Anion-π Interactions Research Articles

Co-assembly of Peptide Amphiphiles and Lipids into Supramolecular Nanostructures Driven by Anion-π Interactions.

Co-assembly of binary systems driven by specific non-covalent interactions can greatly expand the structural and functional space of supramolecular nanostructures. We report here on the self-assembly of peptide amphiphiles and fatty acids driven primarily by anion-π interactions. The peptide sequences investigated were functionalized with a perfluorinated phenylalanine residue to promote anion-π interactions with carboxylate headgroups in fatty acids. These interactions were verified here by NMR and circular dichroism experiments as well as investigated using atomistic simulations. Positioning the aromatic units close to the N-terminus of the peptide backbone near the hydrophobic core of cylindrical nanofibers leads to strong anion-π interactions between both components. With a low content of dodecanoic acid in this position, the cylindrical morphology is preserved. However, as the aromatic units are moved along the peptide backbone away from the hydrophobic core, the interactions with dodecanoic acid transform the cylindrical supramolecular morphology into ribbon-like structures. Increasing the ratio of dodecanoic acid to PA leads to either the formation of large vesicles in the binary systems where the anion-π interactions are strong, or a heterogeneous mixture of assemblies when the peptide amphiphiles associate weakly with dodecanoic acid. Our findings reveal how co-assembly involving designed specific interactions can drastically change supramolecular morphology and even cross from nano to micro scales.

Optimization of Ex-Situ Washing Removal of Polycyclic Aromatic Hydrocarbons from a Contaminated Soil Using Nano-Sulfonated Graphene

Ex-situ soil washing technology offers advantages such as speed and efficiency of remediation and range of application and has been developed to conform with legal requirements and best management practices in USA and some European countries. In this study, nano-sulfonated graphene (SGE) was used as a washing agent to evaluate different processing (washing) parameters for the ectopic leaching removal of polycyclic aromatic hydrocarbons (PAHs) from a coking plant soil. X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectroscopy (FTIR) were used to analyze the characteristics of the SGE tested. The results showed that SGE had a strong adsorption capacity for PAHs through the role of π-π, H-π, and anion-π interactions. The washing parameters, an SGE concentration of 2 000 mg L−1, a liquid/soil (L/S) ratio of 10:1, and 4 cycles of successive washing, were set to arrive to the optimum condition for achieving more than 80% of PAH removal. Under the optimum condition, the PAH removal rate decreased with increasing ring numbers. After one washing cycle at SGE concentration of 2 000 mg L−1 and L/S ratio of 10:1, the PAH removal rate of the SGE tested was much higher than that of Tween 80 (TW80) or methyl-ß-cyclodextrin (MCD) (P < 0.01). Therefore, SGE is a promising material for the nanoremediation of PAH-contaminated soils.

Anion-π weak interactions in a heteroaromatic calixarene receptor. A theoretical investigation

The interactions between a series of differently shaped anions (NO3−, BF4−, PF6−, SCN−) and a heteroaromatic calixarene receptor (tetraoxacalix[2]arene[2]triazine) have been investigated in acetonitrile solution by using density functional (wB97XD and B3LYP-D3 exchange-correlation functionals) and second-order Möller-Plesset perturbation levels of theory in order to strengthen the presence of anion-π interactions in condensed phase. From the computed binding energies, a considerable interaction has been found, whose magnitude seems to depends on the size of considered anions. The obtained geometrical structures well agree with the corresponding X-ray data, while the reproduction of the binding energies depends on the used level of theory.

Lone pair-π interactions in biological systems: occurrence, function, and physical origin.

Lone pair-π interactions are now recognized as a supramolecular bond whose existence in biological systems is documented by a growing number of examples. They are commonly attributed to electrostatic forces. This review attempts to highlight some recent discoveries evidencing the important role which lone pair-π interactions, and anion-π interactions in particular, play in stabilizing the structure and affecting the function of biomolecules. Special attention is paid to studies exploring the physical origin of these at first glance counterintuitive interactions between a lone pair of electrons of one residue and the π-cloud of another. Recent theoretical work went beyond the popular electrostatic model and inquired the extent to which orbital interactions have to be taken into account. In at least one biologically relevant case-that of anion-flavin interactions-a substantial charge-transfer component has been shown to operate.

The non-coordinating anion 1,1,3,3-tetracyano-2-propoxy-propenide as an anion-π donor in cis-diaquabis(2,2′-dipyridylamine)zinc and its cadmium analog: Luminescence properties, Hirshfeld analysis and central-atom induced polymorphism

The synthesis, structure and properties of cis-diaquabis(2,2′-dipyridylamine)zinc bis(1,1,3,3-tetracyano-2-propoxy-propenide), [Zn(H2O)2(2,2′-dpa)2][tcnopr]2, I, and its isomorphous Cd analog, II, are reported. These two coordination compounds were prepared through hydrothermal reactions of Zn(II)/Cd(II) acetate with potassium 1,1,3,3-tetracyano-2-propoxy-propenide (tcnopr–=[(NC)2CC(OPr)C(CN)2]−) in the presence of 2,2′-dipyridylamine (dpa=C10H9N3) as a co-ligand. Both new compounds were fully characterized by elemental analysis, FT-IR spectroscopy, powder XRD and X-ray single-crystal diffraction.Single-crystal X-ray analysis has revealed that compounds I and II are isostructural containing octahedrally coordinated metal ions, and exhibit 3D supramolecular architectures, including multiple anion-π interactions. Furthermore, the photoluminescence properties in the solid state at room temperature and Hirshfeld surfaces analysis to understand the packing have also been investigated in detail.

Interplay between salt bridge, hydrogen bond and anion-π interactions in thiocyanate binding

The L2 ligand (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) is constituted by a tetrazine ring decorated with two morpholine pendants. The crystal structure of the H2L2(SCN)2 anion complex shows the thiocyanate anion interacting with the ligand, protonated on the morpholine groups, through the formation of salt-bridge, anion-π and hydrogen bond interactions. The SCN− anion lies over the tetrazine ring, with the nitrogen atom in proximity to the ring centroid, in a sort of π-π stacking binding mode. Density Functional Theory (DFT) calculations performed on the interaction of SCN− with the plain tetrazine ring showed that the anion-π attraction alone is sufficient to form the complex, even in a simulated implicit water environment. The arrangement of the interacting partners in the DFT-optimized geometry of the most stable complex is very similar to the one actually assumed in the crystal structure. Potentiometric titrations performed in water (0.1M Me4NCl, 298.1±0.1K) revealed that not only the protonated ligand forms, but even the neutral L2 molecule is able to bind SCN− in solution. The stability of the complexes formed is almost insensitive to ligand charge, revealing that, even in water, anion-π interactions are of major importance in the interplay of weak forces contributing to the formation of similar anion complexes.

An Optically Active Polymer for Broad-Spectrum Enantiomeric Recognition of Chiral Acids.

Recognition of enantiomers of chiral acids by anion-π or lone pair-π interactions has not yet been investigated but is a significant and attractive challenge. This study reports an optically active polymer-based supramolecular system with capabilities of discriminating enantiomers of various chiral acids. The polymer featuring alternate π-acidic naphthalenediimides (NDIs) and methyl l-phenylalaninates in the backbone exhibits an unprecedented slow self-assembly process that is susceptible to perturbation by various chiral acids. Thus, the combination of anion-π or lone pair-π interactions and sensitivity of the polymeric self-assembly process to external chiral species endows the system with recognition capabilities. This is the first time that anion-π or lone pair-π interactions have been applied in the recognition of enantiomers of various chiral acids with a single system. The results shed light on new strategies for material design by integrating π-acidic aromatic systems and chiral building blocks to afford relevant advanced functions.

Tuning aerogen bonds via anion-π or lone pair-π interaction: a comparative ab initio study

Using ab initio calculations, cooperative effects between aerogen-bonding and anion-π or lone pair-π interaction is studied in some model complexes. A detailed analysis of the structure, interaction energy, and bonding properties is performed on these systems. The results show that the strength of the aerogen-bonding interaction increases in the presence of an anion-π or a lone pair-π interaction. The estimated cooperative energies are between −0.27 and −11.96 kcal/mol. It is found that the effect of an anion-π interaction on an aerogen-bonding is more important than that of a lone pair-π. The strengthening of the aerogen-bonding in the multi-component complexes depend considerably on the strength of the anion-π or lone pair-π interaction, and it becomes larger in the order X− = F− > Cl− > Br− and HX = HBr > HCl > HF.

Frontispiece: Anion–π Interactions in Flavoproteins Involve a Substantial Charge‐Transfer Component

Frontispiece: Anion–π Interactions in Flavoproteins Involve a Substantial Charge‐Transfer Component

Anion-π interaction in metal-organic networks formed by metal halides and tetracyanopyrazine

Co-crystallization of tetracyanopyrazine, TCP, with the tetraalkylammonium salts of linear [CuBr2]−, planar [PtCl4]2− or [Pt2Br6]2−, or octahedral [PtBr6]2− complexes resulted in formation of the alternating [MlXn]m−/TCP stacks separated by the Alk4N+ cations. These hybrid stacks showed multiple short contacts between halide ligands of the [MlXn]m− complexes and carbon atoms of the TCP acceptor indicating strong anion-π bonding between these species. It confirmed that the anion-π interaction is sufficiently strong to bring together such disparate components as ionic metal complexes and neutral aromatic molecules regardless of the geometry of the coordination compound. Structural features of the solid-state stacks and [MlXn]m−·TCP dyads resulted from the quantum-mechanical computations suggests that the molecular-orbital (weakly-covalent) component play an important role in association of the [MlXn]m− complexes with the TCP acceptor.

Do CH-Anion and Anion-π Interactions Alter the Mechanism of 2:1 Host-Guest Complexation in Arylethynyl Monourea Anion Receptors?

Selective tuning of arylethynyl urea scaffolds for anionic guests requires an understanding of preferred binding motifs of the host-guest interaction. To investigate the binding preference of receptors without a pre-organized binding pocket, two electron-deficient phenylacetylene receptors with a single urea moiety have been prepared and were found to bind halides as 2:1 host-guest complexes that feature key CH-anion or anion-π interactions. These supporting interactions also appear to influence the mechanism of the 2:1 binding event.

Crystal structure of the BoNT/A2 receptor-binding domain in complex with the luminal domain of its neuronal receptor SV2C

A detailed molecular understanding of botulinum neurotoxin (BoNT)/host-cell-receptor interactions is fundamental both for developing strategies against botulism and for generating improved BoNT variants for medical applications. The X-ray crystal structure of the receptor-binding domain (HC) of BoNT/A1 in complex with the luminal domain (LD) of its neuronal receptor SV2C revealed only few specific side-chain – side-chain interactions that are important for binding. Notably, two BoNT/A1 residues, Arg 1156 and Arg 1294, that are crucial for the interaction with SV2, are not conserved among subtypes. Because it has been suggested that differential receptor binding of subtypes might explain their differences in biological activity, we determined the crystal structure of BoNT/A2-HC in complex with SV2C-LD. Although only few side-chain interactions are conserved between the two BoNT/A subtypes, the overall binding mode of subtypes A1 and A2 is virtually identical. In the BoNT/A2-HC – SV2C complex structure, a missing cation-π stacking is compensated for by an additional salt bridge and an anion-π stacking interaction, which explains why the binding of BoNT/A subtypes to SV2C tolerates variable side chains. These findings suggest that motif extensions and a shallow binding cleft in BoNT/A-HC contribute to binding specificity.

Anion-π Interactions in Flavoproteins Involve a Substantial Charge-Transfer Component.

Anion-π interactions have been shown to stabilize flavoproteins and to regulate the redox potential of the flavin cofactor. They are commonly attributed to electrostatic forces. Herein we show that anion-flavin interactions can have a substantial charge-transfer component. Our conclusion emanates from a multi-approach theoretical analysis and is backed by previously reported observations of absorption bands, originating from charge transfer between oxidized flavin and proximate cysteine thiolate groups. This partial covalency of anion-flavin contacts renders classical simulations of flavoproteins questionable.

Anion Recognition Based on a Combination of Double-Dentate Hydrogen Bond and Double-Side Anion-π Noncovalent Interactions.

Anion recognitions between common anions and a novel pincer-like receptor (N,N'-bis(5-fluorobenzoyloxyethyl)urea, BFUR) were theoretically explored, particularly on geometric features of the BFUR@X (X = F-, Cl-, Br-, I-, CO32-, NO3-, and SO42-) systems at a molecular level in this work. Complex structures show that two N-H groups as a claw and two -C6F5 rings on BFUR as a pair of tweezers simultaneously interact with captured anions through cooperative double-dentate hydrogen bond and double-side anion-π interactions. The binding energies and thermodynamic information indicate that the recognitions of the seven anions by BFUR in vacuum are enthalpy-driven and entropy-opposed, which occur spontaneously. Although the binding energy ΔEcp between F- and BFUR is relatively high (289.30 kJ·mol-1), ΔEcp, ΔG, and ΔH of the recognition for CO32- and SO42- are much larger than the cases of F-, Cl-, Br-, I-, and NO3-, suggesting that BFUR is an ideal selective anion receptor for CO32- and SO42-. Additionally, energy decomposition analysis based on localized molecular orbital energy decomposition analysis (LMO-EDA) was performed; electronic properties and behaviors of the present systems were further discussed according to calculations on frontier molecular orbital, UV-vis spectra, total electrostatic potential, and visualized weak interaction regions. The present theoretical exploration of BFUR@X (X = F-, Cl-, Br-, I-, CO32-, NO3-, and SO42-) systems is fundamentally crucial to establish an anion recognition structure-property relationship upon combination of different noncovalent interactions, that is, double-dentate hydrogen bond and double-side anion-π interactions.

Anionic Head Containing Oxacalix[2]arene[2]triazines: Synthesis and Anion-π-Directed Self-Assembly in Solution and Solid State.

A series of oxacalix[2]arene[2]triazines bearing one anionic head such as carboxylate, sulfonate, sulfate, and phosphate were synthesized. With the anionic head and complementary V-shape electron-deficient cavity, these macrocycles can serve as dual building units, and their anion-π directed self-assembly was investigated. The formation of oligomeric aggregates in solution was revealed by nuclear magnetic resonance, dynamic light scattering, and mass spectroscopy. Crystal structures further confirmed chainlike assembly formation directed by anion-π interactions.

Anion-π catalysis: bicyclic products with four contiguous stereogenic centers from otherwise elusive diastereospecific domino reactions on π-acidic surfaces.

Anion-π interactions have been introduced recently to catalysis. The idea of stabilizing anionic intermediates and transition states on π-acidic surfaces is a new fundamental concept. By now, examples exist for asymmetric enolate, enamine, iminium and transamination chemistry, and the first anion-π enzyme has been created. Delocalized over large aromatic planes, anion-π interactions appear particularly attractive to stabilize extensive long-distance charge displacements during domino processes. Moving on from the formation of cyclohexane rings with five stereogenic centers in one step on a π-acidic surface, we here focus on asymmetric anion-π catalysis of domino reactions that afford bicyclic products with quaternary stereogenic centers. Catalyst screening includes a newly synthesized, better performing anion-π version of classical organocatalysts from cinchona alkaloids, and anion-π enzymes. We find stereoselectivities that are clearly better than the best ones reported with conventional catalysts, culminating in unprecedented diastereospecificity. Moreover, we describe achiral salts as supramolecular chirality enhancers and report the first artificial enzyme that operates in neutral water with anion-π interactions, i.e., interactions that are essentially new to enzymes. Evidence in support of contributions of anion-π interactions to asymmetric catalysis include increasing diastereo- and enantioselectivity with increasing rates, i.e., asymmetric transition-state stabilization in the presence of π-acidic surfaces and inhibition with the anion selectivity sequence NO3- > Br- > BF4- > PF6-.

Iodide and triiodide anion complexes involving anion-π interactions with a tetrazine-based receptor.

Protonated forms of the tetrazine ligand L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) interact with iodide in aqueous solution forming relatively stable complexes (ΔG° = -11.6(4) kJ mol-1 for HL2+ + I- = (HL2)I and ΔG° = -13.4(2) kJ mol-1 for H2L22+ + I- = [(H2L2)I]+). When solutions of [(H2L2)I]+ are left in contact with air, crystals of the oxidation product (H2L2)2(I3)3I·4H2O are formed. Unfortunately, the low solubility of I3- complexes prevents the determination of their stability constants. The crystal structures of H2L2I2·H2O (1), H2L2(I3)2·2H2O (2) and (H2L2)2(I3)3I·4H2O (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I- and I3- with H2L22+ is dominated by anion interactions with the π electron density of the receptor. Only in the case of 1, the iodide anions involved in close anion-π interactions with the ligand tetrazine ring form an additional H-bond with the protonated morpholine nitrogen of an adjacent ligand molecule. Conversely, in crystals of 2 and 3 there are alternate segregated planes which contain only protonated ligands hydrogen-bonded to cocrystallized water molecules or I3- and I- forming infinite two-dimensional networks established through short interhalogen contacts, making these crystalline products good candidates to behave as solid conductors. In the solid complexes, the triiodide anion displays both end-on and side-on interaction modes with the tetrazine ring, in agreement with density functional theory calculations indicating a preference for the alignment of the I3- molecular axis with the molecular axis of the ligand. Further information about geometries and structures of triiodide anions in 2 and 3 was acquired by the analysis of their Raman spectra.

アニオン-π触媒

Organocatalyst realizes high yield and high selectivity by using non-covalent bond interactions. In these interactions, anion-π interaction which works between π-acidic surfaces and anions has been conducted to organocatalyst recently since a first report by Matile in 2013. Here, the recent development of anion-π catalysis is introduced from the viewpoints of stabilization mechanism of anionic transition states.

Anion-π recognition between [M(CN)6]3- complexes and HAT(CN)6: structural matching and electronic charge density modification.

Hexacyanidometalates (M = FeIII, CoIII) and multisite anion receptor HAT(CN)6 (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile) recognize each other in acetonitrile solution and self-assemble into the novel molecular networks (PPh4)3[M(CN)6][HAT(CN)6] (M = Fe, 1; Co, 2) and (AsPh4)3[M(CN)6][HAT(CN)6]·2MeCN·H2O (M = Fe, 3; Co, 4). 1-4 contain the stacked columns {[M(CN)6]3-;[HAT(CN)6]}∞ separated by the organic cations. All of the M-C[triple bond, length as m-dash]N vectors point collectively towards the centroids of pyrazine rings on neighboring HAT(CN)6 molecules, with Ncyanidecentroidpyrazine distances that are under 3 Å. The directional character and structural parameters of the new supramolecular synthons correspond to collective triple anion-π interactions between the CN- ligands of the metal complexes and the π-deficient areas of HAT(CN)6. Physicochemical characterisation (IR spectroscopy, UV-Vis spectroscopy, cyclic voltammetry) and dispersion-corrected DFT studies reveal the dominating charge-transfer (CT) and polarisation characters of the interactions. The electronic density flow occurs from the CN- ligands of [M(CN)6]3- to the HAT(CN)6 orbital systems and further, toward the peripheral -CN groups of HAT(CN)6. Solid-state DFT calculations determined the total interaction energy of HAT(CN)6 to be ca. -125 kcal mol-1, which gives ca. -15 kcal mol-1 per one CN-HAT(CN)6 contact after subtraction of the interaction with organic cations. The UV-Vis electronic absorption measurements prove that the intermolecular interactions persist in solution and suggest a 1 : 1 composition of the anion-π {[M(CN)6]3-;[HAT(CN)6]} chromophore, with the formation constant Kadd = (5.8 ± 6) × 102 dm3 mol-1 and the molar absorption coefficient εadd = 180 ± 9 cm-1 dm3 mol-1 at 600 nm, as estimated from concentration-dependent studies.

Anion-π transaminase mimics

The possibility to stabilise anionic transition states on π-acidic aromatic surfaces has been explicitly demonstrated first in 2013. Since then, anion-π catalysis has been introduced to asymmetric enamine and iminium chemistry and to cascade processes, and the first anion-π enzyme has been created. Moving beyond systems that operate with nitronate-π interactions, this report adds transamination to the repertoire of anion-π catalysis. Whereas bioinspired approaches to transamination with pyridoxalphosphate appeared less obvious in this context, the base-catalyzed isomerisation of trifluoromethylimines contains suitable anionic transition states. Run on increasingly π-acidic aromatic surfaces in covalent and supramolecular trifunctional systems, we find that both rate and enantioselectivity of this reaction increase. These results support that anion-π interactions with 2-azaallyl anion intermediates catalyse the isomerisation of trifluoromethylimines by cumulative asymmetric umpolung on π-acidic surfaces.