Cleavage of the peptide bond of beta-alanyl-L-histidine (carnosine) induced by a Co(III)-amine complexes: reaction, structure and mechanism

Cleavage of the peptide bond occurs when beta]-alanyl-L-histidine (carnosine) reacts with [Co(tren)Cl2]+ (tren = tris(2-aminoethyl)amine) to give [Co(tren)(histidine)](2+) 1 and [Co(tren)(beta-alanine)](2+) 2. [Co(tren)(histidine)](2+) 1 crystallizes in the enantiomorphic space group P2(1)2(1)2(1) and 2 crystallizes in the P2(1)/c space group. The mechanism of the cleavage reactions were studied in detail for the precursor [Co(tren)Cl2]+ and [Co(trien)Cl2]+, which convert into [Co(tren)(OH)2]+/[Co(tren)(OH)(OH2)]2+ and [Co(trien)(OH)2]+/[Co(trien)(OH)(OH2)]2+ in water at basic pH (trien = 1,4,7,10-tetraazadecane). At a slightly basic pH, the initial coordination of the substrate (beta-alanyl-L-histidine) is by the carboxylate group for the reaction with [Co(tren)Cl2]+. This is followed by a rate-limiting nucleophilic attack of the hydroxide group at the beta-alanyl-L-histidine carbonyl group. In a strongly basic reaction medium substrate, binding of the metal was through carboxylate and amine terminals. On the other hand, for the reaction between [cis-beta-Co(trien)Cl2]+ and beta-alanyl-L-histidine, the initial coordination of the substrate takes place via an imidazole ring nitrogen, independently, and followed by a nucleophilic attack of the hydroxide group at the beta-alanyl-L-histidine carbonyl group. The circular dichroism spectrum for 1 suggests that a very small extent of racemization of the amino acid (L-histidine) takes place during the cleavage reaction between [Co(tren)Cl2]+ and beta-alanyl-L-histidine. Reaction between [cis-beta-Co(trien)Cl2]+ and beta-alanyl-L-histidine also causes cleavage of the peptide bond, producing a free beta-alanyl molecule and a cationic fragment [cis-alpha-Co(trien)(histidine)](2+) 3 that crystallizes in the optically active space group P2(1)2(1)2(1). Unlike the previous case an appreciable degree of racemization of the L-histidine takes place during the reaction between [cis-beta-Co(trien)Cl2]+ and beta-alanyl-L-histidine. Crystals containing L-histidine and D-histidine fragments in the [cis-alpha-Co(trien)(histidine)]2+ moiety were crystallographically documented by mounting a number of randomly selected crystals.     

Dinuclear Co(III)/Co(III) and Co(II)/Co(III) mixed-valent complexes: synthetic control of the cobalt oxidation level

The reactivity of cobalt(II) salts towards H(3)L (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) was studied in different reaction conditions. Accordingly, the interaction of cobalt(II) acetate with H(3)L in methanol gives rise to the discrete complex [Co(III)(2)L(OAc)(2)(OMe)]*1.5H(2)O.MeOH, 1. Reaction of cobalt(II) acetylacetonate with H(3)L in the presence of dicarboxylic acids was also investigated. Thus, when cobalt(II) acetylacetonate and H(3)L are mixed with terephthalic or malonic acid in 4 : 2 : 1 molar ratios, the mixed valent [Co(II/III)(2)L(acac)(p-O(2)CC(6)H(4)CO(2)H)][Co(II/III)(2)L(acac)(OH)]*2H(2)O*2MeOH, 2 and [Co(II/III)(2)L(acac)(O(2)CCH(2)CO(2)H)][Co(II/III)(2)L(acac)(OH)]*7H(2)O, complexes are isolated. Decreasing the pH of the medium, by addition of a second mol of dicarboxylic acid, leads to [Co(II/III)(2)L(O(2)CCH(2)CO(2))(MeOH)]*2MeOH, 4, while the reaction with terephthalic acid does not proceed. 1, 2 and 4 were crystallographically characterised and all the complexes are dinuclear, with hydrogen bonds that expand the initial nodes. The magnetic characterisation, as well as the NMR spectroscopy, indicates a diamagnetic nature for 1, in agreement with the presence of Co(III), showing the aerial oxidation suffered by the cobalt(II) ions. Nevertheless, are paramagnetic. Temperature variable magnetic measurements were recorded for the crystallographically characterised complexes 2 and 4 and these studies confirm the mixed valence Co(II)/Co(III) nature of the compounds. The best fits of the magnetic data give an axial distortion parameter Delta = 628.7 cm(-1) for 2 and 698.8 cm(-1) for 4, and spin-orbit coupling constant lambda = -117.8 cm(-1) for 2 and -107.0 cm(-1) for 4. Therefore, this study shows that the oxidation degree of the initial cobalt(ii) salt by atmospheric oxygen can be controlled according to the pH of the medium.     

Binding studies of DNA with Co(III) coordination compound

The binding of Co(bpy)₂dppz³⁺ to calf thymus DNA was investigated by using absorption and emission spectroscopy, DNA melting techniques, cyclic voltammetry, viscosity and electro phoresis measurements, where bpy is 2,2′-bipyridyl, dppz is dipyrido[3,2-a:2′,3′-c] phenazine. The binding compound shm absorption hypochromicity, fluorescence enhancement, and increasing of DNA melting temperature and the specific viscosity. CV measurement shows the shifts in oxidation-reduction potential and change in peak current with addition of DNA. The compound ie also shown to be more efficient photosensitisers for strand breaks in plasmid DNA.     

Photoinduced degradation by iron(III): removal of triphenyltin chloride from water

The photoredox process taking place in iron(III) aquacomplexes was used to cause the complete degradation of triphenyltin (TPT). TPT elimination was proved to come only from attack by hydroxyl radicals generated upon irradiation at 365 nm of Fe(H₂O)₅OH²⁺, the iron(III) species present under the experimental conditions ([Fe(III)] in the range (3–6) × 10^?4 mol l^?1). The first step is the formation of an adduct between hydroxyl radicals and the benzene ring. The main process is a stepwise dephenylation of the starting TPT. Hydroxylated phenyltin derivatives were also formed, but only as minor photoproducts. The process was shown to be efficient with artificial light as well as with solar light. Copyright © 2001 John Wiley & Sons, Ltd.     

The Thermokinetics of the Formation Reactions of Cerium(III) n-dodecylbenzene Sulfonate and Cerium(III) Stearate

The thermokinetics of the formation reactions of cerium(III) n-dodecylbenzene sulfonate and cerium(III) stearate are studied by using a microcalorimeter. On the basis of experimental and calculated results, three thermodynamics parameters (the activation enthalpies, the activation entropies, the activation free energies), the rate constant, three kinetic parameters (the activation energies, the pre-exponential constant and the reaction order) and the enthalpies of the reaction of preparing cerium(III) n-dodecylbenzene sulfonate in the temperature range of 20–35°C and cerium(III) stearate in the temperature range of44.6–62.8°C are obtained. The results showed that the title reactions easily took place in the studied temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nitrile hydration by thiolate- and alkoxide-ligated Co-NHase analogues. Isolation of Co(III)-amidate and Co(III)-iminol intermediates

Nitrile hydratases (NHases) are thiolate-ligated Fe(III)- or Co(III)-containing enzymes, which convert nitriles to the corresponding amide under mild conditions. Proposed NHase mechanisms involve M(III)-NCR, M(III)-OH, M(III)-iminol, and M(III)-amide intermediates. There have been no reported crystallographically characterized examples of these key intermediates. Spectroscopic and kinetic data support the involvement of a M(III)-NCR intermediate. A H-bonding network facilitates this enzymatic reaction. Herein we describe two biomimetic Co(III)-NHase analogues that hydrate MeCN, and four crystallographically characterized NHase intermediate analogues, [Co(III)(S(Me2)N(4)(tren))(MeCN)](2+) (1), [Co(III)(S(Me2)N(4)(tren))(OH)](+) (3), [Co(III)(S(Me2)N(4)(tren))(NHC(O)CH(3))](+) (2), and [Co(III)(O(Me2)N(4)(tren))(NHC(OH)CH(3))](2+) (5). Iminol-bound 5 represents the first example of a Co(III)-iminol compound in any ligand environment. Kinetic parameters (k(1)(298 K) = 2.98(5) M(-1) s(-1), ΔH(?) = 12.65(3) kcal/mol, ΔS(?) = -14(7) e.u.) for nitrile hydration by 1 are reported, and the activation energy E(a) = 13.2 kcal/mol is compared with that (E(a) = 5.5 kcal/mol) of the NHase enzyme. A mechanism involving initial exchange of the bound MeCN for OH- is ruled out by the fact that nitrile exchange from 1 (k(ex)(300 K) = 7.3(1) × 10(-3) s(-1)) is 2 orders of magnitude slower than nitrile hydration, and that hydroxide bound 3 does not promote nitrile hydration. Reactivity of an analogue that incorporates an alkoxide as a mimic of the highly conserved NHase serine residue shows that this moiety facilitates nitrile hydration under milder conditions. Hydrogen-bonding to the alkoxide stabilizes a Co(III)-iminol intermediate. Comparison of the thiolate versus alkoxide intermediate structures shows that C≡N bond activation and C═O bond formation proceed further along the reaction coordinate when a thiolate is incorporated into the coordination sphere.     

Oxidation of hydroxynaphthalenes by means of Co(III) acetate

1,4-Dihydroxynaphthalene can be quantitatively oxidized to 1,4-naphthoquinone with Co(III) acetate in glacial acetic acid. Analytical determination can be carried out both directly potentiometrically, as well as indirectly using excess of the oxidant and back titrating the unconsumed amount of the reagent with Fe(II) sulfate.     

Kinetics of Photoinduced Reactions at the Single-Molecule Level: The KACB Method

The kinetics of photoinduced reactions can be approached by laser flash photolysis techniques. Although such techniques allow for a detailed understanding of the important photophysics of molecules, they normally require a substantial amount of sample for measurements (>1 nmol), and thus, they are difficult to apply to analytical and diagnostic applications. The photophysics of a fluorescent molecule can be accessed by monitoring the kinetics of the fluctuation of fluorescence, which is called blinking. Blinking is a phenomenon that can be monitored only if molecules are observed at the single-molecule level. In bulk solution, blinking kinetics can be measured by using fluorescence correlation spectroscopy (FCS), which normally requires more than 10⁵ times less sample than that required for laser flash photolysis. Blinking is controlled to extract fruitful microenvironmental information around a fluorescent molecule, by using a method named kinetic analysis based on the control of fluorescence blinking (KACB). This Concept highlights the adaption of the KACB method to investigate the local conformation of DNA with less than 1 pmol of DNA sample.     

Reverse-phase chromatographic separation of Co(II) and Co(III) as the Co(DEDTC)3 and Co(acac)3 complexes

A reverse-phase chromatographic method is described for the simultaneous determination of Co(II) and Co(III) using gradient elution of the Co(DEDTC)₃ and Co(acac)₃ complexes, respectively, with phosphate buffered acetonitrile water mixtures. The separated Co species are detected spectrophotometrically at 322?nm. The analytical range of the method is 0.1 to 1.25?μg/ml for Co(II) and 0.1 to 1.5?μg/ml for Co(III).     

Cleavage by halogens of carboncobalt bonds in organometallic complexes of cobalt(III) : II. Organobis(dimethylglyoximato)cobalt(III) complexes

The reaction between organocobaloximes and ICl in chloroform has been studied. In absence of an excess of added chloride ion the reaction is electrophilic in character; in presence of an excess of chloride ion both oxidative dealkylation and radical attack can occur.     

Sulfur-bridged Co(III)Pt(II)Co(III) trinuclear complexes with mixed aliphatic and aromatic thiolate ligands: effect of nonbridging ligands on the homochiral linkage of cobalt(III) octahedrons by platinum(II)

The reaction of [Ni[Co(aet)(2)(pyt)](2)](2+) (aet = 2-aminoethanethiolate, pyt = 2-pyridinethiolate) with [PtCl(4)](2)(-) gave an S-bridged Co(III)Pt(II)Co(III) trinuclear complex composed of two [Co(aet)(2)(pyt)] units, [Pt[Co(aet)(2)(pyt)](2)](2+) ([1](2+)). When a 1:1 mixture of [Ni[Co(aet)(2)(pyt)](2)](2+) and [Ni[Co(aet)(2)(en)](2)](4+) was reacted with [PtCl(4)](2)(-), a mixed-type S-bridged Co(III)Pt(II)Co(III) complex composed of one [Co(aet)(2)(pyt)] and one [Co(aet)(2)(en)](+) units, [Pt[Co(aet)(2)(en)][Co(aet)(2)(pyt)]](3+) ([2](3+)), was produced, together with [1](2+) and [Pt[Co(aet)(2)(en)](2)](4+). The corresponding Co(III)Pt(II)Co(III) trinuclear complexes containing pymt (2-pyrimidinethiolate), [Pt[Co(aet)(2)(pymt)](2)](2+) ([3](2+)) and [Pt[Co(aet)(2)(en)][Co(aet)(2)(pymt)]](3+) ([4](3+)), were also obtained by similar reactions, using [Ni[Co(aet)(2)(pymt)](2)](2+) instead of [Ni[Co(aet)(2)(pyt)](2)](2+). While [Pt[Co(aet)(2)(en)](2)](4+) formed both the deltalambda (meso) and deltadelta/lambdalambda (racemic) forms in a ratio of ca. 1:1, the preferential formation of the deltadelta/lambdalambda form was observed for [1](2+) (ca. deltalambda:deltadelta/lambdalambda = 1:3) and [2](3+) (ca. delta(en)lambda(pyt)/lambda(en)delta(pyt):deltadelta/lambdalambda = 1:2). Furthermore, [3](2+) and [4](3+) predominantly formed the deltadelta/lambdalambda form. These results indicate that the homochiral selectivity for the S-bridged Co(III)Pt(II)Co(III) trinuclear complexes composed of two octahedral [Co(aet)(2)(L)](0 or +) units is enhanced in the order L = en < pyt < pymt. The isomers produced were separated and optically resolved, and the crystal structures of the meso-type deltalambda-[1]Cl(2).4H(2)O and the spontaneously resolved deltadelta-[4](ClO(4))(3).H(2)O were determined by X-ray analyses. In deltalambda-[1](2+), the delta and Lambda configurational mer(S).trans(N(aet))-[Co(aet)(2)(pyt)] units are linked by a square-planar Pt(II) ion through four aet S atoms to form a linear-type S-bridged trinuclear structure. In deltadelta-[4](3+), a similar linear-type trinuclear structure is constructed from the delta-mer(S).trans(N(aet))-[Co(aet)(2)(pymt)] and delta-C(2)-cis(S)-[Co(aet)(2)(en)](+) units that are bound by a Pt(II) ion with a slightly distorted square-planar geometry through four aet S atoms.     

Photoinduced transformation of camptothecin in the presence of iron(III) ions

To obtain the information on the photoactivated action of camptothecin (CPT) promoted by transition metals, CPT was UVA irradiated (λ = 365 nm) in dimethylsulfoxide (DMSO) solutions. Fe(III) ions present were efficiently reduced to Fe(II) under argon and also in the presence of oxygen. The photoinduced electron transfer under argon resulted into the generation of carbon-centered radicals identified by EPR spin trapping evidencing the cleavage of CPT skeleton. Whereas the absorption UV/vis experiments with equimolar ratio Fe(III):CPT excluded the formation of charge-transfer complexes, the fluorescence spectra of CPT in the presence of Fe(III) revealed a significant fluorescence quenching indicating the probability of physical association between Fe(III) and CPT species in DMSO solutions confirming Fe(III) involvement in the photoinduced transformation.     

Photoinduced Miyaura Borylation by a Rare‐Earth‐Metal Photoreductant: The Hexachlorocerate(III) Anion

The first photoinduced carbon(sp²)–heteroatom bond forming reaction by a rare‐earth‐metal photoreductant, a Miyaura borylation, has been achieved. This simple, scalable, and novel borylation method that makes use of the hexachlorocerate(III) anion ([Ce^IIICl₆]^3?, derived from CeCl₃) has a broad substrate scope and functional‐group tolerance and can be conducted at room temperature. Combined with Suzuki–Miyaura cross‐coupling, the method is applicable to the synthesis of various biaryl products, including through the use of aryl chloride substrates.     

Two heptadentate Co(III) and Mn(III) complexes with partially deprotonated cyclen derivative bearing four hydroxypropyl pendants: structure,DNA binding and DNA cleavage

Two new complexes, (Co^III)₂(H₃L^?)₂(0.5H₂O)₂(ClO₄)₄ (I) and (Mn^III)₂(H₃L^?)₂(0.5H₂O)₂ (ClO₄)₄ (II), were synthesized and crystallographically characterized [H₄L = 1,4,7,10‐tetra‐(2‐hydroxypropyl)‐l,4,7,10‐tetraazacyclododecane] using electrospray ionization mass spectrometry and X‐ray photoelectron spectrometery. The characterizations confirmed that the valences of the metal ions increased from divalent to trivalent due to deprotonation of one OH group (H₄L was in the form of H₃L^?). Owing to the instability of Co(III) and Mn(III) in both air and in solution, they preferred to exist in divalent form. The two heptadentate complexes are extraordinary in that the chiral pendants of the complexes are different in configuration. Spectroscopic studies, viscosity measurements, thermal denaturation experiments and circular dichroism spectra demonstrated that the complexes were prone to interact with DNA by groove binding. At micromolar concentrations and under physiological conditions, the two complexes were able to oxidatively cleave the supercoiled pUC19 plasmid DNA into its nicked and linear forms. Mechanistic studies using various additives suggest the complexes had structures different from those of other inorganic complexes. These are the first reported inorganic complexes not containing planar aromatic ligands and yet binding at the major groove. Copyright © 2012 John Wiley & Sons, Ltd.     

Reactions of silicon hydrides catalyzed by rhodium(III) sulfoxide complexes

Russian Journal of General Chemistry - Dehydrocondensation reactions of silicon hydrides catalyzed by the rhodium(III) complex [RhCl3(Me2SO)3] in the absence of the second substrate were studied....     

Studies of adsorption and solution of acetylacetonates of Cr(III), Co(III) and Al(III) by gas chromatography

Summary Specific retention volumes, adsorption isotherms, molar heats of solution and changes of the entropy were determined from chromatographic data, which was obtained by the gas chromatographic separation of metal acetylacetonates. The retention data for Cr(III), Co(III) and Al(III) acetylacetonates were measured at different temperatures and different flow rates. From the retention data other values associated with adsorption and solution phenomena were calculated.     

Oxidation of Co(II) by ozone and reactions of Co(III) in solutions of sulfuric acid

Reactions of the oxidation of bivalent cobalt ions by ozone, of the spontaneous decomposition of trivalent cobalt, and of interactions between Co(III) and chloride ions in solutions of sulfuric acid are studied. The order and rate constant of the process of decomposition of Co(III) are determined. Information on the kinetics of the interaction between Co(III) and Cl^– is obtained. Kinetic patterns of the accumulation of Co(III) during the ozonation of solutions of CoSO₄ in sulfuric acid are explained. Molar absorption coefficients of Co(III) and Co²⁺ ions in the visible range of wavelengths are determined.