energy of the two processes is very similar (ranging between 30 and 35 kJ mol^?1). By using dimethylsulfoxide as a radical scavenger and N‐tert‐butyl‐α‐phenylnitrone as a spin trap under aerated conditions, the EPR spectrum of the expected PBN? OCH₃ adduct was detected, supporting the generation of HO^., characteristic of Fenton chemistry in the process. Phenol degradation, on the other hand, exhibits the same activation energy as H₂O₂ decomposition at pH 4 (due to the barrierless attack of HO^. to phenol), but increases the activation energy gradually up to about 90 kJ mol^?1 at pH 7 and then undergoes a subsequent reduction as the pH increases reaching another minimum at pH 8.5 (49 kJ mol^?1).     

A selective ATP chromogenic sensor for use in an indicator displacement assay

We synthesized a dinuclear Zn²⁺ complex that is useful as a sensor for ATP in a DMSO/H₂O (1:9, v/v) solvent system via a simple indicator displacement assay (IDA). This chromogenic sensor method can be used to analyze 0.1-2.0 μM of ATP with no interference from ADP or AMP.     

Kinetics and mechanisms of chlorine transfer from chloramine to amines in aqueous medium

The kinetics and the equilibrium constant of the chlorine transfer reaction between monochloramine NH₂Cl and the amines: C₂H₅NH₂, (CH₃)₂CHNH₂, (CH₃)₂NH, and (C₂H₅)₂NH are investigated by spectrophotometry in aqueous medium at 25°C, in the pH range from 8 to 13 and for an ionic strength equal to 1.03 ± 0.05M. For a concentration of total ammonia equal to 1M, the observed rate constant is pH independent below 8 and above 12.8 and reaches a maximum located between the pK_as of NH₄⁺ and RR'NH₂⁺. From these results and those obtained earlier for NH₂Cl and CH₃NH₂, the reaction is shown to involve an interaction between neutral molecules NH₂Cl and RR'NH, subject to general acid catalysis. The ability of an interaction corresponding to a specific catalysis and involving NH₃Cl⁺ and RR'NH rather than NH₂Cl and RR'NH₂⁺ is also discussed. The activation parameters are given for each reaction.     

Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl^–, NH⁺₄, and N₂O. The rate law is given by ?[d[NH₂Cl]/dt] = k₊[NH₃OH⁺][NH₂Cl] + k₀[NH₂OH][NH₂Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k₊= (1.03 ± 0.06) ×10³ L · mol^?1 · s^?1 and k₀=91 ± 15 L · mol^?1 · s^?1. Isotopic studies demonstrate that both nitrogen atoms in the N₂O come from the NH₂OH/NH₃OH⁺. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be ΔH? = 36 ± 3 kJ · mol^–1 and Δ S? = ?66 ± 9 J · K^?1 · mol^?1, and Δ H? = 12 ± 2 kJ · mol^?1 and Δ S? = ?168 ± 6 J · K^?1 · mol^?1, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006