Kinetics and mechanism of oxidation of d-glucopyranose 6-phosphate and d-ribofuranose 5-phosphate by vanadium(V) in perchloric acid media

The second-order rate constants for the oxidations of d-glucopyranose 6-phosphate and d-ribofuranose 5-phosphate by vanadium(V) in perchloric acid media have been measured spectrophotometrically in the visible region. The order with respect to [H⁺] is less than unity for each reaction. The enthalpy and entropy of activations of the reactions are higher than those for the corresponding two-electron oxidations by chromium(VI). The oxidation involves free-radical intermediates.     

Kinetics and mechanism of oxidation of d-fructose and l-sorbose by chromium(VI) and vanadium(V) in perchloric acid medium

Kinetic data for the oxidations of d-fructose and l-sorbose by chromium(VI) and vanadium(V) in perchloric acid medium are reported. The addition of perchloric acid and sodium perchlorate increases the pseudo-first-order rate constants. Change of the reaction medium from water to deuterium oxide appreciably affects the rates of chromium(VI) oxidations, but does not affect those of vanadium(V) oxidations. The activation parameters are ΔH3 = 46.6 ±3.4 (fructose) and 50.6 ±6.3 (sorbose) kJ.mol^?1, and ΔS3 = ?105 ±11 (fructose) and ?100 ±20 (sorbose) J.deg^?1.mol^?1 for chromium(VI) oxidations, and, for the other reactions, ΔH3 = 53.2 ±4.2 (fructose) and 52.3 ±6.3 (sorbose) kJ.mol^?1, and ΔS3 = ?139.0 ±14 (fructose) and ?137 ±20 (sorbose) J.deg^?1.mol^?1. The kinetics of the oxidations of ketohexoses by chromium(VI) indicate no intermediate-complex formation, whereas those for vanadium(V) indicate the formation of a 1:1 intermediate complex between ketohexoses and vanadium(V).     

Kinetics and mechanism of oxidation of d-erythrose and dL-glyceraldehyde by chromium(VI) and vanadium(V) in perchloric acid medium

The kinetics of oxidation of d-erythrose and dL-glyceraldehyde by chromium (VI) and vanadium(V) in perchloric acid medium have been investigated spectrophotometrically. Each reaction was first-order with respect to [oxidant] and [substrate]. The reactions were catalysed by acid, but their dependence on acidity was complex. Sodium perchlorate accelerated the rate of each reaction. The oxidation rates follow the order glyceraldehyde > erythrose. The activation parameters were calculated and mechanisms consistent with the experimental observations are proposed.     

The kinetics and mechanisms of the reactions of aluminium(III) with gallic acid, gallic acid methyl ester and adrenaline

The kinetics and mechanisms of the reactions of gallic acid, gallic acid methyl ester and adrenaline with aluminium(III) have been investigated in aqueous solution at 25 degrees C and an ionic strength of 0.5 M. A mechanism has been proposed which accounts satisfactorily for the kinetic data. This is consistent with a mechanism in which complex formation takes place almost exclusively by reaction of [Al(H2O)5OH]2+ with the ligands. [Al(H2O)5OH]2+ reacts with gallic acid, gallic acid methyl ester and adrenaline with rate constants of 1145, 1330 and 316 M(-1) s(-1) respectively. These data together with the equilibrium data enable the rate constants for reaction of [Al(H2O)6]3+ with both gallic acid and gallic acid methyl ester to be calculated. In view of the dissociative nature of water exchange on [Al(H2O)6]3+ and [Al(H2O)5(OH)]2+ the complex formation rate constants are discussed in terms of the Eigen-Wilkins-Tamm mechanism. The overall mechanisms have been validated using global analysis. The results are compared with previously published data on the complex formation reactions of aluminium(III). In addition, the rate constants and mechanisms for replacement of maltol by gallic acid methyl ester and diethylenetriaminepentaacetic acid (dtpa) have been investigated.     

Direct and indirect oxidations by peroxynitrite, neither involving the hydroxyl radical

A new mechanism (Mechanism III) that combines features of mechanisms suggested earlier (Goldstein and Czapski, Inorg. Chem. 34:4041–4048; 1995; Pryor, Jin, and Squadrito Proc. Natl. Acad. Sci. USA 91:11173–11177; 1994) is proposed for oxidations by peroxynitrite. In Mechanism III, oxidations by peroxynitrite can take place either directly by ground-state peroxynitrous acid, ONOOH, or indirectly by ONOOH*, where ONOOH* is an activated form of peroxynitrous acid. In the direct oxidation pathway the reaction is first order in peroxynitrite and first order in substrate, and the oxidation yield approaches 100%. In the indirect oxidation pathway the reaction is first order in peroxynitrite and zero order in substrate. In the presence of sufficient concentrations of a substrate that reacts by the indirect oxidation pathway, about 50–60% of the ONOOH directly isomerizes to nitric acid, and about 40–50% of the ONOOH is converted into ONOOH*. Thus, the oxidation yields by the indirect pathway will not exceed 40–50%, and there will always be a residual yield of nitrate even in the presence of very high concentrations of the substrate. Competitive inhibition studies with various free radical scavengers showed that in some cases these scavengers have no effect on oxidation yields. In others, only partial inhibition was observed, far less than that predicted from to the known rate constants for the reactions of these scavengers with the hydroxyl radical. There are some cases where the extent of inhibition correlates well with the known rate constants of the reactions of these scavengers with hydroxyl radical; nevertheless, even in these cases, the involvement of hydroxyl radicals in indirect oxidations by peroxynitrite is ruled out on the basis of kinetics and oxidation yields. Thus, direct oxidations by peroxynitrite are explained in terms of ONOOH, and indirect oxidations in terms of ONOOH*, and substrates can react by one or both of these pathways.     

Light-induced electron-transfer reactions. Part 2. Kinetics of the intra-molecular redox decomposition of tris(oxalato)-cobaltate(III) by irradiation with visible light of aqueous solution containing tris(2,2′-bipyridine)ruthenium(II) ion

The rate of the intra-molecular redox decomposition of the tris(oxalato)cobaltate(III) ion [Co(Ox)₃]^3? is greatly accelerated by irradiation with visible light of aqueous acidic solutions containing the tris(2,2′-bipyridine)ruthenium(II) ion [Ru(bpy)₃]²⁺. The rate of the light-induced reaction in hydrochloric acid with an acidity range 0.05–0.18 mol dm^?3 is of zero-order with respect to the [Co(Ox)₃]^3? ion concentration and is proportional to the light-intensity irradiated and also essentially to the [Ru(bpy)₃]²⁺ ion concentration. Moreover, the rate is independent not only of the oxalate ion concentrations, but also of the acidity over the range 0.05–0.18 mol dm^?3 hydrochloric acid. The ionic-strength dependence, as well as temperature dependence, were extremely small. The [Ru(bpy)₃]²⁺ concentration does not change during the occurrence of the reaction and the tris(2,2′-bipyridine)ruthenium(II) ion acts as a homogeneous catalyzer. However, a dramatic indication that the situation was rather different was found in the stronger acid solutions of 0.5 or 1.0 mol dm^?3 hydrochloric acid, in which the [Ru(bpy)₃]²⁺ concentration decreased greatly immediately after the initiation of reaction and then increased up to the initial concentration. Such a decrease at the initial stage of the reaction disappeared by addition of oxalate before the start of the reaction. A chain mechanism of reaction is proposed to account for these results.     

Kinetics and mechanism of oxidation of some aldoses by vanadium(v) in perchloric acid medium

The kinetics of oxidation of some aldoses by vanadium(V) in perchloric acid media have been investigated. Each reaction is first order with respect to both [Vanadium(V)] and [Aldose]. The reactions are catalysed by acid. The addition of sodium perchlorate accelerates the rate of reaction. Kinetic evidence for the formation of an intermediate compound between vanadium(V) and aldoses is insignificant, and a mechanism is suggested in which vanadium(V) reacts with the aldoses by a fast step to form a transition state, followed by the decomposition of the latter to give the products of reaction in a slow step. The formation of free-radical intermediates has been demonstrated, and one-electron reduction of vanadium(V) by aldoses seems to be the most plausible mechanism. The oxidation rates follow the order: xyloses arabinose galactose mannose. The activation parameters are reported.     

Mechanistic aspects of uncatalyzed and ruthenium(III) catalyzed oxidation of dl-ornithine monohydrochloride by silver(III) periodate complex in aqueous alkaline medium

The oxidation of an amino acid, dl-ornithine monohydrochloride (OMH) by diperiodatoargentate(III) (DPA) was carried out both in the absence and presence of ruthenium(III) catalyst in alkaline medium at 25 °C and a constant ionic strength of 0.10 mol dm⁻³ spectrophotometrically. The reaction was of first order in both catalyzed and uncatalyzed cases, with respect to [DPA] and was less than unit order in [OMH] and negative fraction in [alkali]. The order with respect to [OMH] changes from first order to zero order as the [OMH] increases. The order with respect to Ru(III) was unity. The uncatalyzed reaction in alkaline medium has been shown to proceed via a DPA-OMH complex, which decomposes in a rate determining step to give the products. Where as in catalyzed reaction, it has been shown to proceed via a Ru(III)-OMH complex, which further reacts with two molecules of DPA in a rate determining step to give the products. The reaction constants involved in the different steps of the mechanisms were calculated for both the reactions. The catalytic constant (K_Cat.const.) was also calculated for catalyzed reaction at different temperatures. The activation parameters with respect to slow step of the mechanism and also the thermodynamic quantities were determined.     

Kinetics and mechanism of the Ir(III)-catalyzed oxidation of xylose and maltose by potassium iodate in aqueous alkaline medium

For the first time, the Ir(III) catalysis of the iodate oxidation of xylose and maltose in aqueous alkaline medium has been investigated. The reactions exhibit first-order kinetics with respect to lower [IO(3)(-)] and [OH(-)] and show zero-order kinetics at their higher concentrations. Unity order at low concentrations of maltose becomes zero order at its higher concentrations, whereas zero-order kinetics with respect to [xylose] was observed throughout its variation. The reaction rate is found to be directly proportional to [Ir(III)] in the oxidation of both reducing sugars. Negligible effect of [Cl(-)] and nil effect of ionic strength (mu) on the rate of oxidation have also been noted. The species, [IrCl(3)(H(2)O)(2)OH](-) was ascertained as the reactive species of Ir(III) chloride for both the redox systems. Various activation parameters have been calculated. Formic acid and arabinonic acid for maltose and formic acid and threonic acid for xylose were identified as the main oxidation products of the reactions. Mechanisms consistent with the observed kinetic data and spectral evidence have been proposed for the oxidation of xylose and maltose.     

Mechanistic flexibility in the reduction of copper(II) complexes of aliphatic polyamines by mercapto amino acids

The reactions of copper(II)-aliphatic polyamine complexes with cysteine, cysteine methyl ester, penicillamine, and glutathione have been investigated, with the goal of understanding the relationship between RS- -Cu(II) adduct structure and preferred redox decay pathway. Considerable mechanistic flexibility exists within this class of mercapto amino acid oxidations, as changes in the rate law could be induced by modest variations in reductant concentration (at fixed [Cu(II)]0), pH, and the structure of the redox partners. With excess cysteine present at 25 degrees C, pH 5.0, I = 0.2 M (NaOAc), decay of 1:1 cys-S- -Cu(II) transient adducts was found to be first order in both cys-SH and transient. Second-order rate constants characteristic of Cu(dien)2+(6.1 X 10(3) M-1 sec-1), Cu(Me5dien)2+ (2.7 X 10(3) M-1 sec-1), Cu(en)22+ (2.1 X 10(3) M-1 sec-1), and Cu(dien)22+ (4.7 X 10(3) M-1 sec-1) are remarkably similar, considering substantial differences in the composition and geometry of the oxidant first coordination sphere. A mechanism involving attack of cysteine on the coordinated sulfur atom of the transient, giving a disulfide anion radical intermediate, is proposed to account for these results. Moderate reactivity decreases in the cysteine-Cu(dien)2+, Cu(Me5dien)2+ reactions with increasing [H+] (pH 4-6) reflect partial protonation of the polyamine ligands. A very different rate law, second order in the RS- -Cu(II) transient and approximately zeroth order in mercaptan, applies in the pH 5.0 oxidations of cysteine methyl ester, penicillamine, and glutathione by Cu(dien)2+ and Cu(Me5dien)2+. This behavior suggests the intermediacy of di-mu-mercapto-bridged binuclear Cu(II) species, in which a concerted two-electron change yields the disulfide and Cu(I) products. Similar hydroxo-bridged intermediates are proposed to account for the transition from first- to second-order transient dependence in cysteine oxidations by Cu(dien)2+ and Cu(Me5dien)2+ as the pH is increased from 5 to 7. Yet another rate law, second order in transient and first order in cysteine, applies in the pH 5.0 oxidation of cysteine by Cu(Me6tren)2+ (k(25 degrees C) 7.5 X 10(7) M-2 sec-1, I = 0.2M). Steric rigidity of this trigonal bipyramidal oxidant evidently protects the coordinated sulfur atom from attack in a RSSR- -forming pathway. Formation of a coordinated disulfide in the rate-determining step is proposed, coupled with attack of a noncoordinated cysteine molecule on a vacated coordination position to stabilize the (Me6tren)Cu(I) product.     

Influence of sodium dodecyl sulfate/TritonX-100 micelles on the oxidation of D-fructose by chromic acid in presence of HClO(4)

The kinetics of oxidation of D-fructose by chromic acid in aqueous and aqueous surfactant (sodium dodecyl sulfate, SDS, and alkylphenyl polyethylenglykol, TX-100) media have been investigated in the presence of HClO(4). The reaction is acid catalyzed and is associated with an induction period which is dependent on [H(+)], [surfactant] and temperature. The order of oxidation during induction under [D-fructose]>[chromic acid] conditions is fractional in each reagent in both media. The rate constant was found to increase with [Mn(II)]. A mechanism has been proposed for the reaction. The micelles produce a catalytic effect in the range of SDS and TX-100 concentrations used, and the effect is explained by means of the pseudo-phase mass-action model. In the presence of SDS, the reaction is inhibited by electrolytes (NH(4)Br, NaBr, LiBr), and the inhibition order Na(+)>Li(+)>NH(4)(+) is explained on the basis of electrostatic considerations. The rate constant (k(m)), binding constants (K(S) and K(F)), and corresponding activation parameters (E(a), delta H( not equal ) and delta S( not equal )) have been evaluated and discussed. The order of reactivities of different sugars is found as: D-fructose>D-arabinose>D-xylose approximately D-glucose.     

Porphinatoiron-mediated oxidation of polycyclic aromatic hydrocarbons

Although porphinatoiron complexes have been used extensively as biomimetic catalysts for oxidation of aliphatic and olefinic hydrocarbons, few oxidations of polycyclic aromatic hydrocarbons (PAH) have been reported. In all cases, heterogeneous iodosobenzene/tetraphenylporphinatoiron(III) systems were employed, oxidations were inefficient and control experiments demonstrating the requirement for catalyst were not described. The current study investigates the oxidation of pyrene, benzo[a]pyrene and benzanthracene in a homogeneous m-chloroperoxybenzoic acid/bifacially hindered porphinatoiron system in which the peroxyacid was shown to be unreactive in the absence of catalyst. Pyrene and benzo[a]pyrene were oxidized efficiently, with pyrene yielding mixtures of 1.6- and 1.8-quinones and benzo[a]pyrene yielding mixtures of phenols and quinones. Benzanthracene was oxidized less efficiently, primarily at the meso positions, to give 7.12-quinone. Initial oxidation of meso carbons of benzo[a]pyrene (confirmed by the presence of the 6-hydroxy derivative as a product) and benzanthracene indicates that PAH-to-catalyst charge transfer may be an important oxidation pathway. Oxidation of pyrene was performed by addition of pyrene to observable oxo iron(V) species as well as in a catalytic reaction where excess peroxyacid was added to a solution of pyrene and catalyst and oxo iron(V) is not generated as an observable intermediate. Yields (based on oxidant consumed), were identical under both conditions, strongly supporting oxo iron(V) as a common intermediate.     

Oxidation of d-fructose with vanadium(V): A kinetic approach

The kinetics of the oxidation of d-fructose with vanadium(V) in perchloric acid have been studied. The reaction is of first order with respect to the [Fructose], but the values of the rate constant increase slightly with increasing [V(V)]. In the range from 0.002–0.02m V(V), the inverse of the second-order rate constant is linearly related to the inverse of [V(V)]. Sodium hydrogensulfate and perchlorate accelerate the reaction, the effect of the former salt being greater. At a constant [H⁺] and ionic strength, the reaction is of first order with respect to [HSO₄⁻]. At constant ionic strength, the reaction is of third order with respect to [H⁺]. The activation parameters have been determined. The data obtained have been compared with those for simpler mono- and poly-hydric alcohols. A possible three-step mechanism involving CH bond fission and yielding glucosones as primary products has been suggested.     

Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose and D-fructose by N-bromoacetamide in the presence of perchloric acid: a kinetic and mechanistic study

The kinetics of Pd(II)-catalysed and Hg(II)-co-catalysed oxidation of D-glucose (Glc) and D-fructose (Fru) by N-bromoacetamide (NBA) in the presence of perchloric acid using mercury(II) acetate as a scavenger for Br- ions have been studied. The results show first-order kinetics with respect to NBA at low concentrations, tending to zero order at high concentrations. First-order kinetics with respect to Pd(II) and inverse fractional order in Cl- ions throughout their variation have also been noted. The observed direct proportionality between the first-order rate constant (k1) and the reducing sugar concentration shows departure from the straight line only at very higher concentration of sugar. Addition of acetamide (NHA) decreases the first-order rate constant while the oxidation rate is not influenced by the change in the ionic strength (mu) of the medium. Variation of [Hg(OAc)2] shows a positive effect on the rate of reaction. The observed negative effect in H+ at lower concentrations tends to an insignificant effect at its higher concentrations. The first-order rate constant decreases with an increase in the dielectric constant of the medium. The various activation parameters have also been evaluated. The products of the reactions were identified as arabinonic acid and formic acid for both the hexoses. A plausible mechanism involving HOBr as the reactive oxidising species, Hg(II) as co-catalyst, and [PdCl3.S]-1 as the reactive Pd(II)-sugar complex in the rate-controlling step is proposed.     

Electron paramagnetic resonance, kinetics of formation and decomposition studies of (bis(hydroxyethyl)amino-tris(hydroxymethyl)-methane)oxochromate(V): a model chromium(V) complex for DNA damage studies

A new chromium complex, (bis(hydroxyethyl)amino-tris(hydroxymethyl)methane)oxochromate(V), has been characterized by epr spectroscopy. The chromium(V) complex was formed by the ligand displacement reaction of bis(2-ethyl-2-hydroxybutanato) oxochromate(V) with bis(hydroxyethyl)amino-tris(hydroxy-methyl)methane (BT). Both epr and kinetic data indicate that the reaction proceeds through a chromium(V) intermediate. Kinetics of formation of the intermediate exhibit a rate saturation at higher [BT] (> 30 mM) indicating a rate law constituting an equilibrium between the parent Cr(V) complex and the bis-tris ligand followed by a pure first order process. The g-value of the intermediate is consistent with a Cr(V) complex in which the BT is coordinated in a bidentate fashion replacing a coordinated hydroxy butanoic acid ligand, affording a mixed ligand complex. The equilibrium step (K = 36 M-1) consists of monodentate coordination by the BT ligand and the limiting first order rate constant (1.9 x 10(-2) s-1) manifests the rate of chelation by the polydentate ligand. The intermediate is converted to the product upon further chelation through the complete displacement of the remaining 2-ethyl-2-hydroxy butanoic acid by a first order process (k = 0.023 s-1). The epr data support a pair of products that are in rapid equilibrium. In these products, BT functions either as a tetra or a penta-dentate ligand coordinating through four or five alkoxy sites. The enthalpy and entropy of activations related to the two chelation steps were found to be 32 +/- 2 kJ/mol and -(1.7 +/- 0.2) x 10(2) J/mol K for the intermediate, and 36 +/- 1 kJ/mol and -(1.5 +/- 0.2) x 10(2) J/mol K for the product. Our data support an associative mechanism for the chelation steps. The Cr(V)-BT product is more stable than the parent complex. The second order disproportionation rate constant for the Cr(V)-BT complex was evaluated to be 0.1 M-1 s-1 compared to 8.0 M-1 s-1 for the parent complex. This is the first example of a chromium(V) complex with a non-macrocyclic ligand coordinating through oxygen donor atoms which is stable in aqueous solution at neutral pH over a long period of time.     

Biochemical characterization of mono(ADP-ribosyl)ated poly(ADP-ribose) polymerase

Here, we report the biochemical characterization of mono(ADP-ribosyl)ated poly(ADP-ribose) polymerase (PARP) (EC 2.4.2. 30). PARP was effectively mono(ADP-ribosyl)ated both in solution and via an activity gel assay following SDS-PAGE with 20 microM or lower concentrations of [32P]-3'-dNAD+ as the ADP-ribosylation substrate. We observed the exclusive formation of [32P]-3'-dAMP and no polymeric ADP-ribose molecules following chemical release of enzyme-bound ADP-ribose units and high-resolution polyacrylamide gel electrophoresis. The reaction in solution (i) was time-dependent, (ii) was activated by nicked dsDNA, and (iii) increased with the square of the enzyme concentration. Stoichiometric analysis of the reaction indicated that up to four amino acid residues per mole of enzyme were covalently modified with single units of 3'-dADP-ribose. Peptide mapping of mono(3'-dADP-ribosyl)ated-PARP following limited proteolysis with either papain or alpha-chymotrypsin indicated that the amino acid acceptor sites for chain initiation with 3'-dNAD+ as a substrate are localized within an internal 22 kDa automodification domain. Neither the amino-terminal DNA-binding domain nor the carboxy-terminal catalytic fragment became ADP-ribosylated with [32P]-3'-dNAD+ as a substrate. Finally, the apparent rate constant of mono(ADP-ribosyl)ation in solution indicates that the initiation reaction catalyzed by PARP proceeds 232-fold more slowly than ADP-ribose polymerization.     

The kinetics and mechanisms of reactions of iron(III) with caffeic acid, chlorogenic acid, sinapic acid, ferulic acid and naringin

The kinetics and mechanisms of the reactions of iron(III) with the hydroxy cinnamic acid based ligands caffeic, chlorogenic, sinapic and ferulic acids and the flavonoid naringin have been investigated in aqueous solution. The mechanisms for caffeic and chlorogenic acid are generally consistent with the formation of a 1:1 complex that subsequently decays through an electron transfer reaction. On reaction with iron(III), ferulic and sinapic acids undergo an electron transfer without the prior formation of any complex. There was no evidence of electron transfer occurring in the complex formed when iron(III) is reacted with naringin. Rate constants for k1 (formation) and k(-1) (dissociation) have been evaluated for the complex formation reactions of [Fe(H2O)6(OH)]2+ with caffeic acid, chlorogenic acid and naringin. Analysis of the kinetic data yielded stability constants, equilibrium constants for protonation of the iron(III) chlorogenic acid complex initially formed, together with the rate constants for complex decomposition through intramolecular electron transfers and in the case of caffeic acid and chlorogenic acid, rate constants for the iron(III) assisted decomposition of the initial complex formed. Some of the suggested mechanisms and calculated rate constants are validated by calculations carried out using global analysis of time dependent spectra.     

Studies on some iridium(III) complexes with Schiff bases derived from amino carboxylic acids

The reactions of iridium(III) chloride with different Schiff bases gave complexes of types [Ir(SB)3], [Ir(SB')Cl(H2O)2], [Ir(SB')Cl2]n, [Ir(SB' ')Cl(H2O)]n (SBH = Schiff bases derived from anthranilic acid and benzaldehyde, acetophenone, vanillin, cinnamaldehyde or m-hydroxyacetophenone; SB'H2 = Schiff bases derived from anthranilic acid and salicylaldehyde or o-hydroxyacetophenone; SB'H = Schiff bases derived from p-aminobenzoic acid and benzaldehyde, acetophenone, vanillin, cinnamaldehyde, or m-hydroxyacetophenone; SB' 'H2 = Schiff bases derived from p-aminobenzoic acid and salicylaldehyde or o-hydroxyacetophenone). These complexes have been characterized on the basis of elemental analyses, conductance, magnetic moment, and spectral (electronic, i.r., and 1H n.m.r.) data. The electronic spectra reveals octahedral geometry for these complexes except for [Ir(SB')Cl2]n, which is trigonal bipyramidal. The thermal behavior of these complexes has also been studied by TG, DTG, and DSC techniques. The different kinetic parameters, viz., order of reaction, activation of energy, and heat of reaction were calculated. The antifungal and antiviral activities of the complexes with Schiff bases derived from anthranilic acid have also been investigated.     

Synthesis and nucleic acids binding properties of diastereomeric aminoethylprolyl peptide nucleic acids (aepPNA)

A general synthetic method for Fmoc-protected monomers of all four diastereomeric aminoethyl peptide nucleic acid (aepPNA) has been developed. The key reaction is the coupling of nucleobase-modified proline derivatives and Fmoc-protected aminoacetaldehyde by reductive alkylation. Oligomerization of the aepPNAs up to 10mer was achieved by Fmoc-solid phase peptide synthesis methodology. Preliminary binding studies of these aepPNA oligomers with nucleic acids suggested that the "cis-" homothymine aepPNA decamers with (2'R,4'R) and (2'S,4'S) configurations can bind, albeit with slow kinetics, to their complementary RNA [poly(adenylic acid)] but not to the complementary DNA [poly(deoxyadenylic acid)]. On the other hand, the trans homothymine aepPNA decamers with (2'R,4'S) and (2'S,4'R) configurations failed to form stable hybrid with poly(adenylic acid) and poly(deoxyadenylic acid). No hybrid formation could be observed between a mixed-base (2'R,4'R)-aepPNA decamer with DNA and RNA in both antiparallel and parallel orientations.     

Interpretation of effects of pH on rate constants for the oxidation of three ferrocytochromes c-551 with [Fe(CN)6]3- and [Co(phen)3]3+, and assignment of pKa values

Effects of pH on second-order rate constants, k (25 degrees C), have been determined for the [Fe(CN)6]3- and [Co(phen)3]3+ oxidations of ferrocytochrome c-551 from Pseudomonas aeruginosa, Pseudomonas stutzeri, and Azotobacter vinelandii. For each oxidant similar directional trends are observed. With [Fe(CN)6]3-, rate constants over the pH 4-9.5 range first decrease, and then increase to plateau pH approximately equal to 9 k values of 0.96.10(5), 4.4.10(5) and 1.05.10(5) M-1.s-1, respectively. With [Co(phen)3]3+, rate constants increase in two separate well-defined stages from pH 2.5-9.5 to plateau pH approximately equal to 9 k values of 1.35.10(5), 3.6.10(5) and 1.37.10(5) M-1.s-1, respectively. From these trends, and consistent with previous NMR studies, protein pKa values of 7.16, 8.00 and 6.67, respectively, for the three reduced cytochromes c-551 are assigned to the buried propionic acid at position 7 on the haem ring. Since at pH greater than 6 the trends with pH for both [Fe(CN)6]3- and [Co(phen)3]3+ are in the same direction, it is concluded that this deprotonation results in a decrease in protein reduction potential. At pH less than 6, the trends with [Co(phen)3]3+ and [Fe(CN)6]3- are in opposite directions. Well defined pKa values of 3.6, 3.80 and 3.80 for P. aeruginosa, P. stutzeri and A. vinelandii, respectively, are observed with [Co(phen)3]3+ as oxidant. Upper limits only of pKa values less than 5.0, less than 4.1 and less than 4.5, respectively, are observed with [Fe(CN)6]3- as oxidant, which may or may not be the same as those observed for [Co(phen)3]3+. These latter pKa values are assigned to carboxylate residues at or near to the binding site(s). It is noted that charged residues are invariant on the front face (incorporating the exposed haem edge) of all three cytochromes c-551, and that there are only two carboxylates. One possibility is that the locality including both carboxylates defined by residues Asp-19, Lys-21, Lys-28 and Asp/Glu-29, serves as a binding site for both 3+ and 3- oxidants.