Molecular recognition of oxoanions based on guanidinium receptors

Chromogenic and fluorogenic chemosensors and reagents for anions. A comprehensive review of the year 2009

This critical review is focused on examples reported in the year 2009 dealing with the design of chromogenic and fluorogenic chemosensors or reagents for anions (264 references).

Cryptand-like anion receptors

The design of supramolecular hosts for anions began with simple diaza bicycles, named katapinands, and has evolved over the last 40 years to a number of elegantly designed receptors capable of binding many different anions. About the same time the term cryptand appeared in reference to another bicyclic compound that was selective for alkaline-earth ions. Since the first report these simple bicycles, a vast arena of hosts has appeared, including acyclic, monocyclic, and other multicyclic supramolecular receptors. Studies of these systems have revealed considerable information about anion coordination chemistry, including the fact that many of these complexes mimic their transition-metal corollaries in terms of coordination numbers. However, for anions interactions occur via H-bonding most often, rather than the coordinate covalent or dative bonds observed in transition-metal coordination. This critical review examines the design of enclosed, primarily bicyclic cryptands as hosts for anions, with a small scattering of higher polyhedra when deemed appropriate to the discussion. In order to show the development (evolution) of the field, key examples of early work will be noted and compared with more recent developments (136 references).

Hosting anions. The energetic perspective

Hosting anions addresses the widely spread molecular recognition event of negatively charged species by dedicated organic compounds in condensed phases at equilibrium. The experimentally accessible energetic features comprise the entire system including the solvent, any buffers, background electrolytes or other components introduced for e.g. analysis. The deconvolution of all these interaction types and their dependence on subtle structural variation is required to arrive at a structure-energy correlation that may serve as a guide in receptor construction. The focus on direct host-guest interactions (lock-and-key complementarity) that have dominated the binding concepts of artificial receptors in the past must be widened in order to account for entropic contributions which constitute very significant fractions of the total free energy of interaction. Including entropy necessarily addresses the ambiguity and fuzziness of the host-guest structural ensemble and requires the appreciation of the fact that most liquid phases possess distinct structures of their own. Apparently, it is the perturbation of the intrinsic solvent structure occurring upon association that rules ion binding in polar media where ions are soluble and abundant. Rather than specifying peculiar structural elements useful in anion binding this critical review attempts an illumination of the concepts and individual energetic contributions resulting in the final observation of specific anion recognition (95 references).

Synthesis and study of the use of heterocyclic thiosemicarbazones as signaling scaffolding for the recognition of anions

A family of heterocyclic thiosemicarbazone dyes (1-9) linked to different furan, thiazole, (bi)thiophene, and arylthiophene pi-conjugated bridges were synthesized in good yields, and their response toward anions was studied. Acetonitrile solutions of 1-9 show bands in the 326-407 region that are modulated by the electron-donor or -acceptor strength of the heterocyclic systems appended to the phenylthiosemicarbazone moiety. Anions of different shape such as fluoride, chloride, bromide, iodide, dihydrogen phosphate, hydrogen sulfate, nitrate, acetate, cyanide, and thiocyanate were employed for the recognition studies. From these anions, only fluoride, cyanide, acetate, and dihydrogen phosphate displayed sensing features. Two different effects were observed, (i) a low bathochromic shift of the absorption band due to coordination of the anions with the thiourea protons and (ii) the growth of a new red-shifted band with a concomitant change of the solution from yellow or pale yellow to orange-red due to deprotonation. The extent of each process is a balance between the acidity tendency of the thioureido-NH donors modulated by the donor or acceptor groups in the structure of the receptors and the basicity of the anions. Fluorescence studies were also in agreement with the different effects observed on the UV/vis titrations. Stability constants for the two processes (complex formation + deprotonation) for selected receptors and the anions fluoride and acetate were determined spectrophotometrically using the program HYPERQUAD. Semiempirical calculations to evaluate the hydrogen-donating ability of the dyes and (1)H NMR titrations experiments with fluoride were carried out. A prospective electrochemical characterization of compound 3 in the presence of anions was also performed.

Intracellular delivery of a membrane-impermeable enzyme in active form using functionalized gold nanoparticles

Gold nanoparticles were coated with a short peptide to promote intracellular delivery of membrane-impermeable proteins. Through microscopy and enzyme assays, we demonstrated the particles were able to transport functional enzymes into a variety of cell lines. Significantly, the transported proteins were able to escape from endosomes. Moreover, these particles showed no apparent cytotoxicity.

Mesoporous hybrid materials containing nanoscopic "binding pockets" for colorimetric anion signaling in water by using displacement assays

Mesoporous solids functionalized with anion-binding groups have proved to be suitable anion hosts and have been used in selective colorimetric displacement assays. The material UVM-7, a mesoporous MCM41-type support characterized by the presence of nanometric mesoporous particle conglomerates, was selected as inorganic scaffolding. Reaction of the template-free UVM-7 solid with 3-aminopropyltriethoxysilane (1) yielded solid S1, from which the derivatives S2 and S3 were obtained by reaction with 2-methylthio-2-imidazoline hydroiodide (2) and butyl isocyanate (3), respectively. Solids S4 and S5 were prepared by reaction of the starting mesoporous UVM-7 scaffolding with N-methyl-N'-propyltrimethoxysilyl imidazolium chloride (4) and with 3-(trimethoxysilyl)propyl-N,N,N-trimethylammonium chloride (5), respectively. The solids synthesized contain mesoporous binding pockets that can interact with anions through electrostatic attractive forces (S1, S2, S4, S5) and hydrogen-bonding interactions (S1, S2, S3, S4). These functionalized solids were loaded with a dye (d) capable of interacting coordinatively with the anchored binding sites, in our case 5-carboxyfluorescein, to yield the hybrid materials S1d, S2d, S3d, S4d and S5d. These dye-containing solids are the signaling reporters. Their sensing ability towards a family of carboxylates, namely acetate, citrate, lactate, succinate, oxalate, tartrate, malate, mandelate, glutamate and certain nucleotides, has been studied in pure water at pH 7.5 (Hepes, 0.01 mol dm(-3)). In the sensing protocol, a particular analyte may be bonded preferentially by the nanoscopic functionalized pocket, leading to delivery of the dye to the solution and resulting in colorimetric detection of the guest. The response to a given anion depends on the characteristics of the binding pockets and the specific interaction of the anion with the binding groups in the mesopores. We believe that the possibility of using a wide variety of mesoporous supports that can easily be functionalized with anion-binding sites, combined with suitable dyes as indicators, make this approach significant for opening new perspectives in the design of chromogenic assays for anion detection in pure water.

Application of a library of artificial receptors formed by the self-organization of N-lipidated peptides immobilized on cellulose in studying the effects of the incorporation of a fluorine atom

A library of artificial receptors formed by the self-organization of N-lipidated peptides attached to cellulose via m-aminophenylamino-1,3,5-triazine was used for docking pairs of small colorless N-phenylpiperazines with and without a fluorine atom in the phenyl ring. The interactions of guests with the receptors were visualized by using competitive adsorption-desorption of an appropriate reporter dye. Several library members demonstrated attributes characteristic of the detection of alterations in the guest structure caused by the substitution of one hydrogen atom with fluorine. Analysis of the binding pattern of N-phenylpiperazine derivatives showed two characteristic bonding patterns: one with stronger binding of fluorinated analogues and weaker binding of native phenyl substituted analogues by the most of the receptors studied and another one with stronger binding of native hydrogen substituted compounds and respectively weaker binding of fluorinated analogues of guest molecules by receptors with tryptophan inside the binding pocket.

Chiral guanidine catalyzed enantioselective reactions

Chiral guanidine catalysts share common characteristics such as high pK(a) values and dual hydrogen-bonding modes of activation, and high catalytic activities and enantioselectivities can often be achieved. The utilization of guanidines as catalysts has been growing at a steady pace. In the past few years, it has attracted tremendous attention through several landmark achievements. This article highlights the development of chiral guanidine catalysis in asymmetric synthesis.

Binding mechanisms in supramolecular complexes

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Synthesis of rigidified flavin-guanidinium ion conjugates and investigation of their photocatalytic properties

Flavin chromophores can mediate redox reactions upon irradiation by blue light. In an attempt to increase their catalytic efficacy, flavin derivatives bearing a guanidinium ion as oxoanion binding site were prepared. Chromophore and substrate binding site are linked by a rigid Kemp's acid structure. The molecular structure of the new flavins was confirmed by an X-ray structure analysis and their photocatalytic activity was investigated in benzyl ester cleavage, nitroarene reduction and a Diels-Alder reaction. The modified flavins photocatalyze the reactions, but the introduced substrate binding site does not enhance their performance.

Di-platinum complexes containing thiolato-urea ligands: structural and anion binding studies

The new di-platinum(II) thiolato-bridged complexes [(dppp)Pt{micro-S(CH(2))(n)NHC([double bond, length as m-dash]O)NHR}](2)(OTf)(2) (n = 2, 3; R = Et, Ph) have been synthesized. The X-ray crystal structures of three of these complexes have confirmed their formulation and highlighted the ability of the urea groups on the thiolato bridges to display hydrogen bonding interactions with anions. The binding of anions by these receptors has been quantitatively determined by (1)H NMR spectroscopy and in a qualitative fashion by indicator displacement assays. These studies have shown the receptors to be selective for inorganic phosphate. In addition we have isolated and structurally characterized an unusual 1D coordination polymer with formula {[(dppp)Pt{micro-S(CH(2))(2)NHC([double bond, length as m-dash]O)NHR}][Pb(2)Cl(6)]}(n). The X-ray crystal structure of this inorganic polymer has shown that it consists of units of the di-platinum-thiolato cations bridged by [Pb(2)Cl(6)](2-) moieties.

Phytochemistry of cimicifugic acids and associated bases in Cimicifuga racemosa root extracts

INTRODUCTION

Earlier studies reported serotonergic activity for cimicifugic acids (CA) isolated from Cimicifuga racemosa. The discovery of strongly basic alkaloids, cimipronidines, from the active extract partition and evaluation of previously employed work-up procedures has led to the hypothesis of strong acid/base association in the extract.

OBJECTIVE

Re-isolation of the CAs was desired to permit further detailed studies. Based on the acid/base association hypothesis, a new separation scheme of the active partition was required, which separates acids from associated bases.

METHODOLOGY

A new 5-HT(7) bioassay guided work-up procedure was developed that concentrates activity into one partition. The latter was subjected to a new two-step centrifugal partitioning chromatography (CPC) method, which applies pH zone refinement gradient (pHZR CPC) to dissociate the acid/base complexes. The resulting CA fraction was subjected to a second CPC step. Fractions and compounds were monitored by (1)H NMR using a structure-based spin-pattern analysis facilitating dereplication of the known acids. Bioassay results were obtained for the pHZR CPC fractions and for purified CAs.

RESULTS

A new CA was characterised. While none of the pure CAs was active, the serotonergic activity was concentrated in a single pHZR CPC fraction, which was subsequently shown to contain low levels of the potent 5-HT(7) ligand, N(omega)-methylserotonin.

CONCLUSION

This study shows that CAs are not responsible for serotonergic activity in black cohosh. New phytochemical methodology (pHZR CPC) and a sensitive dereplication method (LC-MS) led to the identification of N(omega)-methylserotonin as serotonergic active principle.

Chiral recognition in bicyclic guanidines

A theoretical study of chiral recognition in bicyclic guanidines has been carried out by means of B3LYP/6-31+G(d,p) DFT calculations. A series of complexes between protonated 4,8-dimethyl-1,5,7-triazabicyclodecene (DTBD) and 2,5-disubtituted chiral cyclopentanones have been evaluated for chiral recognition, both in the gas phase and in benzene solution as per the polarizable continuum model (PCM) and analyzed by AIM and NBO methodologies. An inversion in the sense of chiral recognition has been observed between gas phase and solvated results for cyclopentanone complexes. Among the different correlations found (i.e. between electron density, hydrogen bond distance, second-order perturbation energy), a linear correlation has been established between the chiral recognition energy and different molecular parameters.