Athinoula L. Petrou

Kinetics of the equilibration of oxygen with monomeric and octameric hemerythrin from Themiste zostericola

Single relaxations for the equilibration of O2 with monomeric and octameric deoxy forms of hemerythrin from Themiste zostericola have been observed at 25°C using the temperature-jump technique. At 25°C, pH 8.2 (Tris/H2SO4 and I = 0.10 M (Na2SO4), formation rate constants kon are 7.8 · 107 M−1 · s−1 and 7.5 · 106 M−1 s−1, respectively. The procedure used does not give a precise measure (small intercepts) of dissociation rate constants, koff. These were determined instead by the stopped-flow method using dithionite to induce dissociation of the oxy protein. Values of koff for the monomer (3.1 · 102 s−1) and octamer (82 s−1), in association with kon values, lead to equilibrium constants for the formation of oxyhemerythrin of 2.5 · 105 M−1 and 0.9 · 105 M−1, respectively, at 25°C, pH 8.2 and I = 0.10 M (Na2SO4). These latter are in reasonable agreement with values (1.5 105 M−1 and 1.3 · 105 M−1) determined spectrally on the equilibrated solutions. Using the octameric protein, it was shown that replacement of SO42− by ClO4− or Cl− ions (at a constant I = 0.10 M) led to an approximately 2-fold enhancement of kon but had little effect on koff. The addition of Ca2+ or Mg2+ ions (0.01 M), with or without 0.50 M NaCl, also gives up to 4-fold increases in kon, but unchanged koff values. Oxygen pulse experiments on the octamer show no effect on koff of the degree of oxygenation of the protein. A comparison is made with similar data for hemoglobin, myoglobin and hemocyanin.

Kinetic and Theoretical Studies on the Protonation of [Ni(2-SC6H4N){PhP(CH2CH2PPh2)2}]+: Nitrogen versus Sulfur as the Protonation Site

The complexes [Ni(4-Spy)(triphos)]BPh(4) and [Ni(2-Spy)(triphos)]BPh(4) {triphos = PhP(CH(2)CH(2)PPh(2))(2), 4-Spy = 4-pyridinethiolate, 2-Spy = 2-pyridinethiolate} have been prepared and characterized both spectroscopically and using X-ray crystallography. In both complexes the triphos is a tridentate ligand. However, [Ni(4-Spy)(triphos)](+) comprises a 4-coordinate, square-planar nickel with the 4-Spy ligand bound to the nickel through the sulfur while [Ni(2-Spy)(triphos)](+) contains a 5-coordinate, trigonal-bipyramidal nickel with a bidentate 2-Spy ligand bound to the nickel through both sulfur and nitrogen. The kinetics of the reactions of [Ni(4-Spy)(triphos)](+) and [Ni(2-Spy)(triphos)](+) with lutH(+) (lut = 2,6-dimethylpyridine) in MeCN have been studied using stopped-flow spectrophotometry, and the two complexes show very different reactivities. The reaction of [Ni(4-Spy)(triphos)](+) with lutH(+) is complete within the deadtime of the stopped-flow apparatus (2 ms) and corresponds to protonation of the nitrogen. However, upon mixing [Ni(2-Spy)(triphos)](+) and lutH(+) a reaction is observed (on the seconds time scale) to produce an equilibrium mixture. The mechanistic interpretation of the rate law has been aided by the application of MSINDO semiempirical and ADF calculations. The kinetics and calculations are consistent with the reaction between [Ni(2-Spy)(triphos)](+) and lutH(+) involving initial protonation of the sulfur followed by dissociation of the nitrogen and subsequent transfer of the proton from sulfur to nitrogen. The factors affecting the position of protonation and the coupling of the coordination state of the 2-pyridinethiolate ligand to the site of protonation are discussed.

THE USE OF THE ARRHENIUS EQUATION IN THE STUDY OF DETERIORATION AND OF COOKING OF FOODS SOME SCIENTIFIC AND PEDAGOGIC ASPECTS

Conservation and cooking of foods can be used by students and instructors to demonstrate a fundamental relation of chemical kinetics, the Arrhenius equation. By plotting the logarithms of available conservation and cooking times versus the corresponding inverse temperatures, apparent activation energies for both the deterioration and the cooking of foods of various compositions can be obtained. Such simple applications lead to meaningful results. Examples of deviation from the Arrhenius equation are given by plotting data (shelf-life of certain frozen food at various temperatures) given on the food package. A better fit is obtained by applying a second order polynomial regression to the data. Cooking time (lnt) vs. the inverse of temperature for five categories of foods is also examined and for each category there appears to be a common rate-determining step. Detailed results are presented for the meat category. The pedagogic aspects of the use of Arrhenius equation in the study of deterioration and of cooking of foods are also presented. [Chem. Educ. Res. Pract. Eur.: 2002, 3, 87-97]

Kinetics and Mechanism of the Reaction between Chromium(III) and 3,4-Dihydroxy-Phenyl-Propenoic Acid (Caffeic Acid) in Weak Acidic Aqueous Solutions

Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (I d mechanism) of water molecule from the Cr(H2O)5OH2+ coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are ΔH 2(obs) ≠ = 28.6 ± 2.9 kJ mol−1, ΔS 2(obs) ≠ = −220 ± 10 J K−1mol−1, ΔH 3(obs) ≠ = 62.9 ± 6.7 kJ mol−1 and ΔS 3(obs) ≠ = −121 ± 22 J K−1mol−1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease.

Coordination complexes of 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid) with copper(II), nickel(II), cobalt(II) and iron(III)

SummaryThe preparation of complexes of 3,4-dihydroxyphenylpropoionic acid (hydrocaffeic acid): K2[Cu2(hydcafH)2-Cl2]·2KCl·2MeOH, K2[Co(hydcafH)2]·2KCl, K2[Ni2(hydcafH)2Cl2]·2KCl·2MeOH and Fe2 (hydcafH)2Cl2·2KCl·2H2O was achieved. Spectroscopic and magnetic studies are commensurate with tetrahedral structures for the prepared complexes in which the catechol-like coordination is present.

Binuclear vanadium(V) and vanadium(IV, V) complexes of dihydrocaffeic, caffeic and ferulic acids

SummaryBinuclear complexes of dihydrocaffeic, caffeic and ferulic acids with vanadium were prepared and studied. The suggested square-pyramidal structures with catecholic-type coordination are supported by various spectroscopic, magnetic and thermogravimetric data.

Interactions of aluminum(III) with the biologically relevant ligand d-ribose

Abstract Aluminum(III) can be absorbed when it is appropriately complexed. There are several plasma components which can bind weakly Al(III). Many proteins bind Al(III) in solution quite strongly. Carbohydrates bearing an abundance of electronegative functional groups can interact with metal cations. In solution, d -ribose exists as a mixture at equilibrium of many isomers and only a few of them bear a ‘complexing’ sequence of the hydroxyl groups. The presence of d -ribose in an Al(III) solution experiences a decrease of its Bronsted-acid sites. The lowering of the Bronsted acidity of an Al(III)– d -ribose mixture suggests the existence of attractive interactions (‘association’) between Al(III) ion and the complexing sequence of the hydroxyls of d -ribose. There is enhancement in the stability of the interaction complexes between Al(III) and d -ribose through strong intramolecular hydrogen bonding, which offers the possibility to investigate the kinetics of the subsequent proton release reactions. On the basis of the kinetic results, it may be concluded that proton release reactions, which are associated with the complexation reactions, are associatively activated. The complexes (Al(H 2 O) 6− n ( d -ribose − n H ) (3− n )+ ) resulting from the various ‘complexing’ forms of d -ribose are formed at mainly acidic pH. As the pH increases, the values of the activation enthalpy, Δ H ≠ , are changing, because of the formation of mixed hydroxo-complexes (Al(H 2 O) 6− n − m (OH) m ( d -ribose − n H ) (3− n − m )+ ); finally, OH − displaces d -ribose from the coordination sphere of Al(III) in a rather slow process, i.e. with high values of Δ H ≠ ; the activation enthalpy values, Δ H ≠ , decrease with the progression of the displacement, becoming finally very small due to the formation of a precipitate. Chelate coordination of d -ribose with some divalent and trivalent metal ions has been also reported.

A new series of organochromium complexes formed in aqueous solutions

Abstract Chromous ion reacts with 3-pyridineacrylic acid, 4-pyridineacrylic acid, maleic acid and fumaric acid to give remarkably inert organochromium(III) species. These species were isolated by ion exchange from acidic aqueous media and were characterized by a variety of techniques. An organochromium(III) compound was even isolated from the Cr(III)-3-pyridineacrylic acid aqueous mixture in solid form. In all the organometallic compounds reported in this paper chromium(III) is believed to be σ-bonded to carbonyl by the general scheme .

A meta-analysis and review examining a possible role for oxidative stress and singlet oxygen in diverse diseases.

From kinetic data (k, T) we calculated the thermodynamic parameters for various processes (nucleation, elongation, fibrillization, etc.) of proteinaceous diseases that are related to the β-amyloid protein (Alzheimers), to tau protein (Alzheimers, Picks), to α-synuclein (Parkinsons), prion, amylin (type II diabetes), and to α-crystallin (cataract). Our calculations led to ΔG≠ values that vary in the range 92.8-127 kJ mol-1 at 310 K. A value of ∼10-30 kJ mol-1 is the activation energy for the diffusion of reactants, depending on the reaction and the medium. The energy needed for the excitation of O2 from the ground to the first excited state (1Δg, singlet oxygen) is equal to 92 kJ mol-1 So, the ΔG≠ is equal to the energy needed for the excitation of ground state oxygen to the singlet oxygen (1Δg first excited) state. The similarity of the ΔG≠ values is an indication that a common mechanism in the above disorders may be taking place. We attribute this common mechanism to the (same) role of the oxidative stress and specifically of singlet oxygen, (1Δg), to the above-mentioned processes: excitation of ground state oxygen to the singlet oxygen, 1Δg, state (92 kJ mol-1), and reaction of the empty π* orbital with high electron density regions of biomolecules (∼10-30 kJ mol-1 for their diffusion). The ΔG≠ for cases of heat-induced cell killing (cancer) lie also in the above range at 310 K. The present paper is a review and meta-analysis of literature data referring to neurodegenerative and other disorders.

Reaction of Chromium(III) with 3,4-Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k 1(obs)′ = k 0 + k 1(obs)[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: ΔH 1(obs) ≠ = 51.2 ± 11.5 kJ mol−1, ΔS 1(obs) ≠ = −97.3 ± 28.9 J mol−1 K−1 (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are ΔH 2(obs) ≠ = 44.5 ± 5.0 kJ mol−1, ΔS 2(obs) ≠ = −175.8 ± 70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (I d, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.