Supramolecular Coordination Complexes as Optical Biosensors

Polyphenolic metacyclophane as a radical scavenger for therapeutic activation: a computational study

Modeling antioxidants for improved human health is a prime area of research. Inclusion complexes exhibit antioxidant activity. Supramolecular scaffolds like calixtyrosol are anticipated to have considerable antioxidant and therapeutic activity. In this study, we have designed 30 polyphenolic metacyclophanes and investigated their antioxidant properties. Exceptional O─H bond dissociation energy of 44 kcal/mol is reported for a metacyclophane with acyl urea linkage. This may be explained through a cooperative effect of localization of spin density distribution and an intramolecular hydrogen bonding of the corresponding radical. Further, the pharmacokinetics and toxicity analysis screened eight drug-like candidates. The interaction of the eight screened molecules with the Lysozyme transport protein and SOD protein has been studied using the molecular docking approach. Lastly, the MD simulations are performed to analyze the conformational changes of the transport protein after complexation with the proposed molecules. Comprehensive analyses including density functional studies of physiological parameters, favorable pharmacokinetics, toxicity, molecular docking, and MD simulations affirmed polyphenolic metacyclophane XXI as a radical scavenging and drug-like candidate.

A new biphenol-dipicolylamine based ligand and its dinuclear Zn2+ complex as fluorescent sensors for ibuprofen and ketoprofen in aqueous solution

In this work, the study of the new ligand 3,3'-bis[N,N-bis(pyridine-2-ylmethyl)aminomethyl]-2,2'-dihydroxybiphenyl (L) is reported, where a central 2,2'-biphenol (BPH) fluorophore was functionalized at 3,3'-positions with two dipicolylamine (DPA) side arms as receptor units. Following the synthesis and full chemical-physical characterization, the acid-base and Zn2+-coordination abilities of L were investigated through a combination of potentiometric, UV-Vis, fluorescence, NMR, XRD and DFT measurements. The optical properties of the ligand turned out to be strongly dependent on the pH, being straightforwardly associated with the protonation state of the BPH moiety, whereas its peculiar design allowed to form stable mono and dinuclear Zn2+ complexes. In the latter species, the presence of two Zn2+ ions coordinatively unsaturated and placed at close distance to each other, prompted us to test their usefulness as metallo-receptors for two environmental pollutants of great relevance, ibuprofen and ketoprofen. Potentiometric and fluorescence investigations evidenced that these important non-steroidal anti-inflammatory drugs (NSAIDs) are effectively coordinated by the metallo-receptors and, of relevance, both the stability and the fluorescence properties of the resulting ternary adducts are markedly affected by the different chemical architectures of the two substrates. This study aims at highlighting the promising perspectives arising from the use of polyamino phenolic ligands as chemosensors for H+/Zn2+ and other additional anionic targets in their metal-complexed forms.

Effect of dual modifications with ultrasonication and succinylation on Cicer arietinum protein-iron complexes: Characterization, digestibility, in-vitro cellular mineral uptake and preparation of fortified smoothie

The research aimed to evaluate the effect of ultrasonication and succinylation on the functional, iron binding, physiochemical, and cellular mineral uptake efficacy of chickpea protein concentrate. Succinylation resulted in significant improvements in the water-holding capacity (WHC) (25.47 %), oil-holding capacity (OHC) (31.38 %), and solubility (5.80 %) of the chickpea protein-iron complex. Mineral bioavailability significantly increased by 4.41 %, and there was a significant increase in cellular mineral uptake (64.64 %), retention (36.68 %), and transport (27.96 %). The ferritin content of the succinylated chickpea protein-iron complex showed a substantial increase of 66.31%. Furthermore, the dual modification approach combining ultrasonication and succinylation reduced the particle size of the protein-iron complex with a substantial reduction of 83.25 %. It also resulted in a significant enhancement of 51.5 % in the SH (sulfhydryl) content and 48.92 % in the surface hydrophobicity. Mineral bioavailability and cellular mineral uptake, retention, and transport were further enhanced through dual modification. In terms of application, the addition of single and dual-modified chickpea protein-iron complex to a fruit-based smoothie demonstrated positive acceptance in sensory attributes. Overall, the combined approach of succinylation and ultrasonication to the chickpea protein-iron complex shows a promising strategy for enhancing the physiochemical and techno-functional characteristics, cellular mineral uptake, and the development of vegan food products.

Molecular vessels from preorganised natural building blocks

Supramolecular vessels emerged as tools to mimic and better understand compartmentalisation, a central aspect of living matter. However, many more applications that go beyond those initial goals have been documented in recent years, including new sensory systems, artificial transmembrane transporters, catalysis, and targeted drug or gene delivery. Peptides, carbohydrates, nucleobases, and steroids bear great potential as building blocks for the construction of supramolecular vessels, possessing complexity that is still difficult to attain with synthetic methods - they are rich in functional groups and well-defined stereogenic centers, ready for noncovalent interactions and further functions. One of the options to tame the functional and dynamic complexity of natural building blocks is to place them at spatially designed positions using synthetic scaffolds. In this review, we summarise the historical and recent advances in the construction of molecular-sized vessels by the strategy that couples synthetic predictability and durability of various scaffolds (cyclodextrins, porphyrins, crown ethers, calix[n]arenes, resorcin[n]arenes, pillar[n]arenes, cyclotriveratrylenes, coordination frameworks and multivalent high-symmetry molecules) with functionality originating from natural building blocks to obtain nanocontainers, cages, capsules, cavitands, carcerands or coordination cages by covalent chemistry, self-assembly, or dynamic covalent chemistry with the ultimate goal to apply them in sensing, transport, or catalysis.

54 K Spin Transition Temperature Shift in a Fe6L4 Octahedral Cage Induced by Optimal Fitted Multiple Guests

Spin-crossover (SCO) coordination cages are at the forefront of research for their potential in crafting next-generation molecular devices. However, due to the scarcity of SCO hosts and their own limited cavities, the interplay between the SCO host and the multiple guests binding has remained elusive. In this contribution, we present a family of pseudo-octahedral coordination cages (M6L4, M = ZnII, CoII, FeII, and NiII) assembled from a tritopic tridentate ligand L with metal ions. The utilization of FeII ion leads to the successful creation of the Fe6L4-type SCO cage. Host-guest studies of these M6L4 cages reveal their capacity to encapsulate four adamantine-based guests. Notably, the spin transition temperature T1/2 of Fe6L4 is dependent on the multiple guests encapsulated. The inclusion of adamantine yields an unprecedented T1/2 shift of 54 K, a record shift in guest-mediated SCO coordination cages to date. This drastic shift is ascribed to the synergistic effect of multiple guests coupled with their optimal fit within the host. Through a straightforward thermodynamic cycle, the binding affinities of the high-spin (HS) and low-spin (LS) states are separated from their apparent binding constant. This result indicates that the LS state has a stronger binding affinity for the multiple guests than the HS state. Exploring the SCO thermodynamics of host-guest complexes allows us to examine the optimal fit of multiple guests to the host cavity. This study reveals that the T1/2 of the SCO host can be manipulated by the encapsulation of multiple guests, and the SCO cage is an ideal candidate for determining the multiple guest fit.

Role of the Environment Polarity on the Photophysical Properties of Mesogenic Hetero-Polymetallic Complexes

New hetero-polynuclear coordination complexes based on a pentacoordinated Zn(II) metal center with tridentate terpyridine-based ligands and monoanionic gallates functionalized with long alkyl chains containing ferrocene units were designed, synthesized and characterized using spectroscopic and analytical methods. The complexes are mesomorphic, exhibiting columnar hexagonal mesophases. The photophysical properties in a solution and in an ordered condensed state were accurately investigated and the influence of the polarity of the solvent was evidenced.

Heterometallic cages: synthesis and applications

High symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis and drug delivery. Recently there have been increasing efforts to enhance those applications by generating reduced symmetry MSAs. While there are several emerging methods for generating lower symmetry MSAs, this tutorial review examines the general methods used for synthesizing heterometallic MSAs with a particular focus on heterometallic cages. Additionally, the intrinsic properties of the cages and their potential emerging applications as host-guest systems and reaction catalysts are described.

Diastereoselective Self-Assembly of Low-Symmetry Pdn L2n Nanocages through Coordination-Sphere Engineering

Metal-organic cages (MOCs) are popular host architectures assembled from ligands and metal ions/nodes. Assembling structurally complex, low-symmetry MOCs with anisotropic cavities can be limited by the formation of statistical isomer libraries. We set out to investigate the use of primary coordination-sphere engineering (CSE) to bias isomer selectivity within homo- and heteroleptic Pdn L2n cages. Unexpected differences in selectivities between alternative donor groups led us to recognise the significant impact of the second coordination sphere on isomer stabilities. From this, molecular-level insight into the origins of selectivity between cis and trans diastereoisomers was gained, highlighting the importance of both host-guest and host-solvent interactions, in addition to ligand design. This detailed understanding allows precision engineering of low-symmetry MOC assemblies without wholesale redesign of the ligand framework, and fundamentally provides a theoretical scaffold for the development of stimuli-responsive, shape-shifting MOCs.

Synthesis of 177Lu-Labeled, Somatostatin-2 Receptor-Targeted Metalla-Assemblies: Challenges in the Design of Supramolecular Radiotherapeutics

Self-assembled supramolecular coordination complexes (SCCs) hold promise for biomedical applications in cancer therapy, although their potential in the field of nuclear medicine is still substantially unexplored. Therefore, in this study an exo-functionalized cationic [Pd2L2]4+ metallacycle (L = 3,5-bis(3-ethynylpyridine)phenyl), targeted to the somatostatin-2 receptor (sst2R) and featuring the DOTA chelator (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) in order to bind the β-- and γ-emitter lutetium-177, was synthesized by self-assembly following ligand synthesis via standard solid-phase peptide synthesis (SPPS). This metallacycle was then characterized by reverse-phase high-performance liquid chromatography (RP-HPLC), electrospray ionization mass spectrometry (ESI-MS), and 1H and 1H-DOSY NMR (DOSY = diffusion-ordered spectroscopy). A procedure for the radiolabeling of the metallacycle with 177Lu was also optimized. The resulting [nat/177Lu]Lu-DOTA-metallacycle, termed [nat/177Lu]Lu-Cy, was evaluated concerning its stability and in vitro properties. The compound was more lipophilic compared to the reference [177Lu]Lu-DOTA-TATE (logPOct/H2O = -0.85 ± 0.10 versus -3.67 ± 0.04, respectively). While [natLu]Lu-Cy revealed low stability in a DMEM/F12 GlutaMax medium, it demonstrated good stability in other aqueous media as well as in DMSO. A high sst2R binding affinity (expressed as IC50) was determined in CHOsst2 cells (Chinese hamster ovary cells that were stably transfected with human sst2R). Moreover, the metallacycle exhibited high human serum albumin binding, as assessed by high-performance affinity chromatography (HPAC), and moderate stability in human serum compared to [177Lu]Lu-DOTA-TATE (TATE = (Tyr3)-octreotate). In order to improve stability, a heteroleptic approach was used to develop a less sterically hindered cage-like SCC that is potentially endowed with host-guest chemistry capability, which has been preliminarily characterized by RP-HPLC and ESI-MS. Overall, our initial results encourage future studies on sst2R-directed SCCs and have led to new insights into the chemistry of ss2R-directed SCCs for radiopharmaceutical applications.

Systematic exploration of accessible topologies of cage molecules via minimalistic models

Cages are macrocyclic structures with an intrinsic internal cavity that support applications in separations, sensing and catalysis. These materials can be synthesised via self-assembly of organic or metal-organic building blocks. Their bottom-up synthesis and the diversity in building block chemistry allows for fine-tuning of their shape and properties towards a target property. However, it is not straightforward to predict the outcome of self-assembly, and, thus, the structures that are practically accessible during synthesis. Indeed, such a prediction becomes more difficult as problems related to the flexibility of the building blocks or increased combinatorics lead to a higher level of complexity and increased computational costs. Molecular models, and their coarse-graining into simplified representations, may be very useful to this end. Here, we develop a minimalistic toy model of cage-like molecules to explore the stable space of different cage topologies based on a few fundamental geometric building block parameters. Our results capture, despite the simplifications of the model, known geometrical design rules in synthetic cage molecules and uncover the role of building block coordination number and flexibility on the stability of cage topologies. This leads to a large-scale and systematic exploration of design principles, generating data that we expect could be analysed through expandable approaches towards the rational design of self-assembled porous architectures.

Dual-Emissive Rectangular Supramolecular Pt(II)-p-Biphenyl with 4,4'-Bipyridine Derivative Metallacycles: Stepwise Synthesis and Photophysical Properties

Mixed-ligand tetranuclear supramolecular coordination complexes (SCCs) of Pt(II)-p-biphenyl and bridging ligands derivatives of 4,4'-bypiridine (8)-(10), were synthesized and characterized. The SCCs were synthesized stepwise, starting from the Pt-p-biphenyl -Pt core. The crystal structure of complex {[Pt(2,2'-bpy)]4(μ-bph)2(μ-(4,4'-bpy)2}{PF6}4 (2,2'-bpy = 2,2'-bipyridine, bph = p-biphenyl and 4,4'-bpy = 4,4' bipyridine), was determined using single-crystal diffraction methods. The emission profile of the tetranuclear complexes (8)-(10) was influenced by the length of the bridging ligands and was found to depend on solvent polarity. Dual-emission patterns in methanol-water mixtures were observed only in the cases of complexes (9) and (10), attributed to aggregation-induced emission phenomena.

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Self-Assembly of Two Tubular Metalloligand-Based Palladium-Organic Cages as Hosts for Polycyclic Aromatic Hydrocarbons

Herein we report two tubular metal-organic cages (MOCs), synthesized by the self-assembly of bidentate metalloligands with different lengths and Pd^II. These two MOCs feature Pd₄L₈-type square tubular and Pd₃L₆-type triangular cage structures, respectively. Both MOCs have been fully characterized by NMR spectroscopy, mass spectrometry, and theoretical calculation. Both cages can be employed for encapsulating polycyclic aromatic hydrocarbons and show high binding affinity toward coronene.

PET Imaging of Self-Assembled 18 F-Labelled Pd2 L4 Metallacages for Anticancer Drug Delivery

To advance the design of self-assembled metallosupramolecular architectures as new generation theranostic agents, the synthesis of ¹⁸ F-labelled [Pd₂ L₄ ]⁴⁺ metallacages is reported. Different spectroscopic and bio-analytical methods support the formation of the host-guest cage-cisplatin complex. The biodistribution profiles of one of the cages, alone or encapsulating cisplatin have been studied by PET/CT imaging in healthy mice in vivo, in combination to ICP-MS ex vivo.

A Tetranuclear Copper(II)/Calcium(II) Complex as Dual Chemosensor for Colorimetric and Fluorescent Detection of Non-Steroidal Anti-Inflammatory Drugs

The tetranuclear Cu²⁺ /Ca²⁺ /Ca2⁺ /Cu²⁺ complex based on Malten ligand has been investigated as a platform for anion binding. Simple organic carboxylates and non-steroidal anti-inflammatory drugs (NSAIDs) have been tested, revealing the ability of the platform to bind them. The receiving platform hosts at least two guests in solution although a third anion can be bound, as suggested by X-ray diffraction analysis. The addition of the anions is accompanied by a color change of the solution, making the system a colorimetric sensor for carboxylates (LOD values comprised between 3.6 and 20.7 ppm). A fluorescent system consisting of the 2-(3-oxido-6-oxoxanthen-9-yl)benzoate (fluorescein anion) linked to the tetranuclear platform has been also prepared and used in a chemosensing ensemble approach to signal the presence of the selected anions (Log K between 2.6 and 5.6 for the addition of two guests). The latter also works in a paper strip test, offering the chemosensor a possible practical application.

Bioinorganic supramolecular coordination complexes and their biomedical applications

The field of Bioinorganic Supramolecular Chemistry is an emerging research area including metal-based supramolecules resulting from coordination-driven self-assembly (CDSA), whereby metal ions and organic ligands can be easily linked by metal-ligand bonds via Lewis' acid/base interactions. The focus of this 'In a Nutshell' review will be on the family of supramolecular coordination complexes, discrete entities formed by CDSA, which have recently captured widespread attention as a new class of versatile multifunctional materials with broad biological applications including molecular recognition, biosensing, therapy, imaging and drug delivery. Herein, we provide a summary of the state-of-the-art use of these systems in biomedicine, with some selected representative examples, as well as our visions of the challenges and possible directions in the field.

Anthracene-Containing Metallacycles and Metallacages: Structures, Properties, and Applications

Due to its highly conjugated panel-like structure and unique photophysical and chemical features, anthracene has been widely used for fabricating attractive and functional supramolecular assemblies, including two-dimensional metallacycles and three-dimensional metallacages. The embedded anthracenes in these assemblies often show synergistic effects on enhancing the desired supramolecular and luminescent properties. This review focuses on the metallasupramolecular architectures with anthracene-containing building blocks, as well as their applications in host-guest chemistry, stimulus response, molecular sensing, light harvesting, and biomedical science.

Metal-organic cages against toxic chemicals and pollutants

The continuous release of toxic chemicals and pollutants into the atmosphere and natural waters threatens, directly and indirectly, human health, the sustainability of the planet, and the future of society. Materials capable of capturing or chemically inactivating hazardous substances, which are harmful to humans and the environment, are critical in the modern age. Metal-organic cages (MOCs) show great promise as materials against harmful agents both in solution and in solid state. This Highlight features examples of MOCs that selectively encapsulate, adsorb, or remove from a medium noxious gases, toxic organophosphorus compounds, water pollutant oxoanions, and some emerging organic contaminants. Remarkably, the toxicity of interacting contaminants may be lowered by MOCs as well. Specific cases pertaining to the use of these cages for the chemical degradation of some harmful substances are presented. This Highlight thus aims to provide an overview of the possibilities of MOCs in this area and new methodological insights into their operation for enhancing their activity and the engineering of further remediation applications.

Post-synthetic modifications of metal–organic cages

Metal-organic cages (MOCs) are discrete, supramolecular entities that consist of metal nodes and organic linkers, which can offer solution processability and high porosity. Thereby, their predesigned structures can undergo post-synthetic modifications (PSMs) to introduce new functional groups and properties by modifying the linker, metal node, pore or surface environment. This Review explores current PSM strategies used for MOCs, including covalent, coordination and noncovalent methods. The effects of newly introduced functional groups or generated complexes upon the PSMs of MOCs are also detailed, such as improving structural stability or endowing desired functionalities. The development of the aforementioned design principles has enabled systematic approaches for the development and characterization of families of MOCs and, thereby, provides insight into structure-function relationships that will guide future developments.

Orthogonal binding and displacement of different guest types using a coordination cage host with cavity-based and surface-based binding sites

The octanuclear Co(ii) cubic coordination cage system H (or HW if it bears external water-solubilising substituents) has two types of binding site for guests. These are (i) the partially-enclosed central cavity where neutral hydrophobic organic species can bind, and (ii) the six 'portals' in the centres of each of the faces of the cubic cage where anions bind via formation of a network of CH⋯X hydrogen bonds between the anion and CH units on the positively-charged cage surface, as demonstrated by a set of crystal structures. The near-orthogonality of these guest binding modes provides the basis for an unusual dual-probe fluorescence displacement assay in which either a cavity-bound fluorophore (4-methyl-7-amino-coumarin, MAC; λ_em = 440 nm), or a surface-bound anionic fluorophore (fluorescein, FLU; λ_em = 515 nm), is displaced and has its emission ‘switched on’ according to whether the analyte under investigation is cavity-binding, surface binding, or a combination of both. A completely orthogonal system is demonstrated based using a Hw/MAC/FLU combination: addition of the anionic analyte ascorbate displaced solely FLU from the cage surface, increasing the 515 nm (green) emission component, whereas addition of a neutral hydrophobic guest such as cyclooctanone displaced solely MAC from the cage central cavity, increasing the 440 nm (blue) emission component. Addition of chloride results in some release of both components, and an intermediate colour change, as chloride is a rare example of a guest that shows both surface-binding and cavity-binding behaviour. Thus we have a colourimetric response based on differing contributions from blue and green emission components in which the specific colour change signals the binding mode of the analyte. Addition of a fixed red emission component from the complex [Ru(bipy)₃]²⁺ (Ru) provides a baseline colour shift of the overall colour of the luminescence closer to neutral, meaning that different types of guest binding result in different colour changes which are easily distinguishable by eye.

Orthogonal binding of neutral or anionic fluorophores to the cavity or surface, respectively, of a coordination cage host allows a dual-probe displacement assay which gives a different fluorescence colorimetric response according to where analyte species bind.

Selective formation of a supramolecular coordination complex in the nanometre scale with a ferrocene-based phospholane ligand

A straightforward synthesis of the tetradentate phospholane ligand 1 is reported. The 2 : 1 [M : L] reaction of 1 with [AuCl(tht)] (tht = tetrahydrothiophene) resulted in the 4 : 2 [M : L] supramolecular coordination complex 2 where two ligands 1 are bridging four gold(I) cations. The formation of 2 can be rationalised via a geometrical analysis of the ligand. The coordination mode of the gold atoms was evaluated based on a CSD search, revealing the geometrical changes for a transition from linear to trigonal planar coordination environment.

Evolution of Supramolecular Systems Towards Next-Generation Biosensors

Supramolecular materials, which rely on dynamic non-covalent interactions, present a promising approach to advance the capabilities of currently available biosensors. The weak interactions between supramolecular monomers allow for adaptivity and responsiveness of supramolecular or self-assembling systems to external stimuli. In many cases, these characteristics improve the performance of recognition units, reporters, or signal transducers of biosensors. The facile methods for preparing supramolecular materials also allow for straightforward ways to combine them with other functional materials and create multicomponent sensors. To date, biosensors with supramolecular components are capable of not only detecting target analytes based on known ligand affinity or specific host-guest interactions, but can also be used for more complex structural detection such as chiral sensing. In this Review, we discuss the advancements in the area of biosensors, with a particular highlight on the designs of supramolecular materials employed in analytical applications over the years. We will first describe how different types of supramolecular components are currently used as recognition or reporter units for biosensors. The working mechanisms of detection and signal transduction by supramolecular systems will be presented, as well as the important hierarchical characteristics from the monomers to assemblies that contribute to selectivity and sensitivity. We will then examine how supramolecular materials are currently integrated in different types of biosensing platforms. Emerging trends and perspectives will be outlined, specifically for exploring new design and platforms that may bring supramolecular sensors a step closer towards practical use for multiplexed or differential sensing, higher throughput operations, real-time monitoring, reporting of biological function, as well as for environmental studies.