A highly modular and convergent approach for the synthesis of stimulant-responsive heteroligated cofacial porphyrin tweezer complexes

The synthesis of new hemilabile phosphine ligands and their reaction with [Rh(COE)2Cl]2 to form dissymmetric heteroligated tweezer complexes using a halide-induced ligand rearrangement reaction are reported. These complexes can undergo reactions with small-molecule ligands and elemental anions quantitatively in situ, which serve to regulate the porphyrin-porphyrin distances and interactions within the assembly.     

Nucleotide recognition in water by a guanidinium-based artificial tweezer receptor

The water-soluble tweezer receptor 1 with two symmetric peptidic arms, which are connected by an aromatic scaffold and contain lysine, phenylalanine, and a guanidinium-based anion-binding site as headgroup, has been synthesized. UV/Vis-derived Job plots show that the receptor forms 1:1 complexes with nucleotides and phosphate in buffered water at neutral pH. Binding constants have been determined by fluorescence and UV/Vis spectroscopy. All nucleotides tested were bound very efficiently, even in pure water, with binding constants between 10(4) and 10(5) M(-1) . Interestingly, all mononucleotides were bound much stronger than phosphate by a factor of at least 5 to 10. Furthermore 1 favors the binding of adenosine monophosphate (AMP) over adenosine diphosphate (ADP) and adenosine triphosphate (ATP), which is unprecedented for artificial nucleotide receptors reported so far. According to NMR spectroscopy and molecular modeling studies, the efficient binding is a result of strong electrostatic contacts supported by π-π interactions with the nucleobase within the cavity-shaped receptor.     

Zinc porphyrin tweezer in host-guest complexation: determination of absolute configurations of primary monoamines by circular dichroism

A nonempirical exciton chirality circular dichroic (CD) method for determining the absolute configurations of primary monoamines with amino group directly linked to the stereogenic center is described. Conventional exciton chirality CD method cannot be applied to these compounds since they lack the two sites for attaching the interacting chromophores. This was solved by covalently linking the monoamine to a trifunctional bidentate carrier moiety 1. Treatment of the carrier/monoamine conjugate with the porphyrin tweezer 4 consisting of two pentanediol-linked zinc porphyrins gives rise to 1:1 host-guest macrocyclic complexes that exhibit exciton-coupled CD spectra. The sign of the CD couplet can then be correlated with the absolute configuration of the monoamine as follows: a clockwise arrangement of the L, M, and S (large, medium, small) groups in the Newman projection of the monoamine with the amino group in the rear gives rise to a positive CD couplet, and vice versa; the assignments of L, M, S groups are based on conformational energies (A values). This method is applicable to cyclic and acyclic aliphatic amines, aromatic amines, amino esters, amides, and cyclic amino alcohols, and can be performed at the several microgram level.     

Synthesis of a water-soluble molecular tweezer and a recognition study in an aqueous media

Synthesis of a 1,10-diphenanthryl all-trans fused perhydrophenanthrene derivative and a recognition study in an aqueous media were carried out. A water-soluble derivative recognized certain benzene derivatives with 10⁴-10⁵ M⁻¹ of binding constant.     

A linear supramolecular polymer based on host-guest recognition and metal-ligand coordination

A linear main-chain supramolecular polymer was constructed in aqueous solution via γ-CD host recognition with coumarin unit as well as metalligand coordination between terpyridine unit and Zn²⁺. Besides, the self-assembly behavior and morphological property of this supramolecular polymer system were characterized by DLS and TEM experiments.     

A selective biomimetic tweezer for noradrenaline

The new highly preorganized tweezer molecule 1 binds noradrenaline in polar solvents with unprecedented specificity. It uses a biomimetic recognition pattern and rejects almost all other neurotransmitters. LB experiments on a film balance reflect the same selectivity if 1 is incorporated into a stearic acid monolayer.     

Molecular tweezer and clip in aqueous solution: unexpected self-assembly, powerful host-guest complex formation, quantum chemical 1H NMR shift calculation

The newly prepared water-soluble naphthalene tweezer 2a and anthracene clip 4a (substituted both with lithium methanephosphonate groups in the central spacer unit) undergo an unexpected self-assembly in aqueous solution. The highly ordered intertwined structures of the self-assembled dimers [2a]2 and [4a]2 were elucidated by quantum chemical 1H NMR shift calculations. 2a and 4a form extremely stable host-guest complexes with N-methylnicotinamide 8 in methanol and water as well. According to the thermodynamic parameters determined by 1H NMR titration experiments at various temperatures the self-assembly of 2a and 4a and their strong binding to NMNA 8 observed in aqueous solution are enthalpy driven (DeltaH < 0); the enthalpic driving force is partially compensated by an unfavorable entropy (TDeltaS < 0). Self-assembly and the host-guest binding are therefore beautiful examples of the nonclassical hydrophobic effect.     

Supramolecular recognition forces: an examination of weak metal-metal interactions in host-guest formation

Molecular receptors consisting of two parallel-disposed terpy-M-Cl units (M = Pd2+, Pt2+) are used to form host-guest adducts with aromatic molecules and with neutral square-planar Pt2+ complexes. Host-guest formation is controlled by several factors including pi-pi interactions and, in some cases, weak Pt-Pt interactions between the host and the guest. This latter interaction was examined by comparing the host-guest stability of adducts formed by isoelectronic Pt2+ and Au3+ complexes with the Pt2+ receptor. Consistently, the former is more stable.     

DABCO as a dynamic hinge between cofacial porphyrin panels and its tumbling inside a supramolecular cavity

The heteroleptic supramolecular double-decker porphyrin 1 was synthesized with DABCO as a guest between two cofacial porphyrin units as characterized by (1)H NMR and ESI-MS. While DABCO is not seen to tumble inside the cavity, even at higher temperatures (80 °C), such motion was triggered upon addition of various coordinating ligands (quinuclidine, 4-bromopyridine, or excess of DABCO). Different stoichiometric amounts were needed depending on the n donor quality of the added ligands to initiate tumbling of the "inside" DABCO. As demonstrated in an example with excess DABCO, the tumbling was stopped by lowering the temperature to -50 °C.     

A cyclodextrin host-guest recognition approach to a label-free electrochemical DNA hybridization biosensor

A novel label-free electrochemical DNA hybridization biosensor using a β-cyclodextrin/poly(N-acetylaniline)/carbon nanotube composite modified screen printed electrode (CD/PNAANI/CNT/SPE) has been developed. The proposed DNA hybridization biosensor relies on the intrinsic oxidation signals of guanine (G) and adenine (A) from single-stranded DNA entered into the cyclodextrin (CD) cavity. Due to the binding of G and A bases to complementary cytosine and thymine bases in dsDNA, the signals obtained for ssDNA were much higher than that of dsDNA. The synergistic effect of the multi-walled carbon nanotubes provides a significantly enhanced voltammetric signal, and the CD encapsulation effect makes anodic peaks of G and A shift to less positive potentials than that at the bare SPE. The peak heights of G and A signals are dependent on both the number of the respective bases in oligonucleotides and the concentration of the target DNA sequences. Hybridization of complementary strands was monitored through the measurements of oxidation signal of purine bases, which enabled the detection of target sequences from 0.01 to 1.02 nmol μl(-1) with the detection limit of target DNA as low as 5.0 pmol μl(-1) (S/N = 3). Implementation of label-free and homogeneous electrochemical hybridization detection constitutes an important step toward low-cost, simple, highly sensitive and accurate DNA assay. Discrimination between complementary, noncomplementary, and two-base mismatch targets was easily accomplished using the proposed electrode.     

Molecular recognition. Self-assembly of molecular trigonal prisms and their host-guest adducts

Two supramolecular trigonal prisms, each bearing three molecular clefts are shown to form 1:6 and 1:7 host-guest complexes with 9-methylanthracene and one of the prisms forms a 1:2 host-guest complex with a tritopic tri-anthracene guest that registers with the recognition sites of the host.     

新型脱氧胆酸钳形阴离子受体的设计合成及其对阴离子的识别性能研究

具有对阴离子选择性识别的人工受体的设计合成是生物有机化学和超分子化学前沿富于挑战性的领域之一[1].在许多识别阴离子的人工受体化合物中,脲和硫脲衍生物是重要的中性受体化合物之一.     

Molecular recognition and host-guest interactions in complexes of O-bistren,C-bistren,and bisdien

The host-guest relationships of the complexes of the cryptand ligands O-BISTREN and C-BISTREN, and of the analogous macrocyclic ligand BISDIEN are compared. The affinities of their binuclear copper(II) complexes for the bridging ligands (OH^–, F^–) as cascade type guests (i.e., guests of guests) are reported. The ability of the dicobalt(II)-BISDIEN complex to coordinate dioxygen and an additional bifunctional guest simultaneously, leads to the possibility of a new type of catalysis occurring within the cavity of a macrocyclic complex.     

A molecular tweezer for lysine and arginine

Lysine and arginine play a key role in numerous biological recognition processes controlling, inter alia, gene regulation, glycoprotein targeting and vesicle transport. They are also found in signaling peptide sequences responsible, e.g. for bacterial cell wall biosynthesis, Alzheimer peptide aggregation and skin regeneration. Almost none of all artificial receptor structures reported to date are selective and efficient for lysine residues in peptides or proteins. An artificial molecular tweezer is introduced which displays an exceptionally high affinity for lysine (K(a) approximately 5000 in neutral phosphate buffer). It features an electron-rich torus-shaped cavity adorned with two peripheral anionic phosphonate groups. Exquisite selectivity for arginine and lysine is achieved by threading the whole amino acid side chain through the cavity and subsequent locking by formation of a phosphonate-ammonium/guanidinium salt bridge. This pseudorotaxane-like geometry is also formed in small basic signaling peptides, which can be bound with unprecedented affinity in buffered aqueous solution. NMR titrations, NOESY and VT experiments as well as ITC measurements and Monte Carlo simulations unanimously point to an enthalpy-driven process utilizing a combination of van der Waals interactions and substantial electrostatic contributions for a conformational lock. Since DMSO and acetonitrile compete with the amino acid guest inside the cavity, a simple change in the cosolvent composition renders the whole complexation process reversible.     

Crystal and molecular structures of ionophore-siderophore host-guest supramolecular assemblies relevant to molecular recognition

Ionophore-siderophore host-guest assemblies composed of 18-crown-6 and ferrioxamine B, benzo-18-crown-6 and ferrioxamine B, and cis-syn-cis-dicyclohexano-18-crown-6 and ferrioxamine B were successfully crystallized, and their structures were determined by single-crystal X-ray diffraction. All three crystal lattices also include solvated Mg(II) and perchlorate ions. The ionophore-siderophore host-guest assembly is noncovalently held together by a hydrogen bonding interaction between the pendant protonated amine in the second coordination sphere of ferrioxamine B and the hydrogen bond acceptor oxygen atoms in the crown ether. The crystals of 18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 19.8327(11) A, b = 20.4111(11) A, c = 15.1698(8) A, and beta = 96.435(1) degrees. The crystals of benzo-18-crown-6:ferrioxamine B host-guest assembly are triclinic, with space group P(-)1, and two molecules per unit cell with dimensions a = 11.1747(10) A, b = 16.0580(15) A, c = 18.4175(17) A, alpha = 80.469(3) degrees, beta = 81.481(3) degrees and gamma = 70.212(2) degrees. The crystals of cis-syn-cis-dicyclohexano-18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 20.1473(13) A, b = 21.5778(15) A, c = 14.8013(10) A, and beta = 94.586(2) degrees. The crystal structures of all three host-guest assemblies contain a racemic mixture of Lambda-N-cis, cis and Delta-N-cis, cis coordination isomers of ferrioxamine B. The crystal structures indicate that the steric rigidity of the benzo-18-crown-6 and cis-syn-cis-dicyclohexano-18-crown-6 cavity has a pronounced effect on the conformation of the crown ring and ultimately on the hydrogen bonding interactions between the crown ethers and ferrioxamine B. The structural parameters and the conformational features of the ferrioxamine B guests compare very well with each other and with those of the ferrioxamine B structure obtained in the absence of a host. Structural features relevant to siderophore molecular recognition are discussed.     

Mechanical compression in cofacial porphyrin cyclophane pincers

Intra- and intermolecular interactions are dominating chemical processes, and their concerted interplay enables complex nonequilibrium states like life. While the responsible basic forces are typically investigated spectroscopically, a conductance measurement to probe and control these interactions in a single molecule far out of equilibrium is reported here. Specifically, by separating macroscopic metal electrodes, two π-conjugated, bridge-connected porphyrin decks are peeled off on one side, but compressed on the other side due to the covalent mechanical fixation. We observe that the conductance response shows an exceptional exponential rise by two orders of magnitude in individual breaking events during the stretching. Theoretical studies atomistically explain the measured conductance behavior by a mechanically activated increase in through-bond transport and a simultaneous strengthening of through-space coupling. Our results not only reveal the various interacting intramolecular transport channels in a molecular set of levers, but also the molecules'' potential to serve as molecular electro-mechanical sensors and switches.

A two-order conductance increase upon stretching in porphyrin cyclophane pincer junctions is measured. Atomistic studies explain experimental observations by characteristic intramolecular changes in through-space and through-bond transport.     

Charge transfer excitations in cofacial fullerene-porphyrin complexes

Porphyrin and fullerene donor-acceptor complexes have been extensively studied for their photo-induced charge transfer characteristics. We present the electronic structure of ground states and a few charge transfer excited states of four cofacial porphyrin-fullerene molecular constructs studied using density functional theory at the all-electron level using large polarized basis sets. The donors are base and Zn-tetraphenyl porphyrins and the acceptor molecules are C(60) and C(70). The complexes reported here are non-bonded with a face-to-face distance between the porphyrin and the fullerene of 2.7 to 3.0 A?. The energies of the low lying excited states including charge transfer states calculated using our recent excited state method are in good agreement with available experimental values. We find that replacing C(60) by C(70) in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV. Variation of donor in these complexes has marginal effect on the lowest charge transfer excitation energy. The interfacial dipole moments and lowest charge transfer states are studied as a function of face-to-face distance.     

Effects of host-guest recognition on kinetics of Diels-alder reaction of quinocrown ethers with cyclopentadiene

Diels-Alder reactions of 15-21-membered quinocrown ethers 1a-c and 18-membered quinobenzocrown ether 1d with cyclopentadiene were catalyzed by the addition of alkali, alkaline earth metal and ammonium perchlorates, and scandium trifluoromethane-sulfonate. The alkali metal and ammonium ions brought about a fairly selective rate-acceleration for each crown ether due to the size-fitted ion-in-the-hole complexation. However, such a hole-size-selectivity was not observed for the reactions catalyzed by divalent alkaline-earth (Mg2+ to Ba2+) and trivalent Sc3+ ions. The wrapping complexation played a significant role in rate-acceleration in such a way that the smallest Mg2+ caused 160 times rate-enhancement for the most flexible 1c and the Sc3+ performed maximal 3700 times rate-increment for the 18-membered quinobenzocrown 1d. These effects of cation recognition were rationalized by the reduction of LUMO energy that is favored by the orbital interaction with the HOMO of cyclopentadiene. The magnitude of rate-enhancement was discussed in terms of the cation binding affinity and coordination geometry of quinocrown ethers as well as the valence of cations.     

A chemo-mechanical tweezer for single-molecular characterization of soft materials

A new atomic force microscopy (AFM)-based chemo-mechanical tweezer has been developed that can measure mechanical properties of individual macromolecules in supramolecular assembly and reveal positions of azide-containing polymers. A key feature of the new technology is the use of an AFM tip densely modified with 4-dibenzocyclooctynols (chemo-mechanical tweezer) that can react with multiple azide containing macromolecules of micelles to give triazole "clicked" compounds, which during retracting phases of AFM imaging are removed from the macromolecular assembly thereby providing a surface topographical image and positions of azide-containing polymers. The force-distance curves gave mechanical properties of removal of individual molecules from a supramolecular assembly. The new chemo-mechanical tweezer will make it possible to characterize molecular details of macromolecular assemblies thereby offering new avenues to tailor properties of such assemblies.     

Single-particle detection of cholesterol based on the host-guest recognition induced plasmon resonance energy transfer

Plasmon resonance energy transfer (PRET) occurs between the plasmonic nanoparticles (NPs) and organic dyes forming donor-acceptor pairs, which has great potential in quantitative analytical chemistry because of its excellent sensitivity under dark-field microscopy (DFM). Herein, we introduce supramolecular β-cyclodextrin (β-CD) to design a host-guest recognition plasmonic nano-structure modified gold nanoparticles (GNPs), while GNPs and rhodamine molecule (RB) act as the donor and acceptor, respectively. In the presence of the target cholesterol, due to the stronger binding of cholesterol with β-CD, RB molecules are released, inducing the inhibition of PRET, as well as the increase of the scattering intensity of GNPs. The proposed strategy achieves a linear range from 0.02 µmol/L to 2.0 µmol/L for cholesterol detection, and reaches a limit of detection (LOD) of 6.7 nmol/L. This host-guest recognition strategy can easily integrate receptor-donor pair into one nanoparticle, which simplifies the construction of the PRET platform, and further provides an effective approach for PRET-based analytical applications. Afterwards, the proposed PRET strategy was successfully applied for the detection of cholesterol in serum samples with high sensitivity and specificity. The proposed method provides an effective clinically potential means for the detection of cholesterol and other disease-related biomarkers.