Kinetics of outer-sphere electron transfer reactions involving synthetic models of copper ?blue"? proteins (original) (raw)
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Dalton Transactions, 2003
The outer-sphere electron-transfer reaction between anionic bis(5,6-bis(4-sulfonatophenyl)-3-(2-pyridyl)-1,2,4-triazine)Cu() and cytochrome c II was investigated as a function of pH, ionic strength, concentration, temperature and pressure. The plot of the observed pseudo-first-order rate constant as a function of the Cu() complex concentration showed saturation at higher Cu() concentrations, from which the precursor formation constant and the electron transfer rate constant could be separated (K = (7.7 ± 0.5) × 10 3 M Ϫ1 and k ET = 6.2 ± 0.4 s Ϫ1 at I = 0.2 M, pH 7.4 and 288 K). The pseudo-first-order electron-transfer rate constant was measured as a function of temperature and pressure at (low and) high Cu() concentrations (∆H ‡ = (85 ± 4) 89 ± 4 kJ mol Ϫ1 ; ∆S ‡ = (Ϫ61 ± 13) Ϫ79 ± 15 J K Ϫ1 mol Ϫ1 ; ∆G ‡ (288 K) = (67.6) 66.1 kJ mol Ϫ1 ; ∆V ‡ = (ϩ8.8 ± 0.6) ϩ8.0 ± 0.7 cm 3 mol Ϫ1). Within the volume change for the overall reaction, the volume profile for the electron transfer step is almost symmetrical. The redox process and the change in coordination of the copper centre are proposed to be clearly separated. The back reaction between the Cu() complex and cytochrome c III was investigated as a function of Cu() concentration at pH 7.4 at 1 bar. The observed pseudo-first-order rate constant reaches a saturation at high Cu() concentrations from which the precursor formation constant and the electron-transfer rate constant were estimated (KЈ = (2.0 ± 0.2) × 10 3 M Ϫ1 and kЈ ET = 0.014 ± 0.001 s Ϫ1 at I = 0.2 M, pH 7.4 and 288 K). Simulations of the measured cyclovoltammogramms applying an EC mechanism with two redox systems and two homogeneous chemical reactions were performed. The results are discussed with reference to earlier studies involving Co, Ru and Cr complexes as redox partners for cytochrome c.
Inorg Chem, 1985
donors are imine nitrogens and was held constant at 0.0388 cm-I for each complex. This assumption is based upon crystallographic evidence that the bond lengths remain relatively constant throughout the series. Thus, taking the sign of the hyperfine coupling constant to be negative," the calculated Fermi contact parameters are as follows: [Cu(TC-3,3)], 0.369; [Cu(TC-4,4)], 0.322; [Cu(TC-4,5)], 0.285; [Cu(TC-5,5)], 0.264. From these values a' was calculated by eq 5 to be -0.6. It is therefore apparent that the major factor involved in the attentuation of A,, in the Cu(I1) tropocoronands is the Fermi contact interaction.
Inorganic Chemistry, 2001
The copper(II) and copper(I) complexes of the chelating ligands 2,6-bis(benzimidazol-2′-ylthiomethyl)pyridine (bbtmp) and N,N-bis(benzimidazol-2′-ylthioethyl)methylamine (bbtma) have been isolated and characterized by electronic and EPR spectra. The molecular structures of a redox pair of Cu(II/I) complexes, viz., [Cu(bbtmp)-(NO 3)]NO 3 , 1, and [Cu(bbtmp)]NO 3 , 2, and of [Cu(bbtmp)Cl], 3, have been determined by single-crystal X-ray crystallography. The cation of the green complex [Cu(bbtmp)(NO 3)]NO 3 possesses an almost perfectly square planar coordination geometry in which the corners are occupied by the pyridine and two benzimidazole nitrogen atoms of the bbtmp ligand and an oxygen atom of the nitrate ion. The light-yellow complex [Cu(bbtmp)]NO 3 contains copper(I) with trigonal planar coordination geometry constituted by the pyridine and two benzimidazole nitrogen atoms of the bbtmp ligand. In the yellow chloride complex [Cu(bbtmp)Cl] the asymmetric unit consists of two complex molecules that are crystallographically independent. The coordination geometry of copper(I) in these molecules, in contrast to the nitrate, is tetrahedral, with pyridine and two benzimidazole nitrogen atoms of bbtmp ligand and the chloride ion occupying the apexes. The above coordination structures are unusual in that the thioether sulfurs are not engaged in coordination and the presence of two seven-membered chelate rings facilitates strong coordination of the benzimidazole nitrogens and discourage any distortion in Cu(II) coordination geometry. The solid-state coordination geometries are retained even in solution, as revealed by electronic, EPR, and 1 H NMR spectra. The electrochemical behavior of the present and other similar CuN 3 complexes has been examined, and the thermodynamic aspects of the electrode process are correlated to the stereochemical reorganizations accompanying the redox changes. The influence of coordinated pyridine and amine nitrogen atoms on the spectral and electrochemical properties has been discussed.
The Journal of Chemical Physics, 2010
Structural and energetic reorganizations in redox reaction of type 1 copper proteins are studied by density functional and ab initio molecular orbital calculations. Model complexes of the active site with varying number of ligands, from Cu͑SCH 3 ͒ 0/+ to Cu͑SCH 3 ͒͑Im͒ 2 ͑S͑CH 3 ͒ 2 ͒ 0/+ , where Im denotes imidazole, are investigated. Following the findings of structural instability in Cu͑I͒ ϫ͑SCH 3 ͒͑Im͒ 2 and its stabilization by the addition of the axial methionine ͑Met͒ ligand model, the structure and energetics are examined as functions of the Cu-S Met distance in the range of 2.1-3.3 Å. The reorganization energies in both redox states exhibit a minimum at the Cu-S Met distance of ϳ2.4 Å, whereas the ionization potential increases monotonically. The changes of reorganization energies correlate well with one of the Cu-N His distances rather than the Cu-S Cys distance. The estimated Arrhenius factor for oxidation of plastocyanin by P700 + ͑in photosystem I͒ changes by an order of magnitude when the Cu-S Met distance fluctuates between 2.4 and 3.0 Å, whereas the factor for reduction of plastocyanin by cytochrome f is nearly constant. Together with the data from our previous classical molecular dynamics simulation of solvated protein, we argue that the electron transfer rate is affected, and thus may be controlled, by the fluctuation of a weakly bound axial Met ligand. We also present the assessment of various exchange-correlation functionals, including those with the long-range correction, against the CCSD͑T͒ reference and on the basis of a perturbative adiabatic connection model. For Cu͑SCH 3 ͒ and Cu͑SCH 3 ͒͑Im͒, simple correlations have been found between the reorganization energies and the amount of Hartree-Fock exchange.
Inorganic Chemistry, 1997
Kinetic studies have been conducted in acetonitrile on the electron-transfer reactions of five copper(II/I) complexes involving ligands in which either a benzene or a cyclohexane ring, or both, have been substituted into the ligand backbone of the 14-membered tetrathiamacrocycle [14]aneS 4 . The specific ligands utilized in this work include 2, 3-benzo-1,4,8,11-tetrathiacyclotetradecane (bz-[14]aneS 4 ), 2,3-trans-cyclohexano-1, 4,8,11-tetrathiacyclotetradecane (trans-cyhx-[14]aneS 4 ), 2,3-benzo-9,10-trans-cyclohexano-1, 4,8,11-tetrathiacyclotetradecane (bz,transcyhx-[14]aneS 4 ), 2,3-benzo-9,10-cis-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,cis-cyhx-[14]aneS 4 ), and 2,3-cis-9,10-trans-dicyclohexano-1, 4,8,11-tetrathiacyclotetradecane (cis, trans-dicyhx-[14]aneS 4 ). Each Cu II/I L system has been reacted with three separate reducing agents and three separate oxidizing agents to examine the effect of driving force upon the kinetic parameters. The Marcus relationship has been applied to each crossreaction rate constant to estimate the apparent self-exchange rate constant, k 11 , for each Cu II/I L system. For all but one of the five systems, the k 11 values obtained from the three reduction reactions are in virtual agreement with the corresponding value obtained for the oxidation reaction with the smallest driving force. As the driving force for Cu I L oxidation increases, a smaller k 11 value is calculated for each system. This behavior is consistent with our previously proposed dual-pathway square scheme mechanism in which a significant conformational change occurs as a separate step preceding electron transfer in the case of Cu I L oxidation. Although direct observation of conformationally-gated electron transfer was not attained for any of the five systems included in the current work, limits for the rate constant for conformational change have been estimated from the conditions required to change the apparent pathway for the oxidation kinetics. These limits show that the Cu I L complex involving a single phenyl substituent (bz-[14]aneS 4 ) exhibits a much slower conformational change than do any of the other systems included in this study. The implications of this observation are discussed.
Modelling the impact of geometric parameters on the redox potential of blue copper proteins
Journal of Inorganic Biochemistry, 2006
The synthesis and structure of a homologous series of cationic N 2 S 2 copper(I) Schiff base complexes constructed using o-tert-butylthiobenzaldehyde and a series of terminal diamines (ethane, propane, butane) are reported. The complexes differ only in the length of the methylene chain between the imine groups. This simple modification forces the copper centre to shift geometry from a planar (1,2-diaminoethane) to a more distorted tetrahedral motif (1,4-diaminobutane). The redox potentials of the three cations were measured using cyclic voltammetry in donor (acetonitrile) and non-donor solvents (dichloromethane). The S-Cu-N angles for each complex are correlated against the respective redox potential allowing an analysis of the geometric impact on the redox potential in soft copper centres. The redox potential is observed to increase as the metal centre moves from a planar towards a tetrahedral motif. Comparing this data with the reported structures of the blue copper proteins (rusticyanin and plastocyanin) allows an assessment of the contribution of the geometry of the metal binding site to the operating potential of these proteins to be made.
Electrochemical and spectral characterization of blue copper protein models
Electrochemistry Communications, 2009
Blue copper proteins play a central role in various enzymatic anabolic/catabolic pathways in living cells by virtue of the integrated metal ions. These ions may exist in variable oxidation states, with suitable reduction potentials and fast electron-transfer rates which in turn is a manifestation of their unusual geometry and coordination. We report the electrochemical and spectral characterization of three novel complexes of copper (II) with N 2 S type tridentate chelating agent 2,2 0-dithiodianiline (dta), having structural similarities to the active site of Type I copper proteins. High positive redox potentials in the range of 0.5-0.6 V vs Ag/AgCl electrode of the complexes and the absorption maxima at $550 nm, with high extinction coefficients, correspond well with typical blue copper proteins. The IR and EPR studies support the assigned pseudo tetrahedral structures to the complexes. The diffusion coefficient and rate constant for heterogeneous charge transfer for Cu 2+ /Cu + coordinated in a potentially bio-mimetic Type I site is reported.
Inorganica Chimica Acta, 2005
The macrocycles L 1 -L 3 having N 2 S 2 O-, N 2 S 2 -, and N 2 S 3 -donor sets, respectively, and incorporating the 1,10-phenanthroline unit interact in EtOH and MeCN solutions with Cu II to give 1:1 [M(L)] 2+ complex species. The compounds [Cu(L 1 )(ClO 4 )]ClO 4 (1), [Cu(L 2 )(ClO 4 )]ClO 4 AE 1 2 H 2 O (2) and [Cu(L 3 )](ClO 4 ) 2 (3) were isolated at the solid state and the first two also characterised by Xray diffraction studies. The conformation adopted by L 1 and L 2 in the cation complexes reveals the aliphatic portion of the rings folded over the plane containing the heteroaromatic moiety with the ligands encapsulating the metal centre within their cavity by imposing, respectively, a square-based pyramidal and a square planar geometry. In both complexes, the metal ion completes its coordination sphere by interacting with a ClO 4 À ligand. The compound [Cu(L 3 ) 2 ](PF 6 ) 2 (4) containing a 1:2 cation complex was also isolated at the solid state: EPR spectroscopy measurements suggest the presence of a CuN 4 chromophore in this complex. The EPR and electronic spectral features of 1-4 have been studied and their redox properties examined in comparison with those observed for Type-1 blue copper proteins.
A Structural Strategy for Generating Rapid Electron-Transfer Kinetics in Copper (II/I) Systems
Inorganic …, 1999
Electron-transfer in low molecular weight copper(II/I) systems is generally accompanied by a large reorganization of the inner-coordination sphere. On the basis of recent kinetic studies involving Cu(II/I)-macrocyclic polythiaether complexes, it was hypothesized that forcing Cu(II) out of the macrocyclic cavity (i) decreases the changes in bond angles upon reduction and (ii) obviates any need for donor atom inversion. This should diminish the reorganizational barrier and, thereby, increase the electron self-exchange rate. This hypothesis has now been tested utilizing a somewhat soluble 12-membered macrocyclic tetrathiaether, oxathiane[12]aneS 4 (L). Crystal structures of the Cu II L and Cu I L complexes confirm that, whereas one Cu-S bond dissociates upon reduction, the remaining bond lengths and angles change only minimally. The free ligand, oxathiane[12]aneS 4 , C 10 H 18 OS 4 , crystallizes in the orthorhombic space group Pbca with Z) 8, a) 15.211(2) Å, b) 8.5113(9) Å, c) 20.548(3) Å. The Cu II L complex crystallizes as a 5-coordinate monomer with water as the apical ligand: [CuL(OH 2)]-(ClO 4) 2 ‚H 2 O, C 10 H 22 O 11 S 4 Cl 2 Cu, monoclinic P2(1)/c, Z) 4, a) 15.774(2) Å, b) 8.485(5) Å, c) 16.508(9) Å,) 112.11(6)°. The Cu I L complex crystallizes as a binuclear species: [(CuL) 2 NCCH 3 ](ClO 4) 2 ‚NCCH 3 , C 24 H 42 N 2 O 10 S 8 Cl 2 Cu 2 , in the triclinic space group P1 h with Z) 4, a) 12.5917(2) Å, b) 13.0020(3) Å, c) 14.9285(3) Å, R) 68.356(1)°,) 84.298(1)°, γ) 61.129(1)°. The kinetics of Cu II/I (oxathiane[12]aneS 4) reacting with four selected counter reagentsstwo oxidants and two reductantssyield exceptionally large cross-reaction rate constants. Application of the Marcus cross relation yields calculated self-exchange rate constants ranging from 4 × 10 5 to 8 × 10 5 M-1 s-1 (median: 6 × 10 5 M-1 s-1) for this Cu II/I L redox system at 25°C, µ) 0.10. A comparable result of k 11) (8.4 (0.8) × 10 5 M-1 s-1 has been obtained by NMR line-broadening measurements (at 25°C, corrected to µ) 0.10). This is the largest self-exchange rate constant ever reported for a low molecular weight Cu(II/I) system. Thus, elimination of donor atom inversion coupled with a constrained inner sphere appears to represent a feasible approach for accelerating electron transfer in Cu(II/I) macrocyclic systems.