Kinetics and mechanism of the decomposition of μ-amido-μ-hydroxo(tetraamminecobalt(III))(tetraaquacobalt(III)) ion in acidic aqueous solution

The title complex undergoes decomposition in acidic aqueous solution resulting in equimolar concentration of aquapentaamminecobalt(III) and hexa- aquacobalt(II). The kinetic studies over the ranges of 0.048 M ⩽ [H⁺] ⩽ 0.385 M, 25 ⩽ θ_c ⩽ 41.5°C and at I = 0.5 M reveals that the intricate mechanism involves protonation equilibrium of the title complex, followed by a rate determining bridge cleavage. The further follow-up reaction is a fast electron transfer process to form products. The rate expression derived from the mechanism is k_obs = k₁K₁[H⁺]/(1 + K₁[H⁺]) where the values of k, and K, are found to be 8.9 × 10⁻⁴ s⁻¹ and 3.5 M⁻¹ respectively at 25 °C. The results are compared with that obtained for the decomposition reactions of mononuclear aquaammine complexes of cobalt(III).     

Kinetics and mechanism of the aquation of the trinuclear cation, [μ3-oxo-triaqua-hexakis(acetato)tris(iron(III))]in perchloric acid media

The aquation of the title complex cation in aqueous perchloric acid proceeded via two steps, both postulated to be the proton attack on the oxygen atom which binds the acetate ligand to the metal centre, followed by Fe-O bond cleavage. This was followed by rapid decomposition to produce aqueous iron(III) and acetate ions. The first-order rate constants for the first and second steps at 25 °C are: k₁ = (4.16 ± 0.58) × 10⁻² s⁻¹ and k₂ = (2.09 ± 0.42) × 10⁻³ s⁻¹, respectively, and their corresponding activation parameters are . The spontaneous hydrolysis rate constants for the first and second steps were also determined at 25 °C and ionic strength of 1 mol dm⁻³ and they are k₀ = (3.10 ± 0.82) × 10⁻³ s⁻¹ and , respectively. The corresponding activation parameters are .     

Interactions of bis-[μ-chloro-chlorotricarbonylruthenium(II)] and poly-[μ-dichloro-dicarbonylruthenium(II)] with nucleosides

The interactions of dimeric complex bis-[μ-chloro-chlorotricarbonylruthenium(II)], [Ru(CO)₃Cl₂]₂, and the polymeric complex poly-[μ-dichlorodicarbonylruthenium(II)], [Ru(CO)₂Cl₂]_x, with nucleosides (Nucl) in a 1:1 Ru:Nucl molar ratio for the dimer and 1:2 Ru:Nucl for the polymer, resulted in formation of the monomeric mononucleoside [Ru(CO)₃(Nucl)Cl₂] and bis-nucleoside [Ru(CO)₂(Nucl)₂Cl₂] complexes, respectively. The dimer [Ru(CO)₃Cl₂]₂ also gave the ionic bis-nucleoside complexes [Ru(CO)₃(Nucl)₂Cl]Cl in the molar ratio 1:2 Ru:Nucl. The mononucleoside complexes are stable in solution while the bis-nucleoside complexes tend to lose one nucleoside in strong complexing solvents, probably by solvent substitution. The complexes [Ru(CO)₃(Nucl)Cl₂] and [Ru(CO)₂(Nucl)₂Cl₂] with one N(1)H ionizable imino proton undergo ionization in alkaline solution and the complexes [Ru(CO)₃(NuclH⁺)Cl] and [Ru(CO)₂(NuclH⁺)₂], respectively, were isolated. In these deprotonated complexes the nucleosides behave as bidentate ligands, while in the protonated ones they act as monodentate. All Complexes were characterized by elemental analyses and various spectroscopic methods.     

Thermodynamics of vanadyl(IV)—carboxylate complex formation in aqueous solution

The stability constants and the heats of formation of vanadyl(IV)—acetate, —glycolate, and —glycine complexes have been determined in aqueous solution by means of potentiometric and calorimetric measurements. In the pH range where the protolitic equilibria of VO²⁺ is certainly negligible the acetate forms two mononuclear complexes, the glycolate three whereas the glycine reacts in its zwitterionic form. The stabilities of the glycolate complexes are considerably higher than the acetate ones, in spite of its lower basicity, indicating that the complex formation involves the coordination of the hydroxyl group to the metal ion. The enthalpy changes are positive except for the glycolate where a small negative value is found. For all systems the entropy changes are positive and therefore favourable to the complex formation.     

Kinetics of formation of the 4,4′-bis-dimethylaminodiphenyl carbonium ion (BDC+) and its reaction with sulfhydryl residues

The use of 4,4′-bis-dimethylaminodiphenylcarbinol (BDC-OH) as an analytical reagent for sulfhydryl residues and as a specific chemical modification reagent for proteins is dependent upon the unique properties of the BDC⁺ cation present in aqueous buffers below a pH of 6.5. In the presence of aqueous buffers, pH 5.1, BDC⁺ exhibits a λ_max of 606 nm with an apparent molar absorption coefficient of 10,000 m^?1 cm^?1. Upon the addition of 4m guanidine hydrochloride this apparent coefficient is enhanced to 70,800 m^?1 cm^?1. The true molar extinction coefficient for BDC⁺ was determined to be 128,000 m^?1 cm^?1. The reaction of BDC⁺ with sulfhydryl residues of proteins or simple thiols is rapid and leads to a complex devoid of visible color. In the pH range 3.0–7.0, a complex equilibrium is established among the three species BDC-OH, BDC⁺, BDCH⁺⁺. The formation of this equilibrium is proton mediated, and is discussed in terms of the equilibrium, rate, and acid dissociation constants.     

The mechanism of antimalarial action of the ruthenium(II)–chloroquine complex [RuCl2(CQ)]2

The mechanism of antimalarial action of the ruthenium-chloroquine complex [RuCl(2)(CQ)](2) (1), previously shown by us to be active in vitro against CQ-resistant strains of Plasmodium falciparum and in vivo against P. berghei, has been investigated. The complex is rapidly hydrolyzed in aqueous solution to [RuCl(OH(2))(3)(CQ)](2)[Cl](2), which is probably the active species. This compound binds to hematin in solution and inhibits aggregation to beta-hematin at pH approximately 5 to a slightly lower extent than chloroquine diphosphate; more importantly, the heme aggregation inhibition activity of complex 1 is significantly higher than that of CQ when measured at the interface of n-octanol-aqueous acetate buffer mixtures under acidic conditions modeling the food vacuole of the parasite. Partition coefficient measurements confirmed that complex 1 is considerably more lipophilic than CQ in n-octanol-water mixtures at pH approximately 5. This suggests that the principal target of complex 1 is the heme aggregation process, which has recently been reported to be fast and spontaneous at or near water-lipid interfaces. The enhanced antimalarial activity of complex 1 is thus probably due to a higher effective concentration of the drug at or near the interface compared with that of CQ, which accumulates strongly in the aqueous regions of the vacuole under those conditions. Furthermore, the activity of complex 1 against CQ-resistant strains of P. falciparum is probably related to its greater lipophilicity, in line with previous reports indicating a lowered ability of the mutated transmembrane transporter PfCRT to promote the efflux of highly lipophilic drugs. The metal complex also interacts with DNA by intercalation, to a comparable extent and in a similar manner to uncomplexed CQ and therefore DNA binding does not appear to be an important part of the mechanism of antimalarial action in this case.     

Catecholase activity of a μ-hydroxodicopper(II) macrocyclic complex: structures, intermediates and reaction mechanism

The monohydroxo-bridged dicopper(II) complex (1), its reduced dicopper(I) analogue (2) and the trans-μ-1,2-peroxo-dicopper(II) adduct (3) with the macrocyclic N-donor ligand [22]py4pz (9,22-bis(pyridin-2′-ylmethyl)-1,4,9,14,17,22,27,28,29,30- decaazapentacyclo -[22.2.1^14,7.1^11,14.1^17,20]triacontane-5,7(28),11(29),12,18,20(30), 24(27),25-octaene), have been prepared and characterized, including a 3D structure of 1 and 2. These compounds represent models of the three states of the catechol oxidase active site: met, deoxy (reduced) and oxy. The dicopper(II) complex 1 catalyzes the oxidation of catechol model substrates in aerobic conditions, while in the absence of dioxygen a stoichiometric oxidation takes place, leading to the formation of quinone and the respective dicopper(I) complex. The catalytic reaction follows a Michaelis–Menten behavior. The dicopper(I) complex binds molecular dioxygen at low temperature, forming a trans-μ-1,2-peroxo-dicopper adduct, which was characterized by UV–Vis and resonance Raman spectroscopy and electrochemically. This peroxo complex stoichiometrically oxidizes a second molecule of catechol in the absence of dioxygen. A catalytic mechanism of catechol oxidation by 1 has been proposed, and its relevance to the mechanisms earlier proposed for the natural enzyme and other copper complexes is discussed. Electronic Supplementary Material Supplementary material is available for this article at     

Preparation of polyhydroxyalkyl- and C-glycosylfuran derivatives from free sugars catalyzed by cerium(III) chloride in aqueous solution: an improvement of the Garcia González reaction

Unprotected aldose sugars react smoothly with 1,3-diones or beta-ketoesters in the presence of CeCl(3).7H(2)O in aqueous solution to produce polyhydroxylalkyl- and C-glycosylfuran derivatives in excellent yield. Operationally simple, mild neutral reaction conditions in aqueous solution is the key feature of this methodology.     

Kinetics of the oxidation of bromide ions by bis(2,2′-bipyridine)manganese(III) ions in aqueous perchlorate media

The kinetics of the oxidation of bromide ions by the bis(2,2′-bipyridine)manganese(III) ion have been investigated over a range of acid concentrations for several temperatures. Initial rates of reaction are measured and from their variation with [Mn^III] and [Br−] it is concluded that the observed order in [Mn(bipy)₂³⁺_aq] is one and that in [Br⁻] is intermediate between zero and one. It is shown kinetically that intermediate complexes between Mn^III and Br⁻ ions are involved and from the variation of the rate constant with acidity it is concluded that the decomposition of Mn(bipy)₂Br²⁺_aq is definitely involved as a rate controlling step and that the decomposition of the protonated complex Mn(bipy)(bipyH⁺)Br³⁺_aq is also probably involved as a rate controlling step. The kinetics and mechanism for this oxidation are compared with those found for other cations complexed with bipyridine, for aqua-cations and other complexes of Co^III.     

Structural elucidation of full-length nidogen and the laminin–nidogen complex in solution

Nidogen-1 is a key basement membrane protein that is required for many biological activities. It is one of the central elements in organizing basal laminae including those in the skin, muscle, and the nervous system. The self-assembling extracellular matrix that also incorporates fibulins, fibronectin and integrins is clamped together by networks formed between nidogen, perlecan, laminin and collagen IV. To date, the full-length version of nidogen-1 has not been studied in detail in terms of its solution conformation and shape because of its susceptibility to proteolysis. In the current study, we have expressed and purified full-length nidogen-1 and have investigated its solution behavior using size-exclusion chromatography (SEC), dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). The ab initio shape reconstruction of the complex between nidogen-1 and the laminin γ-1 short arm confirms that the interaction is mediated solely by the C-terminal domains: the rest of the domains of both proteins do not participate in complex formation.     

Synthesis and structure of the complex tetra-μ-prolinatodirhodium(II)

The dinuclear Rh^IIRh^II complex with proline [Rh₂(pro₄][NEt₄]₂ was synthesized and its structure studied by means of spectroscopic (IR, EPR and ESCA) and magnetochemical methods. It was shown that two proline molecules serve as bridging ligands, while the other two are only axially coordinated through their N atoms.     

Structure and mechanism of the γ-secretase intramembrane protease complex

γ-Secretase is a membrane protein complex that proteolyzes within the transmembrane domain of >100 substrates, including those derived from the amyloid precursor protein and the Notch family of cell surface receptors. The nine-transmembrane presenilin is the catalytic component of this aspartyl protease complex that carries out hydrolysis in the lipid bilayer. Advances in cryoelectron microscopy have led to the elucidation of the structure of the γ-secretase complex at atomic resolution. Recently, structures of the enzyme have been determined with bound APP- or Notch-derived substrates, providing insight into the nature of substrate recognition and processing. Molecular dynamics simulations of substrate-bound enzymes suggest dynamic mechanisms of intramembrane proteolysis. Structures of the enzyme bound to small-molecule inhibitors and modulators have also been solved, setting the stage for rational structure-based drug discovery targeting γ-secretase.     

Separation of actinides and lanthanides: Synthesis and molecular structure of a new di-μ-phenoxo-bridged dinuclear bis(dioxouranium(VI)) complex

The new phenoxo-bridged uranyl [(UO₂)₂L₂(thf)₂] [H₂L = N(2-oxyphenyl)-3-methoxy salicylaldiminato (C₁₄H₁₁NO₃)] compound has been synthesized and characterized. The 3D structure of the free ligand is also reported. The complex crystallizes in the monoclinic space group P2₁/c with lattice parameters a = 19.5915(15) Å, b = 10.4096(9) Å, c = 17.5216(14) Å and β = 99.9960(7)° with z = 4. The compound consists of a dinuclear unit composed of two dioxouranium(VI) ions, bridged by two phenoxide oxygens. The coordination around each uranium atom can be regarded as approximately pentagonal bipyramidal. The two uranyl groups are separated by 3.9192(5) Å. The title complex is one of the few examples for bis-uranyl groups bridged by phenoxo ligands. These types of ligands are candidates for the sequestration of the uranium from nuclear waste and provide a good selectivity over competing lanthanide cations in solution. The ligand has been found to selectively bind to a representative actinide rather than to lanthanum (Ln³⁺), which is probably related to the larger ionic radii of Ln(III). So, the ligand is not suitable to clutch two lanthanide metals via the phenolate bridge (Ln-O_(phenol)-Ln). The spherical shape and the larger size of the Ln(III) ions apparently do not allow a fit within the byte angle of phenolate bridge.     

Synthesis,structure and magnetic properties of the binuclear chromium(III) complex di-μ-hydroxobis [(1,4,7,10-tetraazacyclododecane)chromium(III)] dithionate tetrahydrate, [(cyclen)CrOH]2(S206)2·4H2O

The dark blue dimeric complex di-μ-hydroxo- bis [(1,4,7,10-tetraazacyclododecane)chromium(III)] dithionate tetrahydrate, [Cr(C₈H₂₀N₄)OH]₂(S₂O₆)₂· 4H₂O or [Cr(cyclen)OH]₂(S₂O₆)₂·4H₂O, has been synthesized. The crystal structure of the complex has been determined from threeodimensional counter X-ray data. The complex crystallizes in space group P2₁/n of the monoclinic system with two dimeric formula units in a cell of dimensions a = 8.837(5), b = 14.472(8), c= 13.943(6) Å andβ=95.83(4)^o. The structure has been refined by full-matrix least- squares methods to a final value of the weighted R-factor of 0.059 on the basis of 1774 independent intensities. The geometry of the cyclen macrocycle is unsymmetrical, the observed conformations being λδδλ and its enantiomer. The strained ligand conformation leads to significant deviations from octahedral geometry at the chromium centers, and to a bridged geometry in which the CrOCr angle ø and the Cr···Cr separation of 104.1(1)^o and 3.086(2) Å are the largest observed in dimers of this kind. The magnetic susceptibility of the complex indicates antiferromagnetic coupling, with the ground state singlet lying 21.56(6) cm⁻¹ below the lowest lying triplet state. The structural parameters have been used to calculate the triplet energy by means of the Glerup- Hodgson- Pedersen (GHP) model, and the calculated value of 22.3 cme⁻¹ is very similar to the observed value.     

Nuclear overhauser effects for methyl β-maltoside and the conformational states of maltose in aqueous solution

A nuclear Overhauser enhancement in methyl β-maltoside, resulting from pre-irradiation of H-1′ of the non-reducing glucose residue, has been measured and calculated theoretically. Comparison of these data reveals a complicated conformational equilibrium in aqueous solutions of maltose derivatives.     

A new hydroxo-bridged thorium(IV) dimer: Preparation and structure of di-μ-hydroxo-bis[aquanitrato(2,6-diacetylpyridinedisemicarbazone)thorium(IV)] nitrate tetrahydrate

The pentadentate ligand 2,6-diacetylpyridinedisemicarbazone, DAPSC, reacts with Th(NO₃)₄ in ethanolwater mixture and a di-μ-hydroxo Th(IV) dimer is formed. The compound [Th₂(OH)₂(DAPSC)₂(NO₃)₂(H₂O)₂](NO₃)₄·4H₂O (I) is monoclinic, space group P2₁/n with a = 10.705(1), b = 19.008(2), c = 11.782(1) Å, β = 107.82(2)°, V = 2282(1) ų and Z = 2. Detailed X-ray structural analysis showed that each thorium atom in the complex is coordinated to one pentadentate DAPSC ligand, which is subjected to a considerable distortion, one bidentate nitrate group, one water ligand and two bridging hydroxo groups. The coordination number is ten and the best presentation of the polyhedron is that of a distorted bicapped square antiprism. The ThTh separation is 4.0181(6) Å and the average ThO(H) bridge is 2.366 Å. The structure was refined using 3185 reflections to an R value of 5.0%.     

Kinetics and mechanism of reactions of S-protected dithiol monoaminemonoamide (MAMA) ligands with technetium: Characterization of a technetium—thiolate—thioether—MAMA complex,a kinetic intermediate of the reaction

The exchange reactions of S-protected dithiol monoaminemonoamide (MAMA) ligands with Tc(V)-gluconate were investigated. Protection of the mercaptide sulfur atoms with acid, base and metal labile groups permitted complex formation of the MAMA ligands at a range of pHs. In general, the rate of complex formation was faster with the MAMA ligands than with the corresponding diamide dithiol (DADS) ligands. The rate of Tc complex formation depended on the nature of the sulfur protecting groups and on the position of the amine group with respect to the other donor groups in the ligands. Two isomeric ligands showed different mechanisms of complex formation. The isomer which gave the final Tc-dithiolate-MAMA complex in higher yield was shown to form a Tc-thioether-thiol-MAMA complex as an intermediate prior to metal-assisted S-dealkylation. The formation of the Tc-thioether complex intermediate at a lower temperature may account for the enhanced kinetics of chelation compared to the isomer which did not form the intermediate complex.     

Synthesis and crystal structure of bis{di-μ-hydroxobis[fac-tribromoaquotin(IV)]} heptahydrate

A product isolated from a reaction mixture of Br₂ and Ph₃ Sn(CH₂)₁₃CH₃ (3:1 mole ratio) in CHCl₃ solution in air was bis{di-μ-hydroxobis[fac-tribromoaquotin(IV)]} heptahydrate, 2[Br₃ (H₂O)Sn(μ-OH)₂ Sn(O₂H)Br₃] · 7H₂O, 2[fac-(1: X = Br)] · 7H₂O. Previous reports had indicated that the tin complexes, [fac-(1: X = Cl or Br)], had been obtained in various solvated forms from hydrolysis or oxidation/hydrolysis of appropriate tin(IV) or tin(II) halides. The crystal structure determination, reported here, provides an improved refinement of the core, i.e., [fac-(1: X = Br)], of 2[fac-(1: X = Br)] · 7H₂O compared to previous attempts. The solid state structure consists of a central rhomboidal planar Sn₂O₂ ring. The tin centres have distorted octahedral geometries, with each Br ligand trans to an O atom. The Br ligands, trans to the aqua ligands, form longer bonds to tin at 2.5556(7) and 2.5544(6) Å, than those trans to the bridging OH ligands, between 2.5021(7) and 2.5127(7) Å. The Br, OH and H₂O ligands as well as the solvate water molecules are all involved in an extensive hydrogen bonding system in 2[fac-(1: X = Br)] · 7H₂O.