Electrochemical behavior of the tris(pyridine)-Cu funnel complexes: an overall induced-fit process involving an entatic state through a supramolecular stress

Position of substituents directs the electron transfer properties of entatic state complexes: new insights from guanidine-quinoline copper complexes

In a previous study, we showed that the properties and the ability as an entatic state model of copper guanidine quinoline complexes are significantly influenced by a methyl or methyl ester substituent in the 2-position. To prove the importance of the 2-position of the substituent, two novel guanidine quinoline ligands with a methyl or methyl ester substituent in the 4-position and the corresponding copper complexes were synthesized and characterized in this study. The influence of the substituent position on the copper complexes was investigated with various experimental and theoretical methods. The molecular structures of the copper complexes were examined in the solid state by single-crystal X-ray diffraction (SCXRD) and by density functional theory (DFT) calculations indicating a strong dependency on the substituent position compared to the systems substituted in the 2-position from the previous study. Further, the significantly different influence on the donor properties in dependency on the substituent position was analyzed with natural bond orbital (NBO) calculations. By the determination of the redox potentials, the impact on the electrochemical stabilization was examined. With regard to further previously analyzed guanidine quinoline copper complexes, the electrochemical stabilization was correlated with the charge-transfer energies calculated by NBO analysis and ground state energies, revealing the substituent influence and enabling a comparatively easy and accurate possibility for the theoretical calculation of the relative redox potential. Finally, the electron transfer properties were quantified by determining the electron self-exchange rates via the Marcus theory and by theoretical calculation of the reorganization energies via Nelsen's four-point method. The results gave important insights into the dependency between the ability of the copper complexes as entatic state model and the type and position of the substituent.

Biological concepts for catalysis and reactivity: empowering bioinspiration

Biological systems provide attractive reactivity blueprints for the design of challenging chemical transformations. Emulating the operating mode of natural systems may however not be so easy and direct translation of structural observations does not always afford the anticipated efficiency. Metalloenzymes rely on earth-abundant metals to perform an incredibly wide range of chemical transformations. To do so, enzymes in general have evolved tools and tricks to enable control of such reactivity. The underlying concepts related to these tools are usually well-known to enzymologists and bio(inorganic) chemists but may be a little less familiar to organometallic chemists. So far, the field of bioinspired catalysis has greatly focused on the coordination sphere and electronic effects for the design of functional enzyme models but might benefit from a paradigm shift related to recent findings in biological systems. The goal of this review is to bring these fields closer together as this could likely result in the development of a new generation of highly efficient bioinspired systems. This contribution covers the fields of redox-active ligands, entatic state reactivity, energy conservation through electron bifurcation, and quantum tunneling for C-H activation.

Three polyoxometalates-based organic–inorganic hybrids decorated with Cu–terpyridine complexes exhibiting dual functional electro-catalytic behaviors

Three new polyoxometalates-based organic–inorganic hybrids decorated with Cu–terpyridine complexes were prepared by using one-pot methods. Compounds 1–3 demonstrate discrepant dual functional electro-catalytic activities toward reduction of nitrite and oxidation of ascorbic acid.

A Trishistidine Pseudopeptide with Ability to Remove Both CuΙ and CuΙΙ from the Amyloid-β Peptide and to Stop the Associated ROS Formation

The pseudopeptide L, derived from a nitrilotriacetic acid scaffold and functionalized with three histidine moieties, is reminiscent of the amino acid side chains encountered in the Alzheimer's peptide (Aβ). Its synthesis and coordination properties for CuΙ and CuΙΙ are described. L efficiently complex CuΙΙ in a square-planar geometry involving three imidazole nitrogen atoms and an amidate-Cu bond. By contrast, CuΙ is coordinated in a tetrahedral environment. The redox behavior is irreversible and follows an ECEC mechanism in accordance with the very different environments of the two redox states of the Cu center. This is in line with the observed resistance of the CuΙ complex to oxidation by oxygen and the CuΙΙ complex reduction by ascorbate. The affinities of L for CuΙΙ and CuΙ at physiological pH are larger than that reported for the Aβ peptide. Therefore, due to its peculiar Cu coordination properties, the ligand L is able to target both redox states of Cu, redox silence them and prevent reactive oxygen species production by the CuAβ complex. Because reactive oxygen species contribute to the oxidative stress, a key issue in Alzheimer's disease, this ligand thus represents a new strategy in the long route of finding molecular concepts for fighting Alzheimer's disease.

Supramolecular modeling of mono-copper enzyme active sites with calix[6]arene-based funnel complexes

Supramolecular bioinorganic chemistry is a natural evolution in biomimetic metallic systems since it constitutes a further degree of complexity in modeling. The traditional approach consisting of mimicking the first coordination sphere of metal sites proved to be very efficient, because valuable data are extracted from these examples to gain insight in natural systems mechanisms. But it does not reproduce several specific aspects of enzymes that can be mimicked by the implementation of a cavity embedding the labile active site and thus controlling the properties of the metal ion by noncovalent interactions. This Account reports on a strategy aimed at reproducing some supramolecular aspects encountered in the natural systems. The cavity complexes described herein display a coordination site constructed on a macrocycle. Thanks to a careful design of the cavity-based ligands, complexes orienting their labile site specifically toward the inside of the macrocycle were obtained. The supramolecular systems are based on the flexible calix[6]arene core that surrounds the metal ion labile site, thereby constraining exogenous molecules to pass through the conic funnel to reach the metal center. Such an architecture confers to the metal ion very unusual properties and behaviors, which in many aspects are biologically relevant. Three generations of calix[6]-based ligands are presented and discussed in the context of modeling the monocopper sites encountered in some enzymes. A wide range of phenomena are highlighted such as the impact that the size and shape of the access channel to the metal center have on the selectivity and rate of the binding process, the possible remote control of the electronics through small modifications operated on the cavity edges, induced-fit behavior associated with host-guest association (shoe-tree effect) that affects the redox properties of the metal ion and the electron exchange pathway, consequences of forbidden associative ligand exchange allowing a redox switch to drive an "antithermodynamic" ligand exchange, drastic effects of the full control of the second coordination sphere, and dioxygen activation in a confined chamber conducted to a selective and unusual four-electron redox process. All these findings bring new clues for better understanding the control exerted by the proteic environment on a metal center, allow the identification of new reaction pathways, and lead to new proposals for enzymatic catalytic cycle (such as the formation of an alkylhydroperoxide intermediate for mononuclear Cu-hydroxylases). The supramolecular systems may also be exploited for designing highly selective and sensitive probes for molecules of particular function and shape or to design new selective catalysts.

Insights into water coordination associated with the CuII/CuI electron transfer at a biomimetic Cu centre

The coordination chemistry of an aqua Cu complex was investigated in non-coordinating solvents and in ionic liquids.

Electrochemical and homogeneous electron transfers to the Alzheimer amyloid-beta copper complex follow a preorganization mechanism

Deciphering the electron transfer reactivity characteristics of amyloid β-peptide copper complexes is an important task in connection with the role they are assumed to play in Alzheimer's disease. A systematic analysis of this question with the example of the amyloid β-peptide copper complex by means of its electrochemical current-potential responses and of its homogenous reactions with electrogenerated fast electron exchanging osmium complexes revealed a quite peculiar mechanism: The reaction proceeds through a small fraction of the complex molecules in which the peptide complex is "preorganized" so as the distances and angles in the coordination sphere to vary minimally upon electron transfer, thus involving a remarkably small reorganization energy (0.3 eV). This preorganization mechanism and its consequences on the reactivity should be taken into account for reactions involving dioxygen and hydrogen peroxide that are considered to be important in Alzheimer's disease through the production of harmful reactive oxygen species.

Monocopper center embedded in a biomimetic cavity: from supramolecular control of copper coordination to redox regulation

The electrochemical behavior of diversely substituted Cu-N3-calix[6]arene, enzyme-like, "funnel" complexes is analyzed. The Cu(II)/Cu(I) redox process is regulated by the supramolecular organization of the Cu coordination. The presence of a "shoetree" alkyl nitrile guest molecule inside the host cavity is a prerequisite for a dynamic redox behavior. Combination of supramolecular CH-pi weak interactions with the calixarene cavity and electronic/steric effects from the N3 substituting groups (pyridine, imidazole, pyrrolidine) enforces the preferential geometrical pattern adopted by Cu. This dictates the pathway of the electron-transfer process and, thus, the thermodynamics and kinetics of the redox reaction in the framework of a square-scheme mechanism. The present observations recall strongly the redox control exerted by the protein matrix on copper proteins through biological concepts such as induced fit mechanism, protein foldings, and entatic and allosteric effects.

Insights into the binding properties of a cuprous ion embedded in the tren cap of a calix[6]arene and supramolecular trapping of an intermediate

Coordination of Cu(I) to a tren unit that is covalently linked to a calix[6]arene has been explored. The resulting complex revealed itself very stable in solution under an inert atmosphere, but extremely sensitive to O2 in solution as well as in the solid state. Therefore, its binding properties towards non-redox ligands have been studied in detail. The electron-rich metal center displays moderate affinity for nitrilo ligands compared to the calix[6]tris-pyridine ligand. Indeed, the binding enthalpy with acetonitrile is only -30 kJ mol(-1), whereas it is -72 kJ mol(-1) with the tris-pyridine system. In contrast, CO binding is relatively strong due to important pi-back donation from the metal center, as evidenced by the CO stretch, which was found to be less energetic (2075 cm(-1)) than that measured for ligands based on aromatic donors such as imidazole or pyridine. The conformational and dynamic properties of this calix-system have also been studied in detail. With an empty cavity or with the very small CO guest-ligand, the calix-core undergoes partial self-inclusion leading to dissymmetrical conformations. In contrast, nitrilo ligands act as "shoe-trees" that maintain the calix-core in a C(3v) symmetrical cone conformation. Very interestingly, the variable T study relative to the ligand exchange process highlighted a two-step dissociative pathway, where Cu-N bond cleavage/formation is differentiated from the nitrilo guest expulsion/inclusion from/into the calixarene cavity.

Effect of the Methyl-Coenzyme-M Reductase Protein Matrix on the Hole-Size and Nonplanar Deformations of Coenzyme F430

Methyl-coenzyme-M reductase (MCR) is a key enzyme common to all methane-producing pathogens. It catalyses the final step in methane synthesis. Each MCR contains two noncovalently bound molecules of cofactor F430. Normal-coordinate structural decomposition, hole-size analysis, and molecular mechanics calculations were undertaken to examine the effect of MCR on the hole-size and nonplanar deformations of coenzyme F430. In MCR, the protein prevents F430 from undergoing nonplanar deformations, which results in a more rigid tetrahydrocorphinoid cofactor that has a shorter ideal metal-nitrogen distance in the MCR protein matrix than it does in solution. Changing the coordination number of the nickel ion in F430 has a very small effect on the ideal hole size; however, it has a significant effect on the nonplanar deformations the coenzyme undergoes upon contraction and expansion. In all complexes we examined, cofactor F430 undergoes more nonplanar deformations when it contains a four-coordinate metal ion than it does when it contains a six-coordinate metal ion. Clearly, MCR moderates the hole-size and the nonplanar deformations of coenzyme F430, which are known to affect redox potentials and axial ligand affinities. This suggests that the protein environment may be responsible for tuning the chemistry of the active-site nickel ion.

Calix[6]tren and copper(II): a third generation of funnel complexes on the way to redox calix-zymes

Mono-copper enzymes play an important role in biology and their functionality is based on Cu(II)/Cu(I) redox processes. Modeling a mono-nuclear site remains a challenge for a better understanding of its intrinsic reactivity. The first member of a third generation of calixarene-based mono-copper "funnel" complexes is described. The ligand is a calix[6]arene capped by a tren unit, hence presenting a N(4) coordination site confined in a cavity. Its Cu(II) complexes were characterized by electronic and EPR spectroscopies. The x-ray structure of one of them shows a five-coordinated metal ion in a slightly distorted trigonal bipyramidal geometry thanks to its coordination to a guest ligand L (ethanol). The latter sits in the heart of the hydrophobic calixarene cone that mimics the active site chamber and the hydrophobic access channel of enzymes. Competitive binding experiments showed a preference order dimethylformamide > ethanol > MeCN for L binding at the single exchangeable metal site. Cyclic voltammetry studies showed irreversible redox processes in CH(2)Cl(2) when L is an oxygen donor caused by the redox-driven ejection of the guest at the Cu(I) level. In the presence of MeCN, a pseudoreversible process was obtained, owing to a fast equilibrium between a four and a five-coordinate Cu(I) species. Finally, a redox-driven ligand interchange of dimethylformamide for MeCN at the Cu(I) state allowed the trapping of the thermodynamically less stable Cu(II)-MeCN adduct. Hence, this work represents an important step toward the elaboration of a functional supramolecular model for redox mono-copper enzymes, named redox calix-zymes.