Rates of electron-transfer reactions of some copper(II)-phenanthroline complexes with cytochrome c(II) and tris(phenanthroline)cobalt(II) ion

Augustin, Mary Ann; Yandell, John K.

doi:10.1021/ic50193a011

Cited by 45 publications

(28 citation statements)

References 4 publications

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“…From analysis of the ET kinetics, a reorganization energy of 2.1-2.3 eV was calculated. This relatively high λ is consistent with values for Cu(II)/(I) reorganization energies in unconstrained complexes; for example, λ for the Cu(II)/(I) (1,10-phenanthroline) 2 complex is 2.4 eV (35), and λ for unfolded WT Az is approximately the same (36). In contrast, intramolecular ET in the type 0 Cu Az mutant, Cys112Asp/Met121Leu, with a structurally more constrained copper site, is characterized by a much smaller reorganization energy (0.9-1.1 eV).…”

Section: Discussionsupporting

confidence: 78%

Designed azurins show lower reorganization free energies for intraprotein electron transfer

Farver

Marshall

Wherland

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Low reorganization free energies are necessary for fast electron transfer (ET) reactions. Hence, rational design of redox proteins with lower reorganization free energies has been a long-standing challenge, promising to yield a deeper understanding of the underlying principles of ET reactivity and to enable potential applications in different energy conversion systems. Herein we report studies of the intramolecular ET from pulse radiolytically produced disulfide radicals to Cu(II) in rationally designed azurin mutants. In these mutants, the copper coordination sphere has been fine-tuned to span a wide range of reduction potentials while leaving the metal binding site effectively undisrupted. We find that the reorganization free energies of ET within the mutants are indeed lower than that of WT azurin, increasing the intramolecular ET rate constants almost 10-fold: changes that are correlated with increased flexibility of their copper sites. Moreover, the lower reorganization free energy results in the ET rate constants reaching a maximum value at higher driving forces, as predicted by the Marcus theory.cupredoxin | Type 1 copper | blue copper | reorganization energy | Marcus inverted region E lectron transfer (ET) through proteins is central to biological energy conversion processes, from photosynthesis to respiration (1). ET rates between redox centers are tightly controlled and tuned in biological systems for efficiency and to avoid formation of deleterious products (2-5). An important parameter determining the ET rate is the reorganization free energy of the redox sites (6-8), which for biochemical ET reactions has evolved to low values, usually providing high rate constants. Despite many years of work, few efforts have succeeded in designing protein ET sites having reorganization free energies lower than those observed for the native ones. For example, numerous mutations proximal to the type 1 (T1) blue copper site have been introduced into azurin (Az) (9-19), a bacterial electron-mediating protein, but those that have lowered this barrier have not been reported yet.Prominent among the theories developed to rationalize the control of ET reaction rates (20-22) is that of R. A. Marcus (23). A particularly intriguing element of this theory predicts that the ET rate reaches its maximum when the driving force of the reaction equals the reorganization free energy, after which an even higher driving force decreases ET rate, reaching the "Marcus inverted region." It has been proposed that the dramatic difference in rates between some of the forward and backward ET reactions in the photosynthetic reaction center is a result of the latter being in the inverted region, which is reached because of exceptionally low reorganization free energies of the relevant ET sites (24).In the present study, we aim at examining how the major changes in modifying the redox potential of the type 1 copper site in Az achieved by Marshall et al. (25) affect its ET reactivity. A major obstacle in rationally fine-tuning the properties of t...

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Section: Discussionsupporting

confidence: 78%

Designed azurins show lower reorganization free energies for intraprotein electron transfer

Farver

Marshall

Wherland

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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“…Such ET behavior is exemplified by the coordination complex [Cu(phen) 2 ] 2+ (phen = 1,10-phenanthroline), with λ= 2.4 eV. 1 Nature has overcome this problem to allow copper to perform redox functions in living systems: type 1 sites in proteins have dramatically lowered λ's (~ 0.6–0.8 eV), 2 with self-exchange ET rate constants as much as ~ 10 6 higher than those of Cu II/I model complexes.…”

Section: Introductionmentioning

confidence: 99%

Outer-Sphere Contributions to the Electronic Structure of Type Zero Copper Proteins

et al. 2012

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Bioinorganic canon states that active-site thiolate coordination promotes rapid electron transfer (ET) to and from type 1 copper proteins. In recent work, we have found that copper ET sites in proteins also can be constructed without thiolate ligation (called “type zero” sites). Here we report multifrequency electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and nuclear magnetic resonance (NMR) spectroscopic data together with density functional theory (DFT) and spectroscopy-oriented configuration interaction (SORCI) calculations for type zero Pseudomonas aeruginosa azurin variants. Wild-type (type 1) and type zero copper centers experience virtually identical ligand fields. Moreover, O-donor covalency is enhanced in type zero centers relative that in the C112D (type 2) protein. At the same time, N-donor covalency is reduced in a similar fashion to type 1 centers. QM/MM and SORCI calculations show that the electronic structures of type zero and type 2 are intimately linked to the orientation and coordination mode of the carboxylate ligand, which in turn is influenced by outer-sphere hydrogen bonding.

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“…[3, 7] Typically, the ligand frameworks for these studies exploit steric interactions or a macrocyclic ligand to achieve fast ET rates. [8] In contrast to the covalent coordination strategies most commonly employed by small molecule models, synthetic frameworks incorporating secondary sphere H-bonding interactions as a means to stabilize entatic states are exceedingly rare, yet are critical to the ‘rack’ in BCPs. [9] …”

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confidence: 99%

“…[3, 30] A plot of [Cu II ] vs π ΔW (W=line width in Hz) provided a slope that correlates to an ET self-exchange rate of 2.4 × 10 5 M −1 s −1 for 1 in THF at room temperature (see SI), which is the same order of magnitude as the fasted, reported synthetic systems as well as BCPs (10 4 –10 6 M −1 s −1 ). [8a, 8b, 31] …”

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confidence: 99%

Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square‐Planar Copper(I) Complex

Dahl

Szymczak

2016

Angew Chem Int Ed

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6,6″-bis(2,4,6-trimethylanilido)terpyridine (H2TpyNMes) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H2TpyNMes)copper(I)-halide (Cl, Br and I) complexes is dictated by the strength of the NH-halide hydrogen bond. The Cu(I)Cl and Cu(II)Cl complexes are nearly isostructural, the former presenting a highly unusual square planar geometry about Cu(I). The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid CuI/II electron transfer self-exchange rates. Cu(H2TpyNMes)Cl shows similar fast electron transfer (~105 M−1s−1) which is the same order of magnitude as biological systems.

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Rates of electron-transfer reactions of some copper(II)-phenanthroline complexes with cytochrome c(II) and tris(phenanthroline)cobalt(II) ion

Cited by 45 publications

References 4 publications

Designed azurins show lower reorganization free energies for intraprotein electron transfer

Designed azurins show lower reorganization free energies for intraprotein electron transfer

Outer-Sphere Contributions to the Electronic Structure of Type Zero Copper Proteins

Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square‐Planar Copper(I) Complex

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