Dynamic cyclen-metal complexes for molecular sensing and chirality signaling

Cyclen-Based Octaamine Ligand Supporting the Formation of Dinuclear Metal Compounds

A series of divalent first-row dinuclear transition metal complexes─LCr2, LMn2, LFe2, LCo2, and LZn2─are synthesized and characterized using a 1,4,7,10-tetraazacyclododecane (cyclen)-derived octaamine ligand (LH4) as a dinucleating platform. The ligand scaffold stabilizes these complexes without the need of exogenous ligands to complete the coordination sphere, giving rise to coordinatively unsaturated complexes. Crystallographic analysis reveals that the Mn, Fe, Co, and Zn complexes are isostructural, adopting coordination environments with the metal atoms situated in pseudotetrahedral and square pyramidal environments. In contrast, the Cr complex exhibits a structure where the two metal atoms reside in identical and cofacial pseudo-square planar geometries. DFT calculations, electron localization function analysis, and Wiberg bond indices suggest varying degrees of metal-metal bonding interactions across all complexes described here. In LCr2, the short Cr-Cr distance of 1.9609(7) Å is consistent with a quadrupole bond, which is supported by DFT calculations. These results demonstrate the utility of this cyclen-based ligand scaffold in templating the synthesis of dinuclear complexes establishing a range of weak, in the case of the isostructural LMn2, LFe2, LCo2, and LZn2, to strong metal-metal interactions in LCr2. The dinuclear complexes supported by weak field amido and amine donors in LH4 represent a promising platform to investigate biomimetic cooperative small molecule activation.

Selective binding and fluorescence sensing of Zn(II)/Cd(II) using macrocyclic tetra-amines with different fluorophores: insights into the design of selective chemosensors for transition metals

Selective binding and optical sensing of Zn(II) and Cd(II) by L1, HL2, L3, H2L4 and H2L5 receptors were analysed in aqueous solutions by coupling potentiometric, UV-vis absorption and fluorescence emission measurements, with the aim to determine the effect of complex stability on selective signalling of metals with similar electronic configurations. All receptors share the same cyclic tetra-amine binding unit attached to a single quinoline (Q) or 8-hydroxyquinoline (8-OHQ) unit (L1 and HL2, respectively), two Q or 8-OHQ moieties (L3 and H2L4, respectively), and, finally, two Q and two acetate groups (H2L5). The crystal structures of the Cd(II) and Zn(II) complexes show that L3 and H2L4 feature a cavity in which the larger Cd(II) complex is better fitted than the Zn(II) complex, leading to the formation of more stable Cd(II) complexes. In turn, Zn(II) forms more stable complexes with L1 and HL2, owing to its high tendency to give 5-coordinated complexes. Considering optical selectivity, Zn(II) gives the most emissive complex with L3, while the corresponding Cd(II) complex is basically quenched. The gathered structure of the Zn(II) complex, in which the two Q units are associated with one another-a structural motif not observed in the [CdL3]2+ complex-leads to poor solvation of the Q units, favouring complex emission. Among 8-OHQ-containing receptors, the most emissive complex is formed by Cd(II) with HL2, containing a single 8-OHQ moiety. H2L4 forms non-emissive complexes: the presence of two coordinating 8-OHQ moieties weakens metal interactions with the tetra-amine unit, favouring PET to the excited fluorophore that quench the emission.

Efficient PFAS Removal Using Reusable and Non-Toxic 3D Printed Porous Trianglamine Hydrogels

Per- and polyfluoroalkyl substances (PFAS) are now a paramount concern in water remediation. Nowadays, urgent action is required for the development of advanced technologies aimed at capturing PFAS and mitigating their impact. To offer a solution, a functional 3D printed hydrogel tailored is designed to trap a broad spectrum of PFAS contaminants. The hydrogel is made of a photo-crosslinked dimethacrylate-ureido-trianglamine (DMU-Δ) and Pluronic P123 dimethacrylate (PDM) fabricated by stereolithography (SLA). With the aid of 3D-printing, porous and nonporous hydrogels (3D-PSHΔ, 3D-SHΔ) as well as quaternized hydrogels (3D-PSHΔQ+) are prepared. These tailored hydrogels, show high uptake capacities and fast removal kinetics for PFAS from aqueous sources. The PFAS removal efficiency of these hydrogels are then compared to P123 hydrogels with no trianglamine (3D-SH). The 3D-SH hydrogel shows no affinity to PFAS, proving that the sorption is due to the interaction between the trianglamine (Δ) and PFAS. Metadynamic simulations also confirmed this interaction. The porous matrices showed the fastest and highest uptake capacity. 3D-PSHΔ is able to capture ≈ 91% of PFAS within 5 h using initial concentrations of 5 and 0.5 ppm in both deionized and river water. The sorption of PFAS is further enhanced by introducing permanent positive charges to the structure of the porous hydrogels, resulting in even faster sorption kinetics for both long and short PFAS chains with diverse polar heads. Besides the remarkable efficiency in capturing PFAS, these designed hydrogels are non-toxic and have outstanding chemical and thermal stability, making them a brilliant candidate for mass use in the combat against PFAS pollution.

MR Imaging Reveals Dynamic Aggregation of Multivalent Glycoconjugates in Aqueous Solution

Glycoconjugates forming from the conjugation of carbohydrates to other biomolecules, such as proteins, lipids, or other carbohydrates, are essential components of mammalian cells and are involved in numerous biological processes. Due to the capability of sugars to form multiple hydrogen bonds, many synthetic glycoconjugates are desirable biocompatible platforms for imaging, diagnostics, drugs, and supramolecular self-assemblies. Herein, we present a multimeric galactose functionalized paramagnetic gadolinium (Gd(III)) chelate that displays spontaneous dynamic aggregation in aqueous conditions. The dynamic aggregation of the Gd(III) complex was shown by the concentration-dependent magnetic resonance (MR) relaxation measurements, nuclear magnetic resonance dispersion (NMRD) analysis, and dynamic light scattering (DLS). Notably, these data showed a nonlinear relationship between magnetic resonance relaxation rate and concentrations (0.03-1.35 mM), and a large DLS hydrodynamic radius was observed in the high-concentration solutions. MR phantom images were acquired to visualize real-time dynamic aggregation behaviors in aqueous solutions. The in situ visualization of the dynamic self-assembling process of multivalent glycoconjugates has rarely been reported.

Facile Peptide Macrocyclization and Multifunctionalization via Cyclen Installation

Cyclen-peptide bioconjugates are usually prepared in multiple steps that require individual preparation and purification of the cyclic peptide and hydrophilic cyclen derivatives. An efficient strategy is discovered for peptide cyclization and functionalization toward lanthanide probe via three components intermolecular crosslinking on solid-phase peptide synthesis with high conversion yield. Multifunctionality can be conferred by introducing different modular parts or/and metal ions on the cyclen-embedded cyclopeptide. As a proof-of-concept, a luminescent Eu3+ complex and a Gd3+-based contrasting agent for in vitro optical imaging and in vivo magnetic resonance imaging, respectively, are demonstrated through utilizing this preparation of cyclen-embedded cyclic arginylglycylaspartic acid (RGD) peptide.

Angular-resolved Rabi oscillations of orthorhombic spins in a Co(II) molecular qubit

Magnetic molecules are promising candidates for quantum information processing (QIP) due to their tunable electron structures and quantum properties. A high spin Co(II) complex, CoH2dota, is studied for its potential to be used as a quantum bit (qubit) utilizing continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopy at low temperature. On the X-band microwave energy scale, the system can be treated as an effective spin 1/2 with a strongly anisotropic g-tensor resulting from the significant spin-orbital coupling. An experimental and theoretical study is conducted to investigate the anisotropic Rabi oscillations of the two magnetically equivalent spin centres with different orientations in a single crystal sample, which aims to verify the relationship between the Rabi frequency and the orientation of the g-tensor. The findings of this study show that an effective quantum manipulation method is developed for orthorhombic spin systems.

Crown-hydroxylamines are pH-dependent chelating N,O-ligands with a potential for aerobic oxidation catalysis

Despite the rich coordination chemistry, hydroxylamines are rarely used as ligands for transition metal coordination compounds. This is partially because of the instability of these complexes that undergo decomposition, disproportionation and oxidation processes involving the hydroxylamine motif. Here, we design macrocyclic poly-N-hydroxylamines (crown-hydroxylamines) that form complexes containing a d-metal ion (Cu(II), Ni(II), Mn(II), and Zn(II)) coordinated by multiple (up to six) hydroxylamine fragments. The stability of these complexes is likely to be due to a macrocycle effect and strong intramolecular H-bonding interactions between the N-OH groups. Crown-hydroxylamine complexes exhibit interesting pH-dependent behavior where the efficiency of metal binding increases upon deprotonation of the hydroxylamine groups. Copper complexes exhibit catalytic activity in aerobic oxidation reactions under ambient conditions, whereas the corresponding complexes with macrocyclic polyamines show poor or no activity. Our results show that crown-hydroxylamines display anomalous structural features and chemical behavior with respect to both organic hydroxylamines and polyaza-crowns.

Self-assembled lanthanide-based helixes: synthetic control of the helical handedness by chirality of the ligand

The control of the self-assembly of lanthanide helical chain and their helical handedness have been investigated for the first time. Δ- and Λ-form lanthanide chain complexes were obtained by introducing thiazolidine ligands that were synthesised from L- and D-cysteine, respectively, and shared the same formula: [Ln₂(L)₃(H₂O)₅]_∞·3H₂O (Ln: Sm and Eu) (L: 2-(2-hydroxy-3,5-dinitrophenyl)thiazolidine-4-carboxylic acid). The crystallographic, circular dichroism, and luminescence properties of these novel lanthanide chain complexes were studied.

Structural dynamism of chiral sodium peraza-macrocycle complexes derived from cyclic peptoids

A variety of cyclen and hexacyclen derivatives decorated with (S)-1-phenylethyl side chains or (S)-pyrrolidine units have been prepared via a reductive approach from the corresponding cyclic peptoids containing N-(S)-(1-phenylethyl)glycine and l-proline residues. Spectroscopic and DFT studies on their Na+ complexes show that point chirality and ring size play a crucial role in controlling the structural dynamism of 1,2-diaminoethylene units and pendant arms. The detection of highly symmetric C4- and C3-symmetric metalated species demonstrates that a full understanding of the relationship between the structure and conformational properties of peraza-macrocyclic metal complexes is possible.

The Synthesis of a Bis(thiosemicarbazone) Macrocyclic Ligand and the Mn(II), Co(II), Zn(II) and 68Ga(III) Complexes

A 1,4,7,10-tetraazacyclododecane (cyclen) variant bearing two thiosemicarbazone pendant groups has been prepared. The ligand forms complexes with Mn2+, Co2+ and Zn2+. X-ray crystallography of the Mn2+, Co2+ and Zn2+ complexes showed that the ligand provides a six-coordinate environment for the metal ions. The Mn2+ and Zn2+ complexes exist in the solid state as racemic mixtures of the Δ(δ,δ,δ,δ)/Λ(λ,λ,λ,λ) and Δ(λ,λ,λ,λ)/Λ(δ,δ,δ,δ) diastereomers, and the Co2+ complex exists as the Δ(δ,δ,δ,δ)/Λ(λ,λ,λ,λ) and Δ(λ,λ,λ,δ)/Λ(δ,δ,δ,λ) diastereomers. Density functional theory calculations indicated that the relative energies of the diastereomers are within 10 kJ mol-1. Magnetic susceptibility of the complexes indicated that both the Mn2+ and Co2+ ions are high spin. The ligand was radiolabelled with gallium-68, in the interest of developing new positron emission tomography imaging agents, which produced a single species in high radiochemical purity (>95%) at 90 °C for 10 min.

Protonation of cyclen-based chelating agents containing fluorescent moieties

The fluorescence emission properties of 1,4,7,10-tetraazacyclododecane-based receptors with appended heteroaromatic fluorophores are tuned by photoinduced electron and proton transfer processes.

Design and applications of metal-based molecular receptors and probes for inorganic phosphate

Inorganic phosphate has numerous biomedical functions. Regulated primarily by the kidneys, phosphate reaches abnormally high blood levels in patients with advanced renal diseases. Since phosphate cannot be efficiently removed by dialysis, the resulting hyperphosphatemia leads to increased mortality. Phosphate is also an important component of the environmental chemistry of surface water. Although required to secure our food supply, inorganic phosphate is also linked to eutrophication and the spread of algal blooms with an increasing economic and environmental burden. Key to resolving both of these issues is the development of accurate probes and molecular receptors for inorganic phosphate. Yet, quantifying phosphate in complex aqueous media remains challenging, as is the development of supramolecular receptors that have adequate sensitivity and selectivity for use in either blood or surface waters. Metal-based receptors are particularly well-suited for these applications as they can overcome the high hydration enthalpy of phosphate that limits the effectiveness of many organic receptors in water. Three different strategies are most commonly employed with inorganic receptors for anions: metal extrusion assays, responsive molecular receptors, and indicator displacement assays. In this review, the requirements for molecular receptors and probes for environmental applications are outlined. The different strategies deployed to recognize and sense phosphate with metal ions will be detailed, and their advantages and shortfalls will be delineated with key examples from the literature.

Silver ion-induced chiral enhancement by argentivorous molecules

Optically active tetra-armed cyclens with an asymmetric chiral centre in the cyclen moiety were synthesized and were shown to enhance chirality and control of enantiomers on complexation with Ag⁺.

Inclusion of alkyl nitriles by tetra-armed cyclens with styrylmethyl groups

A new technique for the assignment of the absolute configurations of low [α]_D alkyl-nitriles using a Ag⁺ complex with a tetra-armed cyclen is reported.

Toward novel sulphur-containing derivatives of tetraazacyclododecane: synthesis, acid–base properties, spectroscopic characterization, DFT calculations, and cadmium(ii) complex formation in aqueous solution

New sulphur derivatives of cyclen, with potential complementary properties with respect to known compounds, have been synthesized and studied.

Steering the Achiral into Chiral with a Self-Assembly Strategy

Chirality transfer from self-assembly of achiral titanyl phthalocyanine (TiOPc) to its top-sitting TiOPc molecule has been successfully achieved. The TiOPc molecules first assemble into a porous network on Au(111) that contains periodic chiral voids, each being fenced by four axially rotating TiOPc molecules in upward adsorption geometry where their ending O atoms exclusively point away from the substrate. The additional top-sitting TiOPc molecule turns out to be chiral upon adsorption on a chiral void with its ending O atom toward the substrate. The chirality of the top-sitting TiOPc is associated with a charge transfer between its indole rings and the ending O atoms of the underlying TiOPc molecules that form the chiral void, resulting in asymmetric electronic density of the indole rings in the top-sitting molecule and accordingly the chirality of the molecular orbitals. Such a scenario also validates other planar achiral metallophthalocyanines such as copper phthalocyanine that become chiral upon adsorption on the chiral voids in the underlying TiOPc assembly, indicating that the chirality transfer mechanism from assembly to the top-sitting molecule is not uncommon.

Solution structure and structural rearrangement in chiral dimeric ytterbium(iii) complexes determined by paramagnetic NMR and NIR-CD

Chiral lanthanide complexes are attracting interest in enantioselective catalysis and due to their unique optical and magnetic properties. Here, we investigate the chiral ytterbium complex [Yb((S)-THP)] ((S)-THP = ((1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane), which has found applications in catalysis and as a CEST agent in MRI, by means of near-IR circular dichroism (NIR CD), NMR, and mass spectrometry, in different solvents. The NMR analysis revealed that this complex, different from the analogues including early lanthanides, is not axially coordinated by the solvent. In non-protic solvents, and in the presence of bases, [Yb((S)-THP)]3+ dimerizes to [Yb((S)-H2THP)]22+. The careful analysis of the paramagnetic contributions in the NMR spectra allowed us to determine the structure of the dimeric species in solution, revealing a structural rearrangement of the coordination cage following the dimerization process.

An efficient approach to modulate the coordination number of yttrium ions for diverse network formation

Hidden role of water! It acts as a key to determine the coordination number of yttrium by controlling the active ligand concentration.

A study of the Group 1 metal tetra-aza macrocyclic complexes [M(Me4cyclen)(L)]+ using electronic structure calculations

Metal-cyclen complexes have a number of important applications. However, the coordination chemistry between metal ions and cyclen-based macrocycles is much less well studied compared to their metal ion-crown ether analogues. This work, which makes a contribution to address this imbalance by studying complex ions of the type [M(Me₄cyclen)(L)]⁺, was initiated by results of an experimental study which prepared some Group 1 metal cyclen complexes, namely [Li(Me₄cyclen)(H₂O)][BAr^F] and [Na(Me₄cyclen)(THF)][BAr^F] and obtained their X-ray crystal structures [J. M. Dyke, W. Levason, M. E. Light, D. Pugh, G. Reid, H. Bhakhoa, P. Ramasami, and L. Rhyman, Dalton Trans., 2015, 44, 13853]. The lowest [M(Me₄cyclen)(L)]⁺ minimum energy structures (M = Li, Na, K, and L = H₂O, THF, DEE, MeOH, DCM) are studied using density functional theory (DFT) calculations. The geometry of each [M(Me₄cyclen)(L)]⁺ structure and, in particular, the conformation of L are found to be mainly governed by steric hindrance which decreases as the size of the ionic radius increases from Li⁺ → Na⁺ → K⁺. Good agreement of computed geometrical parameters of [Li(Me₄cyclen)(H₂O)]⁺ and [Na(Me₄cyclen)(THF)]⁺ with the corresponding geometrical parameters derived from the crystal structures [Li(Me₄cyclen)(H₂O)]⁺[BAr^F]^- and [Na(Me₄cyclen)(THF)]⁺[BAr^F]^- is obtained. Bonding analysis indicates that the stability of the [M(Me₄cyclen)(L)]⁺ structures originates mainly from ionic interaction between the Me₄cyclen/L ligands and the M⁺ centres. The experimental observation that [M(Me₄cyclen)(L)]⁺[BAr^F]^- complexes could be prepared in crystalline form for M⁺ = Li⁺ and Na⁺, but that experiments aimed at synthesising the corresponding K⁺, Rb⁺, and Cs⁺ complexes failed resulting in formation of [Me₄cyclenH][BAr^F] is investigated using DFT and explicitly correlated calculations, and explained by considering production of [Me₄cyclenH]⁺ by a hydrolysis reaction, involving traces of water, which competes with [M(Me₄cyclen)(L)]⁺ formation. [Me₄cyclenH]⁺ formation dominates for M⁺ = K⁺, Rb⁺, and Cs⁺ whereas formation of [M(Me₄cyclen)(L)]⁺ is energetically favoured for M⁺ = Li⁺ and Na⁺. The results indicate that the number and type of ligands, play a key role in stabilising the [M(Me₄cyclen)]⁺ complexes and it is hoped that this work will encourage experimentalists to prepare and characterise other [M(Me₄cyclen)(L)]⁺ complexes.

Chiral two bladed ML2 metallamacrocycles: design, structures and solution behavior

Chiral two bladed complexes of [Co(L)₂][BF₄]₂ (1) and [Zn(L)₂][BF₄]₂ (2) containing an atropoisomeric semi-rigid bidentate ligand L were synthesized and characterized. They are obtained as homochiral species but in a racemic mixture via a ligand self-sorting mechanism. These enantiomers can be differentiated in solution using optically active anions.

Luminescence sensing of weakly-hydrated anions in aqueous solution by self-assembled europium(iii) complexes

Perchlorate anions in water could be sensed by luminescence decrease of europium(iii) using a self-assembly system.

Selective Sensing of Tyrosine Phosphorylation in Peptides Using Terbium(III) Complexes

Phosphorylation of tyrosine residues in proteins, as well as their dephosphorylation, is closely related to various diseases. However, this phosphorylation is usually accompanied by more abundant phosphorylation of serine and threonine residues in the proteins and covers only 0.05% of the total phosphorylation. Accordingly, highly selective detection of phosphorylated tyrosine in proteins is an urgent subject. In this review, recent developments in this field are described. Monomeric and binuclear Tb(III) complexes, which emit notable luminescence only in the presence of phosphotyrosine (pTyr), have been developed. There, the benzene ring of pTyr functions as an antenna and transfers its photoexcitation energy to the Tb(III) ion as the emission center. Even in the coexistence of phosphoserine (pSer) and phosphothreonine (pThr), pTyr can be efficintly detected with high selectivity. Simply by adding these Tb(III) complexes to the solutions, phosphorylation of tyrosine in peptides by protein tyrosine kinases and dephosphorylation by protein tyrosine phosphatases can be successfully visualized in a real-time fashion. Furthermore, the activities of various inhibitors on these enzymes are quantitatively evaluated, indicating a strong potential of the method for efficient screening of eminent inhibitors from a number of candidates.

Can Cyclen Bind Alkali Metal Azides? A DFT Study as a Precursor to Synthesis

Can cyclen (1,4,7,10-tetraazacyclododecane) bind alkali metal azides? This question is addressed by studying the geometric and electronic structures of the alkali metal azide-cyclen [M(cyclen)N3] complexes using density functional theory (DFT). The effects of adding a second cyclen ring to form the sandwich alkali metal azide-cyclen [M(cyclen)2N3] complexes are also investigated. N3(-) is found to bind to a M(+) (cyclen) template to give both end-on and side-on structures. In the end-on structures, the terminal nitrogen atom of the azide group (N1) bonds to the metal as well as to a hydrogen atom of the cyclen ring through a hydrogen bond in an end-on configuration to the cyclen ring. In the side-on structures, the N3 unit is bonded (in a side-on configuration to the cyclen ring) to the metal through the terminal nitrogen atom of the azide group (N1), and through the other terminal nitrogen atom (N3) of the azide group by a hydrogen bond to a hydrogen atom of the cyclen ring. For all the alkali metals, the N3-side-on structure is lowest in energy. Addition of a second cyclen unit to [M(cyclen)N3] to form the sandwich compounds [M(cyclen)2N3] causes the bond strength between the metal and the N3 unit to decrease. It is hoped that this computational study will be a precursor to the synthesis and experimental study of these new macrocyclic compounds; structural parameters and infrared spectra were computed, which will assist future experimental work.

Multiresponsive Dynamic Covalent Assemblies for the Selective Sensing of Both Cu(2+) and CN(-) in Water

The use of dynamic assembly for molecular sensing is an intensive area of research in supramolecular chemistry. However, the development of self-assembly architectures for the detection of multiple signals remains challenging. Here, we present dynamic covalent assemblies with multiresponsive properties that also show unique selectivity profiles in water. The receptors were generated in a single step with modularly designed building blocks through acylhydrazone linkages, and their orthogonal assembly with a series of external stimuli was investigated. Notably, the system exhibits responses toward cations, anions, solvents, pH, and amphiphilicity. The discrimination of Cu(2+) from other divalent metal ions was achieved by simply changing the solvent and was evidenced by a Cu(2+)-induced pKa shift. The selective recognition of CN(-) in pure aqueous media was also accomplished through a cooperative effect in conjunction with Zn(2+). Furthermore, the assembly and its responsiveness are functional both in solution and the solid state. The aggregation ability of these dynamic covalent systems supports their binding and sensing properties.

Ras Conformational Ensembles, Allostery, and Signaling

Ras proteins are classical members of small GTPases that function as molecular switches by alternating between inactive GDP-bound and active GTP-bound states. Ras activation is regulated by guanine nucleotide exchange factors that catalyze the exchange of GDP by GTP, and inactivation is terminated by GTPase-activating proteins that accelerate the intrinsic GTP hydrolysis rate by orders of magnitude. In this review, we focus on data that have accumulated over the past few years pertaining to the conformational ensembles and the allosteric regulation of Ras proteins and their interpretation from our conformational landscape standpoint. The Ras ensemble embodies all states, including the ligand-bound conformations, the activated (or inactivated) allosteric modulated states, post-translationally modified states, mutational states, transition states, and nonfunctional states serving as a reservoir for emerging functions. The ensemble is shifted by distinct mutational events, cofactors, post-translational modifications, and different membrane compositions. A better understanding of Ras biology can contribute to therapeutic strategies.

Synthesis and Structural Characterization of a Cyclen-Derived Molecular Cage

Reaction of a tetrafunctionalized cyclen derivative containing four aldehyde groups with an appropriate diamine followed by reduction and demetalation highly efficiently affords a bis(cyclen)-derived molecular cage. Potentiometric investigations show that this compound forms dimetallic complexes with copper(II), with the two metal ions selectively coordinated to the cyclen units. X-ray crystallography indicates that these complexes could give rise to new cascade complexes after incorporation of anions between the metal centers.

Macrocyclic metal complexes for metalloenzyme mimicry and sensor development

Examples of proteins that incorporate one or more metal ions within their structure are found within a broad range of classes, including oxidases, oxidoreductases, reductases, proteases, proton transport proteins, electron transfer/transport proteins, storage proteins, lyases, rusticyanins, metallochaperones, sporulation proteins, hydrolases, endopeptidases, luminescent proteins, iron transport proteins, oxygen storage/transport proteins, calcium binding proteins, and monooxygenases. The metal coordination environment therein is often generated from residues inherent to the protein, small exogenous molecules (e.g., aqua ligands) and/or macrocyclic porphyrin units found, for example, in hemoglobin, myoglobin, cytochrome C, cytochrome C oxidase, and vitamin B12. Thus, there continues to be considerable interest in employing macrocyclic metal complexes to construct low-molecular weight models for metallobiosites that mirror essential features of the coordination environment of a bound metal ion without inclusion of the surrounding protein framework. Herein, we review and appraise our research exploring the application of the metal complexes formed by two macrocyclic ligands, 1,4,7-triazacyclononane (tacn) and 1,4,7,10-tetraazacyclododecane (cyclen), and their derivatives in biological inorganic chemistry. Taking advantage of the kinetic inertness and thermodynamic stability of their metal complexes, these macrocyclic scaffolds have been employed in the development of models that aid the understanding of metal ion-binding natural systems, and complexes with potential applications in biomolecule sensing, diagnosis, and therapy. In particular, the focus has been on "coordinatively unsaturated" metal complexes that incorporate a kinetically inert and stable metal-ligand moiety, but which also contain one or more weakly bound ligands, allowing for the reversible binding of guest molecules via the formation and dissociation of coordinate bonds. With regards to mimicking metallobiosites, examples are presented from our work on tacn-based complexes developed as simplified structural models for multimetallic enzyme sites. In particular, structural comparisons are made between multinuclear copper(II) complexes formed by such ligands and multicopper enzymes featuring type-2 and type-3 copper centers, such as ascorbate oxidase (AO) and laccase (Lc). Likewise, with the aid of relevant examples, we highlight the importance of cooperativity between either multiple metal centers or a metal center and a proximal auxiliary unit appended to the macrocyclic ligand in achieving efficient phosphate ester cleavage. Finally, the critical importance of the Zn(II)-imido and Zn(II)-phosphate interactions in Zn-cyclen-based systems for delivering highly sensitive electrochemical and fluorescent chemosensors is also showcased. The Account additionally highlights some of the factors that limit the performance of these synthetic nucleases and the practical application of the biosensors, and then identifies some avenues for the development of more effective macrocyclic constructs in the future.

Optical recognition of alkyl nitrile by a homochiral iron(ii) spin crossover host

In this work, a chiral mononuclear spin crossover host1·MeCN was found to have the ability to optically recognize lactonitrile (LN) and methylglutaronitrile (MGN).

Argentivorous molecules bearing three aromatic side arms: synthesis of triple-armed cyclens and their complexing property towards Ag+

Triple-armed cyclens bearing three aromatic side-arms were prepared, and the Ag⁺-ion-induced¹H NMR and UV-vis spectral changes and X-ray structures suggested that the new armed cyclens behave like an insectivorous plant (Venus flytrap).