Dioxygen reactivity of copper and heme-copper complexes possessing an imidazole-phenol cross-link

A Bioinspired Nonheme FeIII-(O22-)-CuII Complex with an St = 1 Ground State

Cytochrome c oxidase (CcO) is a heme copper oxidase (HCO) that catalyzes the natural reduction of oxygen to water. A profound understanding of some of the elementary steps leading to the intricate 4e-/4H+ reduction of O2 is presently lacking. A total spin St = 1 FeIII-(O22-)-CuII (IP) intermediate is proposed to reduce the overpotentials associated with the reductive O-O bond rupture by allowing electron transfer from a tyrosine moiety without the necessity of any spin-surface crossing. Direct evidence of the involvement of IP in the CcO catalytic cycle is, however, missing. A number of heme copper peroxido complexes have been prepared as synthetic models of IP, but all of them possess the catalytically nonrelevant St = 0 ground state resulting from antiferromagnetic coupling between the S = 1/2 FeIII and CuII centers. In a complete nonheme approach, we now report the spectroscopic characterization and reactivity of the FeIII-(O22-)-CuII intermediates 1 and 2, which differ only by a single -CH3 versus -H substituent on the central amine of the tridentate ligands binding to copper. Complex 1 with an end-on peroxido core and ferromagnetically (St = 1) coupled FeIII and CuII centers performs H-bonding-mediated O-O bond cleavage in the presence of phenol to generate oxoiron(IV) and exchange-coupled copper(II) and PhO• moieties. In contrast, the μ-η2:η1 peroxido complex 2, with a St = 0 ground state, is unreactive toward phenol. Thus, the implications for spin topology contributions to O-O bond cleavage, as proposed for the heme FeIII-(O22-)-CuII intermediate in CcO, can be extended to nonheme chemistry.

Heme-copper and Heme O2-derived synthetic (bioinorganic) chemistry toward an understanding of cytochrome c oxidase dioxygen chemistry

Cytochrome c oxidase (CcO), also widely known as mitochondrial electron-transport-chain complex IV, is a multi-subunit transmembrane protein responsible for catalyzing the last step of the electron transport chain, dioxygen reduction to water, which is essential to the establishment and maintenance of the membrane proton gradient that drives ATP synthesis. Although many intermediates in the CcO catalytic cycle have been spectroscopically and/or computationally authenticated, the specifics regarding the IP intermediate, hypothesized to be a heme-Cu (hydro)peroxo species whose O-O bond homolysis is supported by a hydrogen-bonding network of water molecules, are largely obscured by the fast kinetics of the A (FeIII-O2•-/CuI/Tyr) → PM (FeIV=O/CuII-OH/Tyr•) step. In this review, we have focused on the recent advancements in the design, development, and characterization of synthetic heme-peroxo‑copper model complexes, which can circumvent the abovementioned limitation, for the investigation of the formation of IP and its O-O cleavage chemistry. Novel findings regarding (a) proton and electron transfer (PT/ET) processes, together with their contributions to exogenous phenol induced O-O cleavage, (b) the stereo-electronic tunability of the secondary coordination sphere (especially hydrogen-bonding) on the geometric and spin state alteration of the heme-peroxo‑copper unit, and (c) a plausible mechanism for the Tyr-His cofactor biogenesis, are discussed in great detail. Additionally, since the ferric-superoxide and the ferryl-oxo (Compound II) species are critically involved in the CcO catalytic cycle, this review also highlights a few fundamental aspects of these heme-only (i.e., without copper) species, including the structural and reactivity influences of electron-donating trans-axial ligands and Lewis acid-promoted H-bonding.

Tryptophan Can Promote Oxygen Reduction to Water in a Biosynthetic Model of Heme Copper Oxidases

Heme-copper oxidases (HCOs) utilize tyrosine (Tyr) to donate one of the four electrons required for the reduction of O₂ to water in biological respiration, while tryptophan (Trp) is speculated to fulfill the same role in cyt bd oxidases. We previously engineered myoglobin into a biosynthetic model of HCOs and demonstrated the critical role that Tyr serves in the oxygen reduction reaction (ORR). To address the roles of Tyr and Trp in these oxidases, we herein report the preparation of the same biosynthetic model with the Tyr replaced by Trp and further demonstrate that Trp can also promote the ORR, albeit with lower activity. An X-ray crystal structure of the Trp variant shows a hydrogen-bonding network involving two water molecules that are organized by Trp, similar to that in the Tyr variant, which is absent in the crystal structure with the native Phe residue. Additional electron paramagnetic resonance measurements are consistent with the formation of a Trp radical species upon reacting with H₂O₂. We attribute the lower activity of the Trp variant to Trp's higher reduction potential relative to Tyr. Together, these findings demonstrate, for the first time, that Trp can indeed promote the ORR and provides a structural basis for the observation of varying activities. The results support a redox role for the conserved Trp in bd oxidase while suggesting that HCOs use Tyr instead of Trp to achieve higher reactivity.

Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems

Activation and reduction of O₂ and H₂O₂ by synthetic and biosynthetic iron porphyrin models have proved to be a versatile platform for evaluating second-sphere effects deemed important in naturally occurring heme active sites. Advances in synthetic techniques have made it possible to install different functional groups around the porphyrin ligand, recreating artificial analogues of the proximal and distal sites encountered in the heme proteins. Using judicious choices of these substituents, several of the elegant second-sphere effects that are proposed to be important in the reactivity of key heme proteins have been evaluated under controlled environments, adding fundamental insight into the roles played by these weak interactions in nature. This review presents a detailed description of these efforts and how these have not only demystified these second-sphere effects but also how the knowledge obtained resulted in functional mimics of these heme enzymes.

Heme-Cu Binucleating Ligand Supports Heme/O2 and FeII-CuI/O2 Reactivity Providing High- and Low-Spin FeIII-Peroxo-CuII Complexes

The focus of this study is in the description of synthetic heme/copper/O₂ chemistry employing a heme-containing binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O₂ (-80 °C, tetrahydrofuran (THF) solvent) to the reduced heme compound (P^ImH)Fe^II (1), gives the oxy-heme adduct, formally a heme-superoxide complex Fe^III-(O₂^•-) (2) (resonance Raman spectroscopy (rR): ν_O-O, 1171 cm^-1 (Δ¹⁸O₂, -61 cm^-1); ν_Fe-O, 575 cm^-1 (Δ¹⁸O₂, -24 cm^-1)). Simple warming of 2 to room temperature regenerates reduced complex 1; this reaction is reversible, as followed by UV-vis spectroscopy. Complex 2 is electron paramagnetic resonance (EPR)-silent and exhibits upfield-shifted pyrrole resonances (δ 9.12 ppm) in ²H NMR spectroscopy, indicative of a six-coordinate low-spin heme. The coordination of the tethered imidazolyl arm to the heme-superoxide complex as an axial base ligand is suggested. We also report the new fully reduced heme-copper complex [(P^ImH)Fe^IICu^I]⁺ (3), where the copper ion is bound to the tethered tridentate portion of P^ImH. This reacts with O₂ to give a distinctive low-temperature-stable, high-spin (S = 2, overall) peroxo-bridged complex [(P^ImH)Fe^III-(O₂^2-)-Cu^II]⁺ (3a): λ_max, 420 (Soret), 545, 565 nm; δ_pyrr, 93 ppm; ν_O-O, 799 cm^-1 (Δ¹⁸O₂, -48 cm^-1); ν_Fe-O, 524 cm^-1 (Δ¹⁸O₂, -23 cm^-1). To 3a, the addition of dicyclohexylimidazole (DCHIm), which serves as a heme axial base, leads to low-spin (S = 0 overall) species complex [(DCHIm)(P^ImH)Fe^III-(O₂^2-)-Cu^II]⁺ (3b): λ_max, 425 (Soret), 538 nm; δ_pyrr, 10.2 ppm; ν_O-O, 817 cm^-1 (Δ¹⁸O₂, -55 cm^-1); ν_Fe-O, 610 cm^-1 (Δ¹⁸O₂, -26 cm^-1). These investigations into the characterization of the O₂-adducts from (P^ImH)Fe^II (1) with/without additional copper chelation advance our understanding of the dioxygen reactivity of heme-only and heme/Cu-ligand heterobinuclear system, thus potentially relevant to O₂ reduction in heme-copper oxidases or fuel-cell chemistry.

Formation and Reactivity of New Isoporphyrins: Implications for Understanding the Tyr-His Cross-Link Cofactor Biogenesis in Cytochrome c Oxidase

Cytochrome c oxidase (CcO) catalyzes the reduction of dioxygen to water utilizing a heterobinuclear active site composed of a heme moiety and a mononuclear copper center coordinated to three histidine residues, one of which is covalently cross-linked to a tyrosine residue via a post-translational modification (PTM). Although this tyrosine-histidine moiety has functional and structural importance, the pathway behind this net oxidative C-N bond coupling is still unknown. A novel route employing an iron(III) meso-substituted isoporphyrin derivative, isoelectronic with Cmpd-I ((Por^•+)Fe^IV═O), is for the first time proposed to be a key intermediate in the Tyr-His cofactor biogenesis. Newly synthesized iron(III) meso-substituted isoporphyrins were prepared with azide, cyanide, and substituted imidazole functionalities, by adding nucleophiles to an iron(III) π-dication species formed via addition of trifluoroacetic acid to F₈Cmpd-I (F₈ = (tetrakis(2,6-difluorophenyl)porphyrinate)). Isoporphyrin derivatives were characterized at cryogenic temperatures via ESI-MS and UV-vis, ²H NMR, and EPR spectroscopies. Addition of 1,3,5-trimethoxybenzene or 4-methoxyphenol to the imidazole-substituted isoporphyrin led to formation of the organic product containing the imidazole coupled to aromatic substrate via a new C-N bond, as detected via cryo-ESI-MS. Experimental evidence for the formation of an imidazole-substituted isoporphyrin and its promising reactivity to form the imidazole-phenol coupled product yields viability to the herein proposed pathway behind the PTM (i.e., biogenesis) leading to the key covalent Tyr-His cross-link in CcO.

Geometric and Electronic Structure Contributions to O-O Cleavage and the Resultant Intermediate Generated in Heme-Copper Oxidases

This study investigates the mechanism of O-O bond cleavage in heme-copper oxidase (HCO) enzymes, combining experimental and computational insights from enzyme intermediates and synthetic models. It is determined that HCOs undergo a proton-initiated O-O cleavage mechanism where a single water molecule in the active site enables proton transfer (PT) from the cross-linked tyrosine to a peroxo ligand bridging the heme Fe^III and Cu^II, and multiple H-bonding interactions lower the tyrosine p K_a. Due to sterics within the active site, the proton must either transfer initially to the O(Fe) (a high-energy intermediate), or from another residue over a ∼10 Å distance to reach the O(Cu) atom directly. While the distance between the H⁺ donor (Tyr) and acceptor (O(Cu)) results in a barrier to PT, this separation is critical for the low barrier to O-O cleavage as it enhances backbonding from Fe into the O₂^2- σ* orbital. Thus, PT from Tyr precedes O-O elongation and is rate-limiting, consistent with available kinetic data. The electron transfers from tyrosinate after the barrier via a superexchange pathway provided by the cross-link, generating intermediate P_M. P_M is evaluated using available experimental data. The geometric structure contains an Fe^IV═O that is H-bonded to the Cu^II-OH. The electronic structure is a singlet, where the Fe^IV and Cu^II are antiferromagnetically coupled through the H-bond between the oxo(Fe) and hydroxo(Cu) ligands, while the Cu^II and Tyr^• are ferromagnetically coupled due their delocalization into orthogonal magnetic orbitals on the cross-linked His residue. These findings provide critical insights into the mechanism of efficient O₂ reduction in HCOs, and the nature of the P_M intermediate that couples this reaction to proton pumping.

Spin Interconversion of Heme-Peroxo-Copper Complexes Facilitated by Intramolecular Hydrogen-Bonding Interactions

Synthetic peroxo-bridged high-spin (HS) heme-(μ-η2:η1-O22-)-Cu(L) complexes incorporating (as part of the copper ligand) intramolecular hydrogen-bond (H-bond) capabilities and/or steric effects are herein demonstrated to affect the complex's electronic and geometric structure, notably impacting the spin state. An H-bonding interaction with the peroxo core favors a low-spin (LS) heme-(μ-η1:η1-O22-)-Cu(L) structure, resulting in a reversible temperature-dependent interconversion of spin state (5 coordinate HS to 6 coordinate LS). The LS state dominates at low temperatures, even in the absence of a strong trans-axial heme ligand. Lewis base addition inhibits the H-bond facilitated spin interconversion by competition for the H-bond donor, illustrating the precise H-bonding interaction required to induce spin-crossover (SCO). Resonance Raman spectroscopy (rR) shows that the H-bonding pendant interacts with the bridging peroxide ligand to stabilize the LS but not the HS state. The H-bond (to the Cu-bound O atom) acts to weaken the O-O bond and strengthen the Fe-O bond, exhibiting ν(M-O) and ν(O-O) values comparable to analogous known LS complexes with a strong donating trans-axial ligand, 1,5-dicyclohexylimidazole, (DCHIm)heme-(μ-η1:η1-O22-)-Cu(L). Variable-temperature (-90 to -130 °C) UV-vis and 2H NMR spectroscopies confirm the SCO process and implicate the involvement of solvent binding. Examining a case of solvent binding without SCO, thermodynamic parameters were obtained from a van't Hoff analysis, accounting for its contribution in SCO. Taken together, these data provide evidence for the H-bond group facilitating a core geometry change and allowing solvent to bind, stabilizing a LS state. The rR data, complemented by DFT analysis, reveal a stronger H-bonding interaction with the peroxo core in the LS compared to the HS complexes, which enthalpically favors the LS state. These insights enhance our fundamental understanding of secondary coordination sphere influences in metalloenzymes.

Influence of intramolecular secondary sphere hydrogen-bonding interactions on cytochrome c oxidase inspired low-spin heme-peroxo-copper complexes

Dioxygen reduction by heme-copper oxidases is a critical biochemical process, wherein hydrogen bonding is hypothesized to participate in the critical step involving the active-site reductive cleavage of the O-O bond. Sixteen novel synthetic heme-(μ-O2 2-)-Cu(XTMPA) complexes, whose design is inspired by the cytochrome c oxidase active site structure, were generated in an attempt to form the first intramolecular H-bonded complexes. Derivatives of the "parent" ligand (XTMPA, TMPA = (tris((2-pyridyl)methyl)amine)) possessing one or two amine pendants preferentially form an H-bond with the copper-bound O-atom of the peroxide bridge. This is evidenced by a characteristic blue shift in the ligand-to-metal charge transfer (LMCT) bands observed in UV-vis spectroscopy (consistent with lowering of the peroxo π* relative to the iron orbitals) and a weakening of the O-O bond determined by resonance Raman spectroscopy (rR), with support from Density Functional Theory (DFT) calculations. Remarkably, with the TMPA-based infrastructure (versus similar heme-peroxo-copper complexes with different copper ligands), the typically undetected Cu-O stretch for these complexes was observed via rR, affording critical insights into the nature of the O-O peroxo core for the complexes studied. While amido functionalities have been shown to have greater H-bonding capabilities than their amino counterparts, in these heme-peroxo-copper complexes amido substituents distort the local geometry such that H-bonding with the peroxo core only imparts a weak electronic effect; optimal H-bonding interactions are observed by employing two amino groups on the copper ligand. The amino-substituted systems presented in this work reveal a key orientational anisotropy in H-bonding to the peroxo core for activating the O-O bond, offering critical insights into effective O-O cleavage chemistry. These findings indirectly support computational and protein structural studies suggesting the presence of an interstitial H-bonding water molecule in the CcO active site, which is critical for the desired reactivity. The results are evaluated with appropriate controls and discussed with respect to potential O2-reduction capabilities.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Critical Aspects of Heme-Peroxo-Cu Complex Structure and Nature of Proton Source Dictate Metal-O(peroxo) Breakage versus Reductive O-O Cleavage Chemistry

The 4H+/4e- reduction of O2 to water, a key fuel-cell reaction also carried out in biology by oxidase enzymes, includes the critical O-O bond reductive cleavage step. Mechanistic investigations on active-site model compounds, which are synthesized by rational design to incorporate systematic variations, can focus on and resolve answers to fundamental questions, including protonation and/or H-bonding aspects, which accompany electron transfer. Here, we describe the nature and comparative reactivity of two low-spin heme-peroxo-Cu complexes, LS-4DCHIm, [(DCHIm)F8FeIII-(O22-)-CuII(DCHIm)4]+, and LS-3DCHIm, [(DCHIm)F8FeIII-(O22-)-CuII(DCHIm)3]+ (F8 = tetrakis(2,6-difluorophenyl)-porphyrinate; DCHIm = 1,5-dicyclohexylimidazole), toward different proton (4-nitrophenol and [DMF·H+](CF3SO3-)) (DMF = dimethyl-formamide) or electron (decamethylferrocene (Fc*)) sources. Spectroscopic reactivity studies show that differences in structure and electronic properties of LS-3DCHIm and LS-4DCHIm lead to significant differences in behavior. LS-3DCHIm is resistant to reduction, is unreactive toward weakly acidic 4-NO2-phenol, and stronger acids cleave the metal-O bonds, releasing H2O2. By contrast, LS-4DCHIm forms an adduct with 4-NO2-phenol, which includes an H-bond to the peroxo O-atom distal to Fe (resonance Raman (rR) spectroscopy and DFT). With addition of Fc* (2 equiv overall required), O-O reductive cleavage occurs, giving water, Fe(III), and Cu(II) products; however, a kinetic study reveals a one-electron rate-determining process, ket = 1.6 M-1 s-1 (-90 °C). The intermediacy of a high-valent [(DCHIm)F8FeIV═O] species is thus implied, and separate experiments show that one-electron reduction-protonation of [(DCHIm)F8FeIV═O] occurs faster (ket2 = 5.0 M-1 s-1), consistent with the overall postulated mechanism. The importance of the H-bonding interaction as a prerequisite for reductive cleavage is highlighted.

An FT-IR and DFT study of the free and solvated 4-(imidazol-1-yl)phenol

In this study, FT-IR spectrum of 4-(imidazol-1-yl)phenol was recorded. Its vibrational frequencies and modes were determined. Vibrational assignments were proposed with the help of B3LYP/6-311++G(d,p) level of calculations. Three possible dimeric forms of the molecule were investigated theoretically. Besides, solvent effects on the structure, vibrational frequencies, and atomic charges were studied theoretically. Water, dimethyl sulfoxide, and ethanol were the solvents considered. Experimental FT-IR spectrum in DMSO solution was recorded and compared with solid phase experimental data. DFT B3LYP combined with polarized continuum model (PCM) was employed to characterize the solvent effects.

A biosynthetic model of cytochrome c oxidase as an electrocatalyst for oxygen reduction

Creating an artificial functional mimic of the mitochondrial enzyme cytochrome c oxidase (CcO) has been a long-term goal of the scientific community as such a mimic will not only add to our fundamental understanding of how CcO works but may also pave the way for efficient electrocatalysts for oxygen reduction in hydrogen/oxygen fuel cells. Here we develop an electrocatalyst for reducing oxygen to water under ambient conditions. We use site-directed mutants of myoglobin, where both the distal Cu and the redox-active tyrosine residue present in CcO are modelled. In situ Raman spectroscopy shows that this catalyst features very fast electron transfer rates, facile oxygen binding and O-O bond lysis. An electron transfer shunt from the electrode circumvents the slow dissociation of a ferric hydroxide species, which slows down native CcO (bovine 500 s(-1)), allowing electrocatalytic oxygen reduction rates of 5,000 s(-1) for these biosynthetic models.

Heme-copper-dioxygen complexes: toward understanding ligand-environmental effects on the coordination geometry, electronic structure, and reactivity

The nature of the ligand is an important aspect of controlling the structure and reactivity in coordination chemistry. In connection with our study of heme-copper-oxygen reactivity relevant to cytochrome c oxidase dioxygen-reduction chemistry, we compare the molecular and electronic structures of two high-spin heme-peroxo-copper [Fe(III)O(2)(2-)Cu(II)](+) complexes containing N(4) tetradentate (1) or N(3) tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to density functional theory calculations, we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish mu-(O(2)(2-)) side-on to the Fe(III) and end-on to Cu(II) (mu-eta(2):eta(1)) binding for the complex 1 but side-on/side-on (mu-eta(2):eta(2)) mu-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh(3), acid, and phenols. The comparison of a new X-ray structure of mu-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two mu-oxo complexes ( angleFe-O-Cu = 178.2 degrees in 1a and angleFe-O-Cu = 149.5 degrees in 2a). The reaction of 2 with 1 equiv of an exogenous nitrogen-donor axial base leads to the formation of a distinctive low-temperature-stable, low-spin heme-dioxygen-copper complex (2b), but under the same conditions, the addition of an axial base to 1 leads to the dissociation of the heme-peroxo-copper assembly and the release of O(2). 2b reacts with phenols performing H-atom (e(-) + H(+)) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [Fe(IV)=O] complex (2c). The nature of 2c was confirmed by a comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the H-atom abstraction was either (1) directly detected by electron paramagnetic resonance spectroscopy using phenols that produce stable radicals or (2) indirectly detected by the coupling product of two phenoxyl radicals.

Role of copper ion in regulating ligand binding in a myoglobin-based cytochrome C oxidase model

Cytochrome c oxidase (CcO), the terminal enzyme in the mitochondrial respiratory chain, catalyzes the four-electron reduction of dioxygen to water in a binuclear center comprised of a high-spin heme (heme a(3)) and a copper atom (Cu(B)) coordinated by three histidine residues. As a minimum model for CcO, a mutant of sperm whale myoglobin, named Cu(B)Mb, has been engineered, in which a copper atom is held in the distal heme pocket by the native E7 histidine and two nonnative histidine residues. In this work, the role of the copper in regulating ligand binding in Cu(B)Mb was investigated. Resonance Raman studies show that the presence of copper in CO-bound Cu(B)Mb leads to a CcO-like distal heme pocket. Stopped-flow data show that, upon the initiation of the CO binding reaction, the ligand first binds to the Cu(+); it subsequently transfers from Cu(+) to Fe(2+) in an intramolecular process, similar to that reported for CcO. The high CO affinity toward Cu(+) and the slow intramolecular CO transfer rate between Cu(+) and Fe(2+) in the Cu(B)Mb/Cu(+) complex are analogous to those in Thermus thermophilus CcO (TtCcO) but distinct from those in bovine CcO (bCcO). Additional kinetic studies show that, upon photolysis of the NO-bound Cu(B)Mb/Cu(+) complex, the photolyzed ligand transiently binds to Cu(+) and subsequently rebinds to Fe(2+), accounting for the 100% geminate recombination yield, similar to that found in TtCcO. The data demonstrate that the Cu(B)Mb/Cu(+) complex reproduces essential structural and kinetic features of CcO and that the complex is more akin to TtCcO than to bCcO.

A spectroscopic investigation of a tridentate Cu-complex mimicking the tyrosine-histidine cross-link of cytochrome C oxidase

Heme-copper oxidases have a crucial role in the energy transduction mechanism, catalyzing the reduction of dioxygen to water. The reduction of dioxygen takes place at the binuclear center, which contains heme a3 and CuB. The X-ray crystal structures have revealed that the C6' of tyrosine 244 (bovine heart numbering) is cross-linked to a nitrogen of histidine 240, a ligand to CuB. The role of the cross-linked tyrosine at the active site still remains unclear. In order to provide insight into the function of the cross-linked tyrosine, we have investigated the spectroscopic and electrochemical properties of chemical analogues of the CuB-His-Tyr site. The analogues, a tridentate histidine-phenol cross-linked ether ligand and the corresponding Cu-containing complex, were previously synthesized in our laboratory (White, K.; et al. Chem. Commun. 2007, 3252-3254). Spectrophotometric titrations of the ligand and the Cu-complex indicate a pKa of the phenolic proton of 8.8 and 7.7, respectively. These results are consistent with the cross-linked tyrosine playing a proton delivery role at the cytochrome c oxidase active site. The presence of the phenoxyl radical was investigated at low temperature using electron paramagnetic resonance (EPR) and Fourier transform infrared (FT-IR) difference spectroscopy. UV photolysis of the ligand, without bound copper, generated a narrow g=2.0047 signal, attributed to the phenoxyl radial. EPR spectra recorded before and after UV photolysis of the Cu-complex showed a g=2 signal characteristic of oxidized copper, suggesting that the copper is not spin-coupled to the phenoxyl radical. An EPR signal from the phenoxyl radical was not observed in the Cu-complex, either due to spin relaxation of the two unpaired electrons or to masking of the narrow phenoxyl radical signal by the strong copper contribution. Stable isotope (13C) labeling of the phenol ring (C1') Cu-complex, combined with photoinduced difference FT-IR spectroscopy, revealed bands at 1485 and 1483 cm(-1) in the 12C-minus-13C-isotope-edited spectra of the ligand and Cu-complex, respectively. These bands are attributed to the radical v7a stretching frequency and are shifted to 1468 and 1472 cm(-1), respectively, with 13C1' labeling. These results show that a radical is generated in both the ligand and the Cu-complex and support the unambiguous assignment of a vibrational band to the phenoxyl radical v7a stretching mode. These data are discussed with respect to a possible role of the cross-linked tyrosine radical in cytochrome c oxidase.

Functional biomimetic models for the active site in the respiratory enzyme cytochrome c oxidase

A functional analog of the active site in the respiratory enzyme, cytochrome c oxidase (CcO) reproduces every feature in CcO's active site: a myoglobin-like heme (heme a3), a distal tridentate imidazole copper complex (Cu(B)), a phenol (Tyr244), and a proximal imidazole. When covalently attached to a liquid-crystalline SAM film on an Au electrode, this functional model continuously catalyzes the selective four-electron reduction of dioxygen at physiological potential and pH, under rate-limiting electron flux (as occurs in CcO).

Model studies of azide binding to functional analogues of CcO

N3- binding to a functional model of CcO is investigated in its Fe3+, Fe3+Cu+, and Fe3+Cu2+ forms. A combination of EPR and FTIR indicates that N3- binds in a bridging mode in the bimetallic sites and signature N3- bands are identified for several forms of N3- binding to the site. The presence of the distal metal increases the binding affinity of N3-. This bridging enables antiferromagnetic interaction between the two metal centers in the Fe3+Cu2+ state, which results in an EPR-silent ground state.

Synthesis, characterization, spectroscopic and thermodynamic studies of charge transfer interaction of a new water-soluble cobalt(II) Schiff base complex with imidazole derivatives

The water-soluble cobalt(II) tetradentate Schiff base complex [Co(II)L](ClO4)(2), L: (N,N'-bis(5-[(triphenylphosphonium)-methyl]salicylidine)-o-phenylenediamineperchlorate has been synthesized and characterized. This complex forms charge transfer (CT) complexes with imidazole and 1-methylimidazole. The formation constant, molar absorptivity (epsilon'), and thermodynamic parameters for charge transfer complexes formation of cobalt(II) Schiff base complexes with imidazole derivatives were determined by using UV-vis spectrophotometric method in aqueous solutions at constant ionic strength (I=0.2mol dm(-3) KNO3) at pH 6.0 and various temperatures between 292 and 315K.

Synthetic Models of the Active Site of Cytochromec Oxidase: Influence of Tridentate or Tetradentate Copper Chelates Bearing a HisTyr Linkage Mimic on Dioxygen Adduct Formation by Heme/Cu Complexes

Two synthetic models of the active site of cytochrome c oxidase--[(LN4-OH)CuI-FeII(TMP)]+ (1 a) and [(LN3-OH)CuI-FeII(TMP)]+ (2 a)-have been designed and synthesized. These models each contain a heme and a covalently attached copper moiety supported either by a tetradentate N4-copper chelate or by a tridentate N3-copper chelate including a moiety that acts as a mimic of the crosslinked His-Tyr component of cytochrome c oxidase. Low-temperature oxygenation reactions of these models have been investigated by spectroscopic methods including UV/Vis, resonance Raman, ESI-MS, and EPR spectroscopy. Oxygenation of the tetradentate model 1 a in MeCN and in other solvents produces a low-temperature-stable dioxygen-bridged peroxide [(LN4-OH)CuII-O2-FeIII(TMP)]+ {nuO--O=799 (16O2)/752 cm(-1) (18O2)}, while a heme superoxide species [(TMP)FeIII(O2-)CuIILN3-OH] {nuFe--O2: 576 (16O2)/551 cm(-1) (18O2)} is generated when the tridentate model 2 a is oxygenated in EtCN solution under similar experimental conditions. The coexistence of a heme superoxide species [(TMP)FeIII(O2-)CuIILN3-OH] and a bridged peroxide [(LN3-OH)CuII-O2-FeIII(TMP)]+ species in equal amounts is observed when the oxygenation reaction of 2 a is performed in CH2Cl(2)/7 % EtCN, while the percentage of the peroxide (approximately 70 %) in relation to superoxide (approximately 30 %) increases further when the crosslinked phenol moiety in 2 a is deprotonated to produce the bridged peroxide [(LN3-OH)CuII-O2-FeIII(TMP)]+ {nuO--O: 812 (16O2)/765 cm(-1) (18O2)} as the main dioxygen intermediate. The weak reducibility and decreased O2 reactivity of the tricoordinated CuI site in 2 a are responsible for the solvent-dependent formation of dioxygen adducts. The initial binding of dioxygen to the copper site en route to the formation of a bridged heme-O2-Cu intermediate by model 2 a is suggested and the deprotonated crosslinked His-Tyr moiety might contribute to enhancement of the O2 affinity of the CuI site at an early stage of the dioxygen-binding process.

Heme-copper/dioxygen adduct formation, properties, and reactivity

This Account focuses on our recent developments in synthetic heme/copper/O 2 chemistry, potentially relevant to the mechanism of action of heme-copper oxidases (e.g., cytochrome c oxidase) and to dioxygen activation chemistry. Methods for the generation of O 2 adducts, which are high-spin heme(Fe (III))-peroxo-Cu (II) complexes, are described, along with a detailed structural/electronic characterization of one example. The coordination mode of the O 2-derived heme-Cu bridging group depends upon the copper-ligand environment, resulting in micro-(O 2 (2-)) side-on to Fe (III) and end-on to Cu (II) (micro-eta (2):eta (1)) binding for cases having N 4 tetradentate ligands but side-on/side-on (micro-eta (2):eta (2)) micro-peroxo coordination with tridentate copper chelates. The dynamics of the generation of Fe (III)-(O 2 (2-))-Cu (II) complexes are known in some cases, including the initial formation of a short-lived superoxo (heme)Fe (III)(O 2 -) intermediate. Complexes with cross-linked imidazole-phenol "cofactors" adjacent to the copper centers have also been described. Essential investigations of heme-copper-mediated reductive O-O bond cleavage chemistry are ongoing.

A new dinuclear heme-copper complex derived from functionalized protoporphyrin IX

A new biomimetic model for the heterodinuclear heme/copper center of respiratory oxidases is described. It is derived from iron(III) protoporphyrin IX by covalent attachment of a Gly-L-His-OMe residue to one propionic acid substituent and an amino-bis(benzimidazole) residue to the other propionic acid substituent of the porphyrin ring, yielding the Fe(III) complex 1, and subsequent addition of a copper(II) or copper(I) ion, according to needs. The fully oxidized Fe(III)/Cu(II) complex, 2, binds azide more strongly than 1, and likely contains azide bound as a bridging ligand between Fe(III) and Cu(II). The two metal centers also cooperate in the reaction with hydrogen peroxide, as the peroxide adducts obtained at low temperature for 1 and 2 display different optical features. Support to this interpretation comes from the investigation of the peroxidase activity of the complexes, where the activation of hydrogen peroxide has been studied through the phenol coupling reaction of p-cresol. Here the presence of Cu(II) improves the catalytic performance of complex 2 with respect to 1 at acidic pH, where the positive charge of the Cu(II) ion is useful to promote O-O bond cleavage of the iron-bound hydroperoxide, but it depresses the activity at basic pH because it can stabilize an intramolecular hydroxo bridge between Fe(III) and Cu(II). The reactivity to dioxygen of the reduced complexes has been studied at low temperature starting from the carbonyl adducts of the Fe(II) complex, 3, and Fe(II)/Cu(I) complex, 4. Also in this case the adducts derived from the Fe(II) and Fe(II)/Cu(I) complexes, that we formulate as Fe(III)-superoxo and Fe(III)/Cu(II)-peroxo exhibit slightly different spectral properties, showing that the copper center participates in a weak interaction with the dioxygen moiety.

Synthesis and structural characterization of cross-linked histidine–phenol Cu(ii) complexes as cytochrome c oxidase active site models

Tridentate cross-linked histidine-phenol Cu(ii) ether and ester complexes, chemical analogs of the active site of several heme-copper oxidases, have been synthesized and crystallized.

Molecular oxygen activation by a molybdenum(iv) monooxo bis(β-ketiminato) complex

Molybdenum(IV) monooxo compound that contains bis(beta-ketiminato) ligands activates molecular oxygen forming a molybdenum(VI) monooxo peroxo compound, representing a new entry into molybdenum peroxo derivatives.

Evolutionary migration of a post-translationally modified active-site residue in the proton-pumping heme-copper oxygen reductases

In the respiratory chains of aerobic organisms, oxygen reductase members of the heme-copper superfamily couple the reduction of O2 to proton pumping, generating an electrochemical gradient. There are three distinct families of heme-copper oxygen reductases: A, B, and C types. The A- and B-type oxygen reductases have an active-site tyrosine that forms a unique cross-linked histidine-tyrosine cofactor. In the C-type oxygen reductases (also called cbb3 oxidases), an analogous active-site tyrosine has recently been predicted by molecular modeling to be located within a different transmembrane helix in comparison to the A- and B-type oxygen reductases. In this work, Fourier-transform mass spectrometry is used to show that the predicted tyrosine forms a histidine-tyrosine cross-linked cofactor in the active site of the C-type oxygen reductases. This is the first known example of the evolutionary migration of a post-translationally modified active-site residue. It also verifies the presence of a unique cofactor in all three families of proton-pumping respiratory oxidases, demonstrating that these enzymes likely share a common reaction mechanism and that the histidine-tyrosine cofactor may be a required component for proton pumping.

Built-in Axial Base Binding on Phenanthroline-Strapped Zinc(II) and Iron(III) Porphyrins

In addition to the need for functional models of cytochrome c oxidase, structural models are still required for a better understanding of the small reorganizations occurring during the catalytic cycle. An efficient synthetic approach has been designed to prepare several phenanthroline-strapped porphyrins, two of them bearing two pendant imidazoles. These built-in bases are both potentially able to act as axial bases for the metalloporphyrin and as complementary ligands for copper if necessary. Diamagnetic zinc(II) was used to demonstrate that the distal/proximal selectivity demonstrated by exogenic bases binding studies can be extended to the coordination of iron(III). Combination of EPR and paramagnetic 1H NMR shows that the imidazole binding on the zinc species can be further extended to the iron(III) species in dilute conditions.

Identification of a histidine-tyrosine cross-link in the active site of the cbb3-type cytochrome c oxidase from Rhodobacter sphaeroides

The heme-copper oxidases constitute a superfamily of terminal dioxygen-reducing enzymes located in the inner mitochondrial or in the bacterial cell membrane. The presence of a mechanistically important covalent bond between a histidine ligand of the copper ion (Cu(B)) in the active site and a generally conserved tyrosine residue nearby has been shown to exist in the canonical cytochrome c oxidases. However, according to sequence alignment studies, this critical tyrosine is missing from the subfamily of cbb(3)-type oxidases found in certain bacteria. Recently, homology modeling has suggested that a tyrosine residue located in a different helix might fulfill this role in these enzymes. Here, we show directly by methods of protein chemistry and mass spectrometry that there is indeed a covalent link between this tyrosine and the copper-ligating histidine. The identity of the cross-linked tyrosine was determined by showing that the cross-link is not formed when this residue is replaced by phenylalanine, even though structural integrity is maintained. These results suggest a universal functional importance of the histidine-tyrosine cross-link in the mechanism of O(2) reduction by all heme-copper oxidases.

Model studies of the CuBsite of cytochrome c oxidase utilizing a Zn(ii) complex containing an imidazole–phenol cross-linked ligand

Cytochrome c oxidase, the enzyme complex responsible for the four-electron reduction of O2 to H2O, contains an unusual histidine-tyrosine cross-link in its bimetallic heme a3-CuB active site. We have synthesised an unhindered, tripodal chelating ligand, BPAIP, containing the unusual ortho-imidazole-phenol linkage, which mimics the coordination environment of the CuB center. The ligand was used to investigate the physicochemical (pKa, oxidation potential) and coordination properties of the imidazole-phenol linkage when bound to a dication. Zn(II) coordination lowers the pKa of the phenol by 0.6 log units, and increases the potential of the phenolate/phenoxyl radical couple by approximately 50 mV. These results are consistent with inductive withdrawal of electron density from the phenolic ring. Spectroscopic data and theoretical calculations (DFT) were used to establish that the cationic complex [Zn(BPAIP)Br]+ has an axially distorted trigonal bipyramidal structure, with three coordinating nitrogen ligands (two pyridine and one imidazole) occupying the equatorial plane and the bromide and the tertiary amine nitrogen of the tripod in the axial positions. Interestingly, the Zn-Namine bonding interaction is weak or absent in [Zn(BPAIP)Br]+ and the complex gains stability in basic solutions, as indicated by 1H NMR spectroscopy. These observations are supported by theoretical calculations (DFT), which suggest that the electron-donating capacity of the equatorial imidazole ligand can be varied by modulation of the protonation and/or redox state of the cross-linked phenol. Deprotonation of the phenol makes the equatorial imidazole a stronger sigma-donor, resulting in an increased Zn-Nimd interaction and thereby leading to distortion of the axial ligand axis toward a more tetrahedral geometry.

Single-turnover intermolecular reaction between a Feiii–superoxide–Cuicytochrome c oxidase model and exogeneous Tyr244 mimics

An Fe(III)-superoxide-Cu(I) cytochrome c oxidase model reacts intermolecularly with hindered phenols leading to phenoxyl radicals, as was observed in the enzyme and evidence for the formation of an Fe(IV)-oxo is presented.