Tetrazine-based dynamic covalent polymers as degradable extraction materials in sample preparation.

BACKGROUND: Current trends in Analytical Chemistry are highly focused on the introduction of new extraction materials with a high selectivity towards the target analytes, high extraction capacity as well as sustainable characteristics. In this context, the introduction of smart materials able to respond to an external stimulus constitutes a promising approach in the field. However, investigations regarding the development of such stimuli-responsive polymers have been basically centered on their synthesis and the control of their properties, and hardly on exploiting such properties to generate polymers that, once their extraction function is fulfilled, they can be degraded into fragments with little or negligible toxicity, or even into their constituent monomers for an efficient recycling. RESULTS: The applicability of a degradable and recyclable dynamic covalent polymer based on the use of tetrazine as a linker was assessed as sorbent for the extraction of a group of 37 persistent organic pollutants, including 10 polycyclic aromatic hydrocarbons, 11 organochlorine pesticides, 14 polychlorinated biphenyls, and 2 antibacterial agents, from water samples. A microdispersive solid-phase extraction procedure was developed for the selective extraction of the target analytes, while their separation, determination, and quantification were achieved by gas chromatography coupled to mass spectrometry. The optimized procedure was validated for seawater and wastewater obtaining mean relative recovery values between 72 and 112 % for almost all the analytes, with satisfactory relative standard deviation values (<18 %). After extraction, the polymer could be degraded by adding the amino acid L-tyrosine, being possible a quantitative recovery of the initial functional monomer. SIGNIFICANCE: A responsive polymer based on the chemical versatility of the tetrazine ring was used as sorbent in sample preparation providing excellent results, showing good physicochemical properties and the ability to be degraded after use. This polymer constitutes an interesting alternative to reduce chemical waste through the recycling of monomers, contributing to the development of more sustainable analytical methodologies.

Kinetic Control of Complexity in Multiple Dynamic Libraries.

Multiple dynamic libraries of compounds are generated when more than one reversible reaction comes into play. Commonly, two or more orthogonal reversible reactions are used, leading to non-communicating dynamic libraries which share no building blocks. Only a few examples of communicating libraries have been reported, and in all those cases, building blocks are reversibly exchanged from one library to the other, constituting an antiparallel dynamic covalent system. Herein we report that communication between two different dynamic libraries through an irreversible process is also possible. Indeed, alkyl amines cancel the dynamic regime on the nucleophilic substitution of tetrazines, generating kinetically inert compounds. Interestingly, such amine can be part of another dynamic library, an imine-amine exchange. Thus, both libraries are interconnected with each other by an irreversible process which leads to kinetically inert structures that contain parts from both libraries, causing a collapse of the complexity. Additionally, a latent irreversible intercommunication could be developed. In such a way, a stable molecular system with specific host-guest and fluorescence properties, could be irreversibly transformed when the right stimulus was applied, triggering the cancellation of the original supramolecular and luminescent properties and the emergence of new ones.

Synthesis of Structurally Related Coumarin Derivatives as Antiproliferative Agents.

A library of structurally related coumarins was generated through synthesis reactions and chemical modification reactions to obtain derivatives with antiproliferative activity both in vivo and in vitro. Out of a total of 35 structurally related coumarin derivatives, seven of them showed inhibitory activity in in vitro tests against Taq DNA polymerase with IC50 values lower than 250 µM. The derivatives 4-(chloromethyl)-5,7-dihydroxy-2H-chromen-2-one (2d) and 4-((acetylthio)methyl)-2-oxo-2H-chromen-7-yl acetate (3c) showed the most promising anti-polymerase activity with IC50 values of 20.7 ± 2.10 and 48.25 ± 1.20 µM, respectively. Assays with tumor cell lines (HEK 293 and HCT-116) were carried out, and the derivative 4-(chloromethyl)-7,8-dihydroxy-2H-chromen-2-one (2c) was the most promising, with an IC50 value of 8.47 µM and a selectivity index of 1.87. In addition, the derivatives were evaluated against Saccharomyces cerevisiae strains that report about common modes of actions, including DNA damage, that are expected for agents that cause replicative stress. The coumarin derivatives 7-(2-(oxiran-2-yl)ethoxy)-2H-chromen-2-one (5b) and 7-(3-(oxiran-2-yl)propoxy)-2H-chromen-2-one (5c) caused DNA damage in S. cerevisiae. The O-alkenylepoxy group stands out as that with the most important functionality within this family of 35 derivatives, presenting a very good profile as an antiproliferative scaffold. Finally, the in vitro antiretroviral capacity was tested through RT-PCR assays. Derivative 5c showed inhibitory activity below 150 µM with an IC50 value of 134.22 ± 2.37 µM, highlighting the O-butylepoxy group as the functionalization responsible for the activity.

Dynamic Hydroxyl-Yne Reaction with Phenols.

Dynamic Covalent Chemistry (DCvC) has gained increasing importance in supramolecular chemistry and materials science. Herein we prove the dynamic nature of the exchange between phenols and vinyl ethers. Exchange is fast at room temperature and under mild conditions. The equilibrium constants and the electronic effect of the phenol substituents were calculated. This novel incorporation to the DCvC toolbox could be quite useful, and as a proof it was used for the synthesis of a responsive molecular cage.

Chain-Shattering Polymers as Degradable Microdispersive Solid-Phase Extraction Sorbents.

A chain-shattering polymer (CSP) has been proposed as a microdispersive solid-phase extraction (µdSPE) sorbent in a proof-of-concept study of degradable materials for analytical purposes. The responsive CSP was synthesized from 1,3,5-tris(bromomethyl)-2-nitrobenzene acting as the self-immolative trigger responsive unit and 2,6-naphthalenedicarboxylic acid as aromatic linker to enhance noncovalent aromatic interactions with the analytes. The CSP was characterized and applied as a µdSPE sorbent of a group of plasticizers, which were selected as model analytes, from different types of environmental water samples (tap, waste, and spring waters). Gas chromatography coupled to mass spectrometry detection was used for analyte determination. Mean recovery values were in the range of 80%-118% with RSD values below 22%. After the extraction, the polymer could be efficiently degraded by UV irradiation or by chemical reduction, recovering the aromatic linker. This work has proved the potential of CSPs as recyclable sorbents, paving the way to more environmentally benign analytical procedures.

A Fluorescent Cage for Supramolecular Sensing of 3-Nitrotyrosine in Human Blood Serum.

3-Nitrotyrosine (NT) is generated by the action of peroxynitrite and other reactive nitrogen species (RNS), and as a consequence it is accumulated in inflammation-associated conditions. This is particularly relevant in kidney disease, where NT concentration in blood is considerably high. Therefore, NT is a crucial biomarker of renal damage, although it has been underestimated in clinical diagnosis due to the lack of an appropriate sensing method. Herein we report the first fluorescent supramolecular sensor for such a relevant compound: Fluorescence by rotational restriction of tetraphenylethenes (TPE) in a covalent cage is selectively quenched in human blood serum by 3-nitrotyrosine (NT) that binds to the cage with high affinity, allowing a limit of detection within the reported physiological concentrations of NT in chronic kidney disease.

Hydrogen sulphide-triggered theranostic prodrugs based on the dynamic chemistry of tetrazines.

Dynamic nucleophilic aromatic substitution of tetrazines (SNTz) has been employed to build theranostic prodrugs that are activated by hydrogen sulfide. H2S is typically found in high concentrations in some kinds of cancer cells and it is able to trigger the disassembly of tetrazine prodrugs. In such a way, a dual release of drugs and/or fluorescent compounds can be selectively triggered.

Dynamic Nucleophilic Aromatic Substitution of Tetrazines.

A dynamic nucleophilic aromatic substitution of tetrazines (SN Tz) is presented herein. It combines all the advantages of dynamic covalent chemistry with the versatility of the tetrazine moiety. Indeed, libraries of compounds or sophisticated molecular structures can be easily obtained, which are susceptible to post-functionalization by inverse electron demand Diels-Alder (IEDDA) reaction, which also locks the exchange. Additionally, the structures obtained can be disassembled upon the application of the right stimulus, either UV irradiation or a suitable chemical reagent. Moreover, SN Tz is compatible with the imine chemistry of anilines. The high potential of this methodology has been proved by building two responsive supramolecular systems: A macrocycle that displays a light-induced release of acetylcholine; and a truncated [4+6] tetrahedral shape-persistent fluorescent cage, which is disassembled by thiols unless it is post-stabilized by IEDDA.

Application of stimuli-responsive materials for extraction purposes.

Stimuli-responsive materials, frequently designated as "smart/intelligent materials", can modify their structure or properties by either a biological, physical, or chemical stimulus which, if properly controlled, could be used for specific applications. Such materials have been studied and exploited in several fields, like electronics, photonics, controlled drugs administration, imaging and medical diagnosis, among others, as well as in Analytical Chemistry where they have been used as chromatographic stationary phases, as part of sensors and for extraction purposes. This review article pretends to provide an overview of the most recent applications of these materials (mostly polymeric materials) in sample preparation for extraction purposes, as well as to provide a general vision of the current state-of-the-art of this field, their potential use and future applications.

Building Covalent Molecular Capsules by Thiol-Michael Addition Click Reaction.

The thiol-Michael addition (TMA) is a powerful methodology to click several fragments together, despite having been underestimated in the synthesis of complex systems for supramolecular chemistry. Herein, a very fast and efficient method has been developed to make covalent molecular capsules by taking advantage of the TMA click reaction. Several scaffolds commonly used in supramolecular chemistry, such as calixarenes, CTV, or cavitands, have been used to quickly obtain covalent cages. Additionally, a ' click&click' procedure has been also developed, by sequential combination of TMA and CuAAC click reaction, as an easy and quick way to build complex molecular structures.

Mild-Base-Promoted Arylation of (Hetero)Arenes with Anilines.

Transition metal-free radical arylation of heteroarenes is achieved at room temperature by simply adding aqueous sodium carbonate to a solution of the corresponding heteroarene and arenediazonium salt, which can even be formed in situ. Such an easy, inexpensive and mild methodology has been optimized and applied to the expeditious modification of interesting molecular cores like naphthylimide or bisthienylcyclopentenes.

Radical C-H arylations of (hetero)arenes catalysed by gallic acid.

Gallic acid efficiently catalyses radical arylations in water-acetone at room temperature. This methodology proved to be versatile and scalable. Therefore, it constitutes a greener alternative to arylation. Moreover, considering that gallic acid is an abundant vegetable tannin, this work also unleashes an alternative method for the reutilisation of bio-wastes.

Synthesis of New Benzocyclotrimer Analogues: New Receptors for Tetramethylammonium Ion Recognition.

Using a [2 + 2 + 2] cycloaddition/Mitsunobu reaction sequence, a convenient synthesis to access new benzocyclotrimer analogues has been developed. The new receptors have the geometry and functionality capable of recognizing the tetramethylammonium ion in the gas phase and in solution.

Oxidation with air by ascorbate-driven quinone redox cycling.

Transition metal-free oxidation with air at room temperature has been achieved by simply using ascorbate (vitamin C) and catalytic amounts of menadione (vitamin K3). A combination of the mentioned vitamins transforms atmospheric oxygen into hydrogen peroxide, which is able to oxidize arylboronic acids and other chemical moieties.

Synthesis and conformational analysis of cyclic homooligomers from pyranoid ε-sugar amino acids.

New pyranoid ε-sugar amino acids were designed as building blocks, in which the carboxylic acid and the amine groups were placed in positions C2 and C3 with respect to the tetrahydropyran oxygen atom. By using standard solution-phase coupling procedures, cyclic homooligomers containing pyranoid ε-sugar amino acids were synthesized. Conformation analysis was performed by using NMR spectroscopic experiments, FTIR spectroscopic studies, X-ray analysis, and a theoretical conformation search. These studies reveal that the presence of a methoxy group in the position C4 of the pyran ring produces an important structural change in the cyclodipeptides. When the methoxy groups are present, the structure collapses through interresidue hydrogen bonds between the oxygen atoms of the pyran ring and the amide protons. However, when the cyclodipeptide lacks the methoxy groups, a U-shape structure is adopted, in which there is a hydrophilic concave face with four oxygen atoms and two amide protons directed toward the center of the cavity. Additionally, we found important evidence of the key role played by weak electrostatic interactions, such as the five-membered hydrogen-bonded pseudocycles (C5) between the amide protons and the ether oxygen atoms, in the conformation equilibrium of the macrocycles and in the cyclization step of the cyclic tetrapeptides.

Toward metal complexes that can directionally walk along tracks: controlled stepping of a molecular biped with a palladium(II) foot.

We report on the design, synthesis, and operation of a bimetallic molecular biped on a three-foothold track. The "walker" features a palladium(II) complex "foot" that can be selectively stepped between 4-dimethylaminopyridine and pyridine ligand sites on the track via reversible protonation while the walker remains attached to the track throughout by means of a kinetically inert platinum(II) complex foot. The substitution pattern of the three ligand binding sites, together with the kinetic stability of the metal-ligand coordination bonds, affords the two positional isomers a high degree of metastability, meaning that altering the chemical state of the track does not automatically instigate stepping in the absence of an additional stimulus (heat in the presence of a coordinating solvent). The use of metastable metal complexes for foot-track interactions offers a promising alternative to dynamic covalent chemistry for the design of small-molecule synthetic molecular walkers.

Ascorbic acid as an initiator for the direct C-H arylation of (hetero)arenes with anilines nitrosated in situ.

Ascorbic acid (vitamin C) has been used as a radical initiator in a metal-free direct C-H arylation of (hetero)arenes. Starting from an aniline, the corresponding arenediazonium ion is generated in situ and immediately reduced by vitamin C to an aryl radical that undergoes a homolytic aromatic substitution with a (hetero)arene. Notably, neither heating nor irradiation is required. This procedure is mild, operationally simple, and constitutes a greener approach to arylation.

Expedient synthesis of C3-symmetric hexasubstituted benzenes via Nicholas reaction/[2 + 2 + 2] cycloaddition. new platforms for molecular recognition.

An expedient methodology to synthesize macrocyclic compounds in one step based on the Nicholas reaction is disclosed. The key step features two intermolecular reactions followed by an intramolecular reaction from the starting dicobalt hexacarbonyl-propargylic complex. The macrocycles obtained were modified through [2 + 2 + 2] cycloaddition, generating two new C3-symmetric hexasubstituted benzene structures suitable for molecular recognition purposes.

Correlation between conformational equilibria of free host and guest binding affinity in non-preorganized receptors.

Positive cooperativity between host conformational equilibria and guest binding has been widely reported in protein receptors. However, reported examples of this kind of cooperativity in synthetic hosts are scarce and largely serendipitous, among other things because it is hard to envision systems which display this kind of cooperativity. In order to shed some light on the correlation between conformational equilibria of free host and guest binding, selected structural modifications have been performed over a family of nonpreorganized hosts in order to induce conformational changes and to analyze their effect on the binding affinity. The conformational effect was evaluated by a theoretical conformational search and correlated with the ability of the receptors. All data suggest that those receptors that display the best association constants are able to sample folded conformations analogous to the conformational requirements for the binding of the guests. On the contrary, for those receptors where folded conformers are scarce, then the association constant and enantioselectivity clearly drop.

A novel approach for the evaluation of positive cooperative guest binding: kinetic consequences of structural tightening.

Cooperativity is one of the most relevant features displayed by biomolecules. Thus, one of the challenges in the field of supramolecular chemistry is to understand the mechanisms underlying cooperative binding effects. Traditionally, cooperativity has been related to multivalent receptors, but Williams et al. have proposed a different interpretation based on the strengthening of noncovalent interactions within receptors upon binding. According to such an interpretation, positive cooperative binding operates through structural tightening. Hence, a quite counterintuitive kinetic behavior for positively cooperative bound complexes may be postulated: the more stable the complex, the slower it is formed. Such a hypothesis was tested in a synthetic system in which positive cooperative binding was previously confirmed by calorimetric experiments. Indeed, a linear correlation between the thermodynamics (ΔG°) and the kinetics (ΔG(≠)) of guest binding confirmed the expected behavior. These distinctive kinetics provide solid evidence of positive cooperative guest binding, which is particularly useful bearing in mind that kinetic experiments are frequently and accurately carried out in both synthetic and biological systems.