Formation Of Precursor Complex Research Articles

Kinetics and mechanism of oxidation of 8-hydroxyquinoline and its derivatives by cerium(IV) through precursor complex formation

In acid (HClO4) medium CeIV is reduced to CeIII by 8-hydroxyquinoline (1) and its derivatives, the 5-sulphonate (2) and 7-iodo-5-sulphonate (3), through precursor complex formation. The rates of precursor complex formation have been studied by stopped-flow spectrophotometry for both (1) and (2). Formation of the precursor complex occurs in two concurrent paths involving Ceaq4+ and [Ce(OH)]aq3+ reacting by Ia and Id processes respectively. The reaction with (3) is, however, too fast for the stopped-flow method. The rates for subsequent intramolecular electron transfer processes in the precursor complexes are sufficiently slow for all the three species(1)–(3) for evaluation by conventional spectrophotometry. For the electron transfer process the rate varies in the sequence:(2)>(1)≈(3) due to electron-withdrawing character of the substituent SO3− and electron-releasing character of I.

Kinetic salt effects on the reaction between the [1,3,6,8,10,13,16,19-octa-azabicyclo(666)icosane]cobalt(III) ion, Co(sep)3+, and the hexa-aquochromium(II) ion

Highly specific effects are shown by different salts on the title reaction. Some salts increase the rate by a factor between ca. two and four, whereas for others the increase is between 30 and over a 1000 times. This is attributed to the formation of highly reactive ion pairs (or triplets), whose formation constants have been evaluated, in some cases, from the kinetic data at constant ionic strength. These ion pairs involve essentially the CrII reactant, and their high reactivity is related to the rate of formation of the precursor complex and/or to the rate of electron transfer within the precursor complex.

Early events in DNA packaging in a defined in vitro system of bacteriophage T3

We have developed a defined in vitro system for packaging phage T3 DNA which is composed of purified proheads and the noncapsid proteins gp18 and gp19, products of genes 18 and 19. The reaction requires Mg 2+, ATP, and polyethylene glycol and is inhibited by a nonhydrolyzable ATP analog, adenosine-5′- O-(3′-thiotriphosphate)(ATP-γ-S) ( K. Hamada, H. Fujisawa, and T. Minagawa, 1986, Virology 151, 119–123). About 30% of added mature T3 DNA was packaged into heads in the defined system. A complex with a sedimentation coefficient of about 50 S (50 S complex) accumulated in the reaction mixture containing ATP-γ-S. The 50 S complex was DNase sensitive and was converted to filled heads by a second reaction in the presence of ATP without addition of DNA, proheads, gp18, and gp19. These results indicate that during early stages of DNA packaging, formation of precursor complexes proceeds by an allosteric mechanism with ATP acting as effector. The movement of DNA into the head is driven by the energy released by hydrolysis of ATP. gp18 formed a complex with DNA without addition of ATP-γ-S and gp19. gp18-DNA complex was DNase sensitive and did not bind gp19; it was converted to filled heads by way of a second reaction after addition of ATP, gp19, and proheads. gp19 formed a functional complex with prohead in the presence of ATP-γ-S or ATP. The complex did not bind gp18 but was converted to filled heads by incubation with ATP, gp18, and DNA. In the absence of ATP-γ-S, gp19 formed complexes with prohead that were abortive in DNA packaging. Formation of the 50 S complex occurred in a reaction mixture containing gp18-DNA and gp19-prohead complexes in the presence of ATP-γ-S. From these results, we propose details of the molecular mechanism of DNA packaging in the defined in vitro system.

Studies in nickel(IV) chemistry. Kinetics of electron transfer from dimethyl sulfoxide to oxohydroxonickel(IV) in aqueous acid medium

AbstractThe kinetics of decomposition of “oxohydroxonickel(IV)” [Ni(IV)] with concomitant intramolecular electron transfer to produce hexaaquanickel(II) and dioxygen in aqueous acid solutions show pseudo‐first‐order dissappearance of the Ni(IV). The pseudo‐first‐order rate constants for the acid decomposition (kad) satisfy where KMH and kd refer to the equilibrium protonation constant and the decomposition constant of the protonated species of the Ni(IV) respectively. The values of KMH and kd in aqueous medium at 45°C and μ = 2.0M are 25.5 ± 1M−1 and (1.7 ± 0.1) × 10−5 s−1, respectively.The kinetics of the intermolecular electron transfer from dimethyl sulfoxide (DMSO) to the Ni(IV), producing Ni(H2O)62+ and dimethyl sulfone as products, have been investigated by monitoring the formation of Ni(H2O)62+. The pseudo‐first‐order rate constants for the electron transfer kobs are linearly dependent on [DMSO]0 or [H+], attaining limiting values at higher relative [DMSO]0 or [H+], in accordance with where K1c and K2c represent the formation constants of the precursors involving DMSO and the unprotonated and one‐protonated Ni(IV) species, respectively, and k1x and k2x are the corresponding decomposition rate constants of the precursors. The values of K2c and k2x are (2.3 ± 0.1) × 104M−1 and 19 ± 1 s−1, respectively, at 45°C and μ = 1.0M. Results are interpreted in terms of probable mechanisms involving (1) a rate‐determining decomposition of the protonated Ni(IV) followed by rapid product formation steps, and (2) precursor complex formation between DMSO and the unprotonated or the protonated species of the Ni(IV) followed by rate‐determining decomposition with electron transfer.

Studies in nickel(IV) Chemistry. Kinetics of the aquacopper(II)‐catalyzed oxidation of phenylhydrazine by tris(dimethylglyoximato)‐nickelate(IV) in aqueous medium

AbstractThe kinetics of electron transfer in the redox system containing phenylhydrazine (S) and tris(dimethylglyoximato)nickelate(IV), in the presence of catalytic amounts of added Cu(II)aq, have been studied in aqueous medium at an ionic strength of 0.25M in the pH range of 6.01–9.06. The kinetics exhibit pseudo‐zero‐order disappearance of Ni(IV) when an excess of [S]0 and small amounts of Cu(II) are present. While the pseudo‐zero‐order rate constants are almost linearly dependent on [S]0 at constant [Cu(II)] and pH tending to become non‐linearly dependent on higher relative [S]0, they are linearly dependent on [Cu(II)] in a 20‐fold range. The pH‐rate profiles with low [S]0 and [Cu(II)] show a monotonic decrease in rates with increasing pH, the rates tending to attain limiting values at higher relative pH. Results are interpreted in terms of a probable mechanism involving the formation of precursor complexes of phenylhydrazine and Cu(II) species in the medium, followed by the rate‐determining breakdown of the precursors with concomitant electron transfer. The hydrolyzed species of Cu(II) reacts more slowly than does the aquacopper(II). Ni(IV) does not appear to have any kinetic role in the redox system and is involved only in rapid product formation steps. The oxidation product of phenylhydrazine is 4‐hydroxyazobenzene.

Studies in Nickel(IV) Chemistry. V. Kinetics of Electron Transfer in Redox Systems Containing Tris(dimethylglyoximato)nickelate(IV), Hydroxylamine, and Catalytic Amounts of Copper(II) in Aqueous Alkaline Media

Abstract Kinetics of electron transfer in the redox system containing hydroxylamine and tris(dimethylglyoximato)nickelate(IV) ([Ni(dmg)3]2−), in the presence of catalytic amounts of added copper(II) ions, have been studied in aqueous medium in the pH range 8.04–11.81. The kinetics exhibit pseudo-zero-order disappearance of [Ni(dmg)3]2− when excess [NH2OH]0 is present, in the presence and absence of added metal ions. While addition of Cu(II) results in significant catalysis, that of Co(II)/Co(III), Fe(II)/Fe(III), Ni(II) or Zn(II) does not. The pseudo-zero-order rate constants (k0) are found to be linearly dependent on [NH2OH]0 and on added [Cu(II)]. The k0 values register a decrease as [OH−] is increased in the pH range 8.04–ca.9.42, attaining a limiting value in the pH range ca. 9.8–ca. 11.81. Results are interpreted in terms of a probable mechanism involving formation of precursor complexes from hydroxylamine and Cu(II) species, followed by rate-determining breakdown of the precursors with concomitant electron transfer. The Cu(II)·aq species reacts several orders of magnitude faster than the Cu(OH)2·aq species. The Ni(IV) does not appear to have any kinetic role in the redox system and is involved only in rapid product formation steps. The stoichiometric ratios (Δ[Ni(IV)]/Δ[NH2OH]) of ca. 1.0 in the lower pH region (7.8–8.6) and ca. 0.5 in the higher pH region (9.1–11.6) are explained in terms of differential product formation.

Electron transfer in catecholamines - tetrapyridine-iron(III) complex ions anchored to polypeptides

[Fe(tetpy)(OH) 2] + complex ions (tetpy = 2,2′,2′',2‴-tetrapyridyl) anchored to poly(L-glutamate) (FeL) or poly(D-glutamate) (FeD) through the γ-carboxylate groups of the polymers [1] exhibit stereoselective peroxidatic activity with a number of optically active substrates, according to the reaction (pH = 7.0, Tris buffer 0.05 M) [2, 3]: ▪ Progressive binding of complex ions was found to determine a coil-to-α-helix transition in the charged polypeptide matrices, as well as aggregation phenomena with a freezing of iron molecules inbetween helical chains. Under these conditions, electron transfer from catecholamines (L-adrenalin and L-dopa) to the central iron(III) ion, which is rate-determining, proceeds stereoselectively because extensive and possibly specific interactions between substrate molecules and the peptidic residues in the close environment of the active sites different steric constraints for the two diastereomeric precursor complexes. The rate constants of electron transfer processes can be expressed by nuclear and electronic factors which are highly sensitive to the separation of the redox centers [4]. Therefore, even small differences in steric hindrances between the diastereo-isomers would affect differently the optimal mutual orientation of the reacting OH group of the substrates and the peripheral tetrapyridyl ligand of the active sites, whose π-system very likely acts as an electron-transfer agent [5]. Preliminary results on conformational analysis of substrate-catalyst adducts support such conclusions. This is reflected in the rate constants of the electron-transfer step, as illustrated by the Lineweaver-Burk plot of the reaction catalyzed by FeL or FeD systems at a complex to polymer-residue ratio [C]/[P] = 0.20, reported in Fig. 1. From the results, the turnover numbers for the oxidation of L-adrenaline are 0.56±0.07 and 2.04±0.23 min −1, respectively, whereas those for the oxidation of L-dopa are 0.42±0.05 and 1.14±0.12 ▪ min −1 (25.9 °C). On the other hand, the Michaelis constants are K M = (1.30±0.18) × 10 −3 and (1.05±0.15) × 10 −3 M in the former case, and (8.0±1.12) × 10 −4 and (5.50±0.48) × 10 −4 M in the latter. According to Marcus theory [6], the ultimate rate of electron transfer depends on the rate of reorganization of the surrounding medium (precursor and successor complex formation). When the asymmetric polymers play an active role in the reaction, being involved in the formation of the precursor complex the polypeptide itself might experience local conformational changes. Since the time scale of these rearrangements is unknown, it is possible that the polymer reorganization retards the overall rate of the electron transfer process. This could explain the finding that stereoselectivity occurs at the expense of the efficiency of reaction, as shown in Fig. 2. Using catalysts at low [C]/[P] ratio, the active sites are exposed to the bulk solvent and a substrate-coordinated metal chelate forms without any ▪ assistance from the coiled polypeptides [3]. This accounts for the lack of stereoselectivity in the oxidation reaction, which takes place at much higher rate however (Fig. 2).

The relationship between enthalpy changes and oxygenation constants for cobalt and iron porphyrins

The dioxygen uptake ability of Co(T( p-CH 3O)PP)B, where T( p-CH 3O)PP is the dianion of 5,10,15,20-Tetra( p-methoxyphenyl)porphyrin and B is pyridine, piperidine, and N-methylimidazole, was studied in toluene and methylene chloride. A general trend between log K O 2 (standard state of 1 Torr) and ΔH O 2 was found for cobalt synthetic complexes, cobalt ‘natural’ complexes, and iron ‘natural’ complexes. The linear relationships have similar slopes, similar temperature depedences, but different intercepts. The difference in intercepts is attributed to differences in entropies arising from solvation effects. When the latter linear relationship is coupled with the near linear dependence of ΔH for precursor complex formation, Co(T( p-CH 3O)PP) + B ⇄ Co(T( p-CH 3O)PP)B, and ΔH O 2 , a powerful predictive tool arises for the estimation of the oxygen uptake ability of the complex from the enthalpy change of the precursor formation reaction.

The Kinetics and mechanisms of oxidation of dihydroxybenzenes by ethylenediaminetetraacetatomanganate(III)

The kinetics and mechanism of oxidation of 1,4-dihydroxybenzene (H2Q), 2,5-dihydroxybenzenesulphonic acid (H2SQ) and 4,5-dihydroxy-1,3-benzenedisulphonic acid (H2T) to the correspondingortho orPara-quinones by ethylenediaminetetraacetatomanganate (III) has been investigated over the 2.5–6.0 pH range, at 20.0° by the stopped-flow technique. Complex behaviour is observed for H2Q due to the superposition of the precursor complex formation and the redox act, while at higher pH's these reactions separate. For the other substrates, a single second order step (first order in each reactant) was present whose rate showed different pH dependences. All these features are discussed with reference to the reduction potential of the substrates, their acidity and coordinating ability.

Mechanism of the chromium(2+), vanadium(2+), and hexaammineruthenium(2+) reduction of N-bonded glycinato- and .beta.-alaninatopentaamminecobalt(III) complexes. Evidence for paths involving precursor complex formation with outer-sphere electron transfer

Mechanism of the chromium(2+), vanadium(2+), and hexaammineruthenium(2+) reduction of N-bonded glycinato- and .beta.-alaninatopentaamminecobalt(III) complexes. Evidence for paths involving precursor complex formation with outer-sphere electron transfer

Oxidation of cuprous stellacyanin by aminopolycar☐ylatocobaltate(III) complexes

Rate parameters are reported for the oxidation of cuprous stellacyanin by Co(PDTA) − ( k(25.0°) = 17.9 M −1sec −1, Δ H ≠ = 8.5 kcal/mol. Δ S ≠ = −24 cal/mol-deg; pH 7.0, μ 0.5 M) and Co(CyDTA) − ( k(25.1°) = 17.0 M −1 sec −1, Δ H ≠ = 8.7 kcal/mol, Δ S ≠ = −24 cal/mol-deg; pH 7.0, μ 0.5 M). The first order Co(PDTA) − and Co(CyDTA) − dependences observed over wide concentration ranges contrast with the saturation behavior reported previously for Co(EDTA) − as the oxidant. It is concluded that the -CH 3 and -(CH 2) 4-substituents of PDTA and CyDTA, respectively, prevent the alkylated derivatives of Co(EDTA) − from hydrogen bonding with the reduced blue protein, causing precursor complex formation constants to fall far below that of 149 M −1 (25.1°) observed for the EDTA complex. The similarity between Δ H ≠ and Δ S ≠ values for the oxidation of stellacyanin by Co(PDTA) − and Co(CyDTA) − indicates that the size of alkyl substituents linked to the carbon atoms of the EDTA ethylenediamine backbone has little influence on activation requirements for Cu(I) to Co(III) electron transfer. The electron transfer reactivity of aminopolycar☐ylatocobalt(III) complexes with cuprous stellacyanin therefore appears to be linked to the accessibility of one or more of the ligated acetate groups to outer-sphere contact with the type 1 Cu(I) center. Saturation in k obsd vs. [oxidant] plots found for the reactions of Co(PDTA) − and Co(CyDTA) − with stellacyanin at pH 6 and at pH 7 in the presence of EDTA is attributed to the formation of “dead-end” oxidant-protein complexes.

Inner-sphere electron-transfer reactions of cobalt(III). Nonbridging ligand effects, activation energies, and precursor complex formation

Inner-sphere electron-transfer reactions of cobalt(III). Nonbridging ligand effects, activation energies, and precursor complex formation

Evidence for precursor complex formation in Cr2+-Co(III) reactions

Evidence for precursor complex formation in Cr2+-Co(III) reactions