Electrochemical Studies on the Thermodynamics of Electron Transfer and Ligand Binding of Several Metalloporphyrins in Aprotic Solvents

Kadish, Karl M.; Thompson, L. K.; Beroiz, D.; Bottomley, Lawrence A.

doi:10.1021/bk-1977-0038.ch004

Cited by 8 publications

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“…97,98 The thermodynamics of M 3+ /M 2+ redox reactions in M-TPP complexes (M ) Co, Fe) in nonaqueous coordinating solvents (DMSO, pyridine, DMF, butyronitrile) are dominated by solvent binding/dissociation at the axial positions of the metal. 99 It is interesting to note that, analogous to previous observations for all cytochromes c and for most electron transport proteins, the enthalpy and entropy terms of the free energy change of the reduction reaction lead to opposing contributions to E°′. In particular, for AcMP11, both ∆H°′ rc and ∆S°′ rc have an opposite sign as compared to cytc, but again the entropic term is smaller than the enthalpic one.…”

Section: Effects Of Exogenous Axial Ligand Binding On Cytochrome Csupporting

confidence: 63%

“…Such solvent structuring effects provide the most widely accepted explanation for the positive reduction entropies shown by M 3+ /M 2+ aquo couples and their complexes with simple monodentate ligands and planar macrocyclic systems in aqueous solution. ,− Accordingly, the reduction entropy of hemin chloride in mixed water/ N , N ‘-dimethylformamide (DMF) and water/ N , N ‘-dimethylacetamide (DMA) solutions, determined through potentiometric titrations, were found to increase with increasing water content . Water indeed replaces DMF and DMA in axial heme iron coordination. , The thermodynamics of M 3+ /M 2+ redox reactions in M-TPP complexes (M = Co, Fe) in nonaqueous coordinating solvents (DMSO, pyridine, DMF, butyronitrile) are dominated by solvent binding/dissociation at the axial positions of the metal . It is interesting to note that, analogous to previous observations for all cytochromes c and for most electron transport proteins, the enthalpy and entropy terms of the free energy change of the reduction reaction lead to opposing contributions to E °‘.…”

Section: Discussionmentioning

confidence: 99%

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Control of Cytochrome c Redox Potential: Axial Ligation and Protein Environment Effects

et al. 2002

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Axial iron ligation and protein encapsulation of the heme cofactor have been investigated as effectors of the reduction potential (E degrees ') of cytochrome c through direct electrochemistry experiments. Our approach was that of partitioning the E degrees ' changes resulting from binding of imidazole, 2-methyl-imidazole, ammonia, and azide to both cytochrome c and microperoxidase-11 (MP11), into the enthalpic and entropic contributions. N-Acetylmethionine binding to MP11 was also investigated. These ligands replace Met80 and a water molecule axially coordinated to the heme iron in cytochrome c and MP11, respectively. This factorization was achieved through variable temperature E degrees ' measurements. In this way, we have found that (i) the decrease in E degrees ' of cytochrome c due to Met80 substitution by a nitrogen-donor ligand is almost totally enthalpic in origin, as a result of the stronger electron donor properties of the exogenous ligand which selectively stabilize the ferric state; (ii) on the contrary, the binding of the same ligands and N-acetylmethionine to MP11 results in an enthalpic stabilization of the reduced state, whereas the entropic effect invariably decreases E degrees ' (the former effect prevails for the methionine ligand and the latter for the nitrogenous ligands). A comparison of the reduction thermodynamics of cytochrome c and the MP11 adducts offers insight on the effect of changing axial heme ligation and heme insertion into the folded polypeptide chain. Principally, we have found that the overall E degrees ' increase of approximately 400 mV, comparing MP11 and native cytochrome c, consists of two opposite enthalpic and entropic terms of approximately +680 and -280 mV, respectively. The enthalpic term includes contributions from both axial methionine binding (+300 mV) and protein encapsulation of the heme (+380 mV), whereas the entropic term is almost entirely manifest at the stage of axial ligand binding. Both terms are dominated by the effects of water exclusion from the heme environment.

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Section: Effects Of Exogenous Axial Ligand Binding On Cytochrome Csupporting

confidence: 63%

Section: Discussionmentioning

confidence: 99%

Control of Cytochrome c Redox Potential: Axial Ligation and Protein Environment Effects

et al. 2002

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“…Valuable information about the mechanism of E °‘ modulation in heme proteins has been obtained from the enthalpy (Δ H °‘ rc ) and entropy (Δ S °‘ rc ) changes of the reduction reaction, measured through variable-temperature electrochemical and spectroelectrochemical experiments ( − ). This paper reports for the first time on the thermodynamics of Fe(III) to Fe(II) reduction for the high-spin and cyanide-bound form of a member of the animal heme peroxidase superfamily.…”

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Redox Thermodynamics of the Fe(III)/Fe(II) Couple of Human Myeloperoxidase in Its High-Spin and Low-Spin Forms

et al. 2006

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Myeloperoxidase (MPO) (donor, hydrogen peroxide oxidoreductase, EC 1.11.1.7) is the most abundant neutrophil enzyme and catalyzes predominantly the two-electron oxidation of ubiquitous chloride (Cl-), to generate the potent bleaching oxidant hypochlorous acid (HOCl), thus contributing to bacterial killing and inflammatory reactions of neutrophils. Here, the thermodynamics of the one-electron reduction of the ferric heme in its ferric high-spin and cyanide-bound low-spin forms were determined through spectroelectrochemical experiments. The E(o)' values for free and cyanide-bound MPO (5 and -37 mV, respectively, at 25 degrees C and pH 7.0) are significantly higher than those of other heme peroxidases. Variable-temperature experiments revealed that the enthalpic stabilization of ferric high-spin MPO is much weaker than in other heme peroxidases and is exactly compensated by the entropic change upon reduction. In contrast to those of other heme peroxidases, the stabilization of the ferric cyanide-bound MPO is also very weak and fully entropic. This peculiar behavior is discussed with respect to the MPO-typical covalent heme to protein linkages as well as to the published structures of ferric MPO and its cyanide complex and the recently published structure of lactoperoxidase as well as the physiological role of MPO in bacterial killing.

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“…Of interest was how the coordination number of axially bound pyridine depends on the oxidation state of the immobilized cohalt porphyrin and its complex stability constants. A base of comparative electrochemical data for the interaction of pyridine with dissolved cobalt tetraphenylporphyrin is avail~ble [6][7][8].…”

mentioning

confidence: 99%

“…Of interest was how the coordination number of axially bound pyridine depends on the oxidation state of the immobilized cobalt porphyrin and its complex stability constants. A base of comparative electrochemical data for the interaction of pyridine with dissolved cobalt tetraphenylporphyrin is available (6)(7)(8). Also of interest was a better understanding of the previously noted (2) abnormally small electrochemical surface wave for reduction of immobilized C/,~.-Co(III)(NH2)4TPP as compared to C/,x.,Co(II) ( NH2 ) 4TPP.…”

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confidence: 99%

Electron Transfer and Axial Coordination Reactions of Cobalt Tetra(Aminophenyl)Porphyrins Covalently Bonded to Carbon Electrodes

Jester

Rocklin

Murray

1980

J. Electrochem. Soc.

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Electrochemistry of tetra(aminophenyl)porphyrin covalently attached to glassy carbon and then cobalt‐metallated, was investigated in DMSO and CH3CN solvents in the presence of pyridine. Shifts in formal potential were used to measure complex stability constants and coordination number for the pyridine complexes of the immobilized metalloporphyrin. Unusually slow electrochemistry of the Co(III/II) reaction was also studied by cyclic voltammetry and by reflectance spectroelectrochemistry. It is proposed that coordination of cobalt by surface carboxylate groups causes extraordinarily slow electron transfer for the Co (III/II) step.

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Electrochemical Studies on the Thermodynamics of Electron Transfer and Ligand Binding of Several Metalloporphyrins in Aprotic Solvents

Cited by 8 publications

References 0 publications

Control of Cytochrome c Redox Potential: Axial Ligation and Protein Environment Effects

Control of Cytochrome c Redox Potential: Axial Ligation and Protein Environment Effects

Redox Thermodynamics of the Fe(III)/Fe(II) Couple of Human Myeloperoxidase in Its High-Spin and Low-Spin Forms

Electron Transfer and Axial Coordination Reactions of Cobalt Tetra(Aminophenyl)Porphyrins Covalently Bonded to Carbon Electrodes

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