Labile Site Research Articles

TMPA‐based Cavitary Cobalt (II) Funnel Complexes

AbstractThe synthesis of a series of mononuclear, dicationic CoII funnel complexes is reported herein. Three ligands Calix‐TMPAX present a calix[6]arene cone closed at its small rim by a tris(2‐pyridylmethyl)amine (TMPA) unit and differ by the nature of three cavity walls, anisole, phenol or quinone. The X‐ray diffraction structure of [CoII(MeCN)Calix‐TMPAOMe](ClO4)2 displays a trigonal bipyramidal geometry, with Co bound to all 4 nitrogen atoms of the TMPA cap, and to one MeCN guest molecule buried inside the calixarene cavity. All complexes were fully characterized in solution as high spin 5‐coordinate species using various techniques, including 1H NMR spectroscopy. For comparison purpose, an analogous CoII complex based on the TMPACH2OH ligand, devoid of a calixarene core, was synthetized. Its X‐ray structure shows a dicationic 7‐coordinate cobalt(II) center in the N4O3 environment provided by the ligand, leaving no space for exogenous ligand binding. This contrasts with the 5‐coordinate complexes obtained with the calix‐ligands that allow guest‐ligand binding and exchange. A brief overview of the coordination properties of the calix‐complexes, compared to those obtained with TMPA ligands, devoid of a cavity, highlights major differences in terms of complexation kinetics, geometry, coordination to the labile site, anion affinity, nuclearity, and stability.

Selective Metal-ion Complexation of a Biomimetic Calix[6]arene Funnel Cavity Functionalized with Phenol or Quinone.

In the biomimetic context, many studies have evidenced the importance of the 1st and 2nd coordination sphere of a metal ion for controlling its properties. Here, we propose to evaluate a yet poorly explored aspect, which is the nature of the cavity that surrounds the metal labile site. Three calix[6]arene-based aza-ligands are compared, that differ only by the nature of cavity walls, anisole, phenol or quinone (LOMe , LOH and LQ ). Monitoring ligand exchange of their ZnII complexes evidenced important differences in the metal ion relative affinities for nitriles, halides or carboxylates. It also showed a possible sharp kinetic control on both, metal ion binding and ligand exchange. Hence, this study supports the observations reported on biological systems, highlighting that the substitution of an amino-acid residue of the enzyme active site, at remote distance of the metal ion, can have strong impacts on metal ion lability, substrate/product exchange or selectivity.

Well‐defined Nickel P3C Complexes as Hydrogenation Catalysts of N‐Heteroarenes Under Mild Conditions

AbstractThe reactivity of metal complexes can be primarily controlled by their coordinating ligands. Electronic effects and geometric restrictions imposed by ligands, prototypically modify their reactivity, but are not always straightforward accessible. In the course of the study, we explored new C3‐symmetric organometallic nickel (II) complexes ((P3C)Ni)) with the particularity of having a metalated apical carbon trans to a labile coordination site readily available as a catalytic site. The synthetic methodology allowed to tune the electronic properties by introducing substituents on the phosphines. We studied the nature of the C−Ni bond by X‐ray absorption spectroscopy (XAS) and DFT and applied them to the catalytic hydrogenation of quinolines with a good chemical scope and group tolerance. 1H‐NMR monitoring, isotopic labelling, and computational studies are consistent with a heterolytic activation of the dihydrogen molecule after forming a σ‐H2 adduct trans to the metalated carbon.

Asymmetric construction of tetrahedral chiral zinc with high configurational stability and catalytic activity

Chiral metal complexes show promise as asymmetric catalysts and optical materials. Chiral-at-metal complexes composed of achiral ligands have expanded the versatility and applicability of chiral metal complexes, especially for octahedral and half-sandwich complexes. However, Werner-type tetrahedral complexes with a stereogenic metal centre are rarely used as chiral-at-metal complexes because they are too labile to ensure the absolute configuration of the metal centre. Here we report the asymmetric construction of a tetrahedral chiral-at-zinc complex with high configurational stability, using an unsymmetric tridentate ligand. Coordination/substitution of a chiral auxiliary ligand on zinc followed by crystallisation yields an enantiopure chiral-only-at-zinc complex (> 99% ee). The enantiomer excess remains very high at 99% ee even after heating at 70 °C in benzene for one week. With this configurationally stable zinc complex of the tridentate ligand, the remaining one labile site on the zinc can be used for a highly selective asymmetric oxa-Diels-Alder reaction (98% yield, 87% ee) without substantial racemisation.

Can Papain-like Protease Inhibitors Halt SARS-CoV-2 Replication?

SARS-CoV-2 encoded papain-like protease (PLpro) harbors a labile Zn site (Cys189–X–X–Cys192–Xn–Cys224–X–Cys226) and a classic catalytic site (Cys111–His272–Asp286), which play key roles for viral replication and hence represent promising drug targets. In this Viewpoint, both sulfur-based drugs and peptides-based inhibitors may block Cys residues in the catalytic and/or Zn site of CoV-2-PLpro, leading to dysfunction of CoV-2-PLpro and thereby halting viral replication.

Synthesis, Characterization, and Reactivity of a Series of Homo‐ and Hetero‐dinuclear Complexes based on an Asymmetric FloH Ligand System

In a previous communication we reported the site‐directed generation of a heterodinuclear FeIIICuII complex (1) by using an asymmetric dinucleating ligand FloH. The iron(III) ion was introduced first on the preferential metal‐binding site of the ligand that led to the formation of the thermodynamically favored five‐membered chelate ring upon metal‐binding. Copper(II) was introduced in the next step. The stepwise metalation strategy reported previously has now been extended to synthesize a series of heterodinuclear FeIIIMII [M = Mn (2), Fe (3), Co (4), and Ni (5)] and FeIICuI (1a) as well as the homodinuclear CuICuI (6) complexes. The complexes were characterized by X‐ray crystallography (except for 1a and 6), and by a limited number of spectroscopic methods. Complex 1 with a labile solvent binding site at FeIII reacted with H2O2 to form a transient intermediate that showed reactivity typical of metal peroxide complexes. The metal centers in the complexes 2–5 are coordinatively saturated, and hence they showed no reactivity with H2O2. Complex 1a reacted with O2 via an intermolecular pathway to form a μ‐oxo bridged tetrameric complex 1b, which was structurally characterized. This is in contrast to the homodinuclear CuICuI and heme FeIICuI cores, which prefer an intramolecular pathway for O2 activation.

Synthesis and Binding Properties of a Tren-Capped Hexahomotrioxacalix[3]arene.

The straightforward synthesis of a new hexahomotrioxacalix[3]arene-based ligand capped by a tren subunit was developed and the binding properties of the corresponding zinc complex were explored by NMR spectroscopy. Similarly to the closely related calix[6]tren-based systems, the homooxacalixarene core ensures the mononuclearity of the zinc complex and the metal center displays a labile coordination site for exogenous guests. However, very different host-guest properties were observed: i) in CDCl3 , the zinc complex strongly binds a water molecule and is reluctant to recognize other neutral guests, ii) in CD3 CN, the exo-coordination of anions prevails. Thus, in strong contrast to the calix[6]tren-based systems, the coordination of neutral guests that thread through the small rim and fill the polyaromatic cavity was not observed. This unique behaviour is likely due to the fact that the 18-membered ethereal macrocycle is too small to let a molecule threading through it. This work illustrates the key role played by the second coordination sphere in the binding properties of metal complexes.

Crystal structures and supramolecular architectures of ONO donor hydrazone and solvent exchangeable dioxidomolybdenum(VI) complexes derived from 3,5-diiodosalicyaldehyde-4-methoxybenzoylhydrazone: Hirshfeld surface analysis and interaction energy calculations

A tridentate ONO donor aroylhydrazone, 3,5-diiodosalicyaldehyde-4-methoxybenzoylhydrazone (H2DSMB), and two of its cis-MoO2 complexes, [MoO2(DSMB)(H2O)](DMF)2 (1) and [MoO2(DSMB)(py)] (2), have been synthesized and characterized by different physico-chemical methods. The molecular structures were determined by single crystal X-ray diffraction studies. The aroylhydrazone H2DSMB crystallizes in a monoclinic space group with two identical molecules in the asymmetric unit. The complex [MoO2(DSMB)(H2O)](DMF)2 (1) crystallizes in a triclinic space group whereas the other complex, [MoO2(DSMB)(py)] (2), crystallizes in an orthorhombic crystal system. The distorted octahedral geometry around the Mo(VI) central atom is satisfied by the di-deprotonated dianionic hydrazone moiety and two oxido oxygen atoms, the sixth labile coordination site being occupied by oxygen/nitrogen atoms from solvent molecules. The importance of van der Waals interactions in the formation of the elementary structure obtained from the single crystal X-ray diffraction was authenticated by Hirshfeld surface analysis. The individual types of intermolecular contacts and their impact on the crystal packing was investigated by the analysis of the Hirshfeld surfaces and the signatures on the two-dimensional fingerprint plots. Interaction energy calculations reveal that the dispersion energy framework is dominant over other energy frameworks. The thermal behavior and the nature of the water molecule in the complex [MoO2(DSMB)(H2O)](DMF)2 (1) is discussed. Furthermore, in vitro cytotoxicity studies of the synthesized aroylhydrazone and its Mo(VI) complexes against the lymphoma ascites cell line are also included.

Controlling Substrate Binding to Fe4S4 Clusters through Remote Steric Effects.

The extraordinary reactivity exhibited by many Fe-S enzymes is due in large part to the influence of the protein scaffold on substrate binding and activation. In principle, the coordination chemistry of synthetic Fe-S clusters could similarly be controlled through remote steric effects. Toward this end, we report the synthesis of 3:1 site-differentiated [Fe4S4] clusters ligated by N -heterocyclic carbene (NHC) ligands with variable steric profiles: IMes (1,3-dimesitylimidazol-2-ylidene) and I iPrMe (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). Treatment of (IMes)3Fe4S4Cl with NaBArF4 in ethereal solvents (Et2O and THF) leads to the formation of an ether adduct, [(IMes)3Fe4S4(solv)][BArF4]; solvent can be displaced by addition of tBuNC to form the unusual monoisocyanide adduct [(IMes)3Fe4S4(CN tBu)][BArF4]. Carrying out the same reactions with the less sterically encumbered cluster (I iPrMe)3Fe4S4Cl results in more typical reactivity: undesired ligand redistribution to form the homoleptic cluster [(I iPrMe)4Fe4S4][BArF4] and generation of the triisocyanide adduct [(I iPrMe)3Fe4S4(CN tBu)3][BArF4]. The increased steric profile of the IMes ligands disfavors ligand redistribution and defines a binding pocket at the apical Fe, thereby enabling the generation of a coordinatively unsaturated and substitutionally labile Fe site. This method of controlling the coordination chemistry at the apical Fe site by modifying the sterics of ligands bound to adjacent Fe sites complements existing strategies for generating site-differentiated Fe-S clusters and provides new opportunities to direct reactivity at cuboidal metalloclusters.

Design, synthesis and biological evaluation of novel, orally bioavailable pyrimidine-fused heterocycles as influenza PB2 inhibitors

With the aim to identify novel influenza PB2 inhibitors with high potency and excellent pharmacokinetic parameters, we have designed and synthesized two new series of pyrimidine-fused heterocycle derivatives based on two generations of co-crystal structures. Docking studies with the newly disclosed PDB structure guided the second round of rational design and led to the discovery of 25m, 25o and 25p as representative compounds with improved potency (EC50 < 1 nM). After pinpointing the metabolic labile site, the CN replacement of compound 25p successfully produced compound 29c, which demonstrated highly improved PK properties (Cl = 1.3 mL/min/kg, PO AUC = 152 μM h at 10 mpk in mouse, F = 57%) and improved potency, emerging as a promising lead compound for the treatment of influenza A infection.

Analysis of the Active Site Cysteine Residue of the Sacrificial Sulfur Insertase LarE from Lactobacillus plantarum

LarE from Lactobacillus plantarum is an ATP-dependent sulfur transferase that sacrifices its Cys176 sulfur atom to form a dehydroalanine (Dha) side chain during biosynthesis of the covalently linked nickel-pincer nucleotide (NPN) cofactor (pyridinium 3-thioamide-5-thiocarboxylic acid mononucleotide) of lactate racemase. Coenzyme A (CoA) stabilizes LarE and forms a CoA-Cys176 mixed disulfide with the protein. This study presents the crystal structure of the LarE/CoA complex, revealing protein interactions with CoA that mimic those for binding ATP. CoA weakly inhibits LarE activity, and the persulfide of CoA is capable of partially regenerating functional LarE from the Dha176 form of the protein. The physiological relevance of this cycling reaction is unclear. A new form of LarE was discovered, an NPN-LarE covalent adduct, explaining prior results in which activation of the lactate racemase apoprotein required only the isolated LarE. The crystal structure of the inactive C176A variant revealed a fold essentially identical to that of wild-type LarE. Additional active site variants of LarE were created and their activities characterized, with all LarE variants analyzed in terms of the structure. Finally, the L. plantarum LarE structure was compared to a homology model of Thermoanaerobacterium thermosaccharolyticum LarE, predicted to contain three cysteine residues at the active site, and to other proteins with a similar fold and multiple active site cysteine residues. These findings suggest that some LarE orthologs may not be sacrificial but instead may catalyze sulfur transfer by using a persulfide mechanism or from a labile site on a [4Fe-4S] cluster at this position.

Metal Coordination to Unsymmetric 1,n′‐Disubstituted Ferrocene Histidine Peptides

In this study, the coordination chemistry of unsymmetrical 1,n′‐substituted ferrocene (Fc) peptide conjugates is explored, where the peptide substituents are two different histidine‐containing peptides. The resulting Fc‐conjugates serve as ligands for the formation of metal complexes, with the intention to generate complexes that possess a labile coordination site. The following Fc[CO‐His(Trt)‐OMe][COOMe] (1), Fc[CO‐His(Trt)‐Gly‐OMe][CO‐Gly‐OMe] (2), Fc[CO‐His(Trt)‐His(Trt)‐OMe][CO‐His(Trt)‐OMe] (3), and Fc[CO‐His(Trt)‐Gly‐His(Trt)‐OMe][CO‐Gly‐His(Trt)‐OMe] (4) were synthesized and characterized. Each compound forms discrete complexes with metal divalent metal ions Zn2+, Co2+ and Cd2+, where a single metal ion is complexed by the metallo‐ligand, as was confirmed spectroscopically. The redox properties of the Fc group are influenced by metal coordination to the peptide substituents and anodic shifts occur upon the addition of metal ions. These follow the order Zn2+ &gt; Cd2+ &gt; Co2+. NMR experiments indicated imidazole rings are the site of metal coordination. Fc‐peptide conjugates display a switchable axial chirality that is dependent on metal complexation.

Functional modeling of the MnCAT active site with a dimanganese(III) complex of an unsymmetrical polydentate N3O3 ligand

A new diMnIII complex, [Mn2L(OAc)2(H2O)](BPh4)·3H2O (1), obtained with the unsymmetrical N3O3-ligand H3L = 1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N′-(2-hydroxybenzyl),N′-(benzyl)amino]propan-2-ol, has been prepared and characterized. The unsymmetrical hexadentate ligand L3− leads to coordination dissymmetry (dissimilar donor atoms) around each Mn ion (N2O4 and NO4(solvent), respectively) leaving one labile site on one of the two Mn ions that facilitates interaction of the metal center with H2O2, as in Mn catalase. 1 is able to catalyze H2O2 disproportionation in acetonitrile, with second-order rate constant kcat = 23.9(2) M−1 s−1. The accessibility of the MnII2 state and the closeness of the two one-electron reduction processes suggest 1 employs MnIII2/MnII2 oxidation states for catalysis.

A trapped human PPM1A–phosphopeptide complex reveals structural features critical for regulation of PPM protein phosphatase activity

Metal-dependent protein phosphatases (PPM) are evolutionarily unrelated to other serine/threonine protein phosphatases and are characterized by their requirement for supplementation with millimolar concentrations of Mg2+ or Mn2+ ions for activity in vitro The crystal structure of human PPM1A (also known as PP2Cα), the first PPM structure determined, displays two tightly bound Mn2+ ions in the active site and a small subdomain, termed the Flap, located adjacent to the active site. Some recent crystal structures of bacterial or plant PPM phosphatases have disclosed two tightly bound metal ions and an additional third metal ion in the active site. Here, the crystal structure of the catalytic domain of human PPM1A, PPM1Acat, complexed with a cyclic phosphopeptide, c(MpSIpYVA), a cyclized variant of the activation loop of p38 MAPK (a physiological substrate of PPM1A), revealed three metal ions in the active site. The PPM1Acat D146E-c(MpSIpYVA) complex confirmed the presence of the anticipated third metal ion in the active site of metazoan PPM phosphatases. Biophysical and computational methods suggested that complex formation results in a slightly more compact solution conformation through reduced conformational flexibility of the Flap subdomain. We also observed that the position of the substrate in the active site allows solvent access to the labile third metal-binding site. Enzyme kinetics of PPM1Acat toward a phosphopeptide substrate supported a random-order, bi-substrate mechanism, with substantial interaction between the bound substrate and the labile metal ion. This work illuminates the structural and thermodynamic basis of an innate mechanism regulating the activity of PPM phosphatases.

430 Preclinical evaluation of a novel non-steroidal glucocorticoid receptor agonist with low systemic exposure and reduced potential for skin atrophy

Topical corticosteroids are widely used efficacious drugs but are associated with both local and systemic adverse effects. With the aim to provide an effective but safer topical treatment, we here describe a novel non-steroidal glucocorticoid receptor (GR) agonist with low systemic exposure and reduced potential for skin atrophy induction. LEO 134310 was shown to be a full GR agonist with high affinity for GR and low nanomolar potency in an inflammatory functional assay in human PBMCs. GR was the only nuclear hormone receptor affected by LEO 134310 in a panel of nuclear receptors, including the mineralocorticoid receptor, a receptor which has been associated with induction of skin atrophy. In blood and liver LEO 134310 is rapidly inactivated at a labile site, leading to high systemic clearance. In accordance with this profile, topical treatment with LEO 134310 in a TPA-induced dermatitis mouse model resulted in comparable efficacy to betamethasone valerate (BMV), with no systemic adverse effects at maximally effective doses. In contrast, BMV and clobetasol propionate (CP) markedly reduced body weight at maximally effective doses after topical treatment. Furthermore, skin atrophy after 4-week application to minipigs was minimal after treatment with LEO 134310 compared with significant skin atrophy induced by treatment with CP and BMV. LEO 134310 showed similar levels of target engagement as BMV, measured by FKBP5 mRNA, but lower than CP. Finally, topical treatment with LEO 134310 in a human skin explant model resulted in full agonism, measured by GR target engagement, albeit at higher dosage strength than required for BMV and CP. In conclusion, LEO 134310A shows an improved therapeutic index in preclinical studies compared to traditional corticosteroids and may potentially offer an improved topical treatment solution for patients with atopic dermatitis and psoriasis.

Tetrahedral Copper(II) Complexes with a Labile Coordination Site Supported by a Tris-tetramethylguanidinato Ligand.

A new tridentate N3 ligand (TMG3tach) consisting of cis,cis-1,3,5-triaminocyclohexane (tach) and three N,N,N',N'-tetramethylguanidino (TMG) groups has been developed to prepare copper complexes with a tetrahedral geometry and a labile coordination site. Treatment of the ligand with CuIIX2 (X = Cl and Br) gave copper(II)-halide complexes, [CuII(TMG3tach)Cl]+ (2Cl) and [CuII(TMG3tach)Br]+ (2Br), the structures of which have been determined by X-ray crystallographic analysis. The complexes exhibit a four-coordinate structure with C3v symmetry, where the labile halide ligand (X) occupies a position on the trigonal axis. 2Br was converted to a methoxido-copper(II) complex [CuII(TMG3tach)(OMe)](OTf) (2OMe), also having a similar four-coordinate geometry, by treating it with an equimolar amount of tetrabutylammonium hydroxide in methanol. The methoxido-complex 2OMe was further converted to the corresponding phenolato-copper(II) (2OAr) and thiophenolato-copper(II) (2SAr) complexes by ligand exchange reactions with the neutral phenol and thiophenol derivatives, respectively. The electronic structures of the copper(II) complexes with different axial ligands are discussed on the basis of EPR spectroscopy and DFT calculations.

Glycosyltransfer in mutants of putative catalytic residue Glu303 of the human ABO(H) A and B blood group glycosyltransferases GTA and GTB proceeds through a labile active site.

The homologous glycosyltransferases α-1,3-N-acetylgalactosaminyltransferase (GTA) and α-1,3-galactosyltransferase (GTB) carry out the final synthetic step of the closely related human ABO(H) blood group A and B antigens. The catalytic mechanism of these model retaining enzymes remains under debate, where Glu303 has been suggested to act as a putative nucleophile in a double displacement mechanism, a local dipole stabilizing the intermediate in an orthogonal associative mechanism or a general base to stabilize the reactive oxocarbenium ion-like intermediate in an SNi-like mechanism. Kinetic analysis of GTA and GTB point mutants E303C, E303D, E303Q and E303A shows that despite the enzymes having nearly identical sequences, the corresponding mutants of GTA/GTB have up to a 13-fold difference in their residual activities relative to wild type. High-resolution single crystal X-ray diffraction studies reveal, surprisingly, that the mutated Cys, Asp and Gln functional groups are no more than 0.8 Å further from the anomeric carbon of donor substrate compared to wild type. However, complicating the analysis is the observation that Glu303 itself plays a critical role in maintaining the stability of a strained "double-turn" in the active site through several hydrogen bonds, and any mutation other than E303Q leads to significantly higher thermal motion or even disorder in the substrate recognition pockets. Thus, there is a remarkable juxtaposition of the mutants E303C and E303D, which retain significant activity despite disrupted active site architecture, with GTB/E303Q, which maintains active site architecture but exhibits zero activity. These findings indicate that nucleophilicity at position 303 is more catalytically valuable than active site stability and highlight the mechanistic elasticity of these enzymes.

A monofunctional platinum complex coordinated to a rhodium metalloinsertor selectively binds mismatched DNA in the minor groove.

We report the synthesis and characterization of a bimetallic complex derived from a new family of potent and selective metalloinsertors containing an unusual Rh-O axial coordination. This complex incorporates a monofunctional platinum center containing only one labile site for coordination to DNA, rather than two, and coordinates DNA nonclassically through adduct formation in the minor groove. This conjugate displays bifunctional, interdependent binding of mismatched DNA via metalloinsertion at a mismatch as well as covalent platinum binding. DNA sequencing experiments revealed that the preferred site of platinum coordination is not the traditional N7-guanine site in the major groove, but rather N3-adenine in the minor groove. The complex also displays enhanced cytotoxicity in mismatch repair-deficient and mismatch repair-proficient human colorectal carcinoma cell lines compared to the chemotherapeutic cisplatin, and it triggers cell death via an apoptotic pathway, rather than the necrotic pathway induced by rhodium metalloinsertors.

Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn2+, Fe2+, and Cu2+ cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu2+ complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of these complexes were determined by means of solid state magnetic moment, with a low value of μ=3.10μB for the Fe2+ complex suggesting that it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu2+ complex, while the μ=5.52μB value for the Mn2+ complex suggests that it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting that catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe2+ complex as the oxidation catalysts most worthy of continued development.

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.