Receptors ditopic

Hexaammonium macrocycles [32]aneN6 and [38]aneN6 were designed as selective ditopic receptor molecules for dicarboxylates -02C—R—C02- such as succinate, glutarate, or adipate 51). Highest stability of the complex corresponds to the best fit between the substrate R length and the site separation of the receptor II. 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Interesting results were reported with dithiocarbamates derived from diamines. Thus, a series of dinuclear zinc(II), cobalt(II), and nickel(II) dithiocarbamate ditopic macrocyclic receptors containing various spacer groups of different sizes has been reported.412,413,414 Spectroscopic investigations and... 

In particular, for a ditopic receptor that can bind successively two cations (see previous section), the criterion for cooperativity is Kn/Ku > 1/4, i.e. complexation of a second cation is made easier by the presence of a bound cation. For instance, a cooperative effect was observed with fluoroionophore E-l (see Section 10.3.4). 

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

Figure 2.4. Ditopic receptors for the recognition of a,co-alkane diammonium ions.

F. Fages, J.-P. Desvergne, H. Bouas-Laurent, J.-M. Lehn, J. P. Konopelski, P. Marsau, and Y. Barrans, Synthesis and fluorescence emission properties of a bis-anthracenyl macrotricyclic ditopic receptor. Crystal structure ofits dinuclear rubidium cryptate, J. Chem. Soc., Chem. Commun. 655(1990). 

The property of the [Fe (Por)Cl] complex to act as a ditopic receptor towards diatomic anions is very attractive for studying its reaction... 

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

Subsequent to the preparation of 8, a number of other sapphyrin-nucleobase conjugated carriers were synthesized in the authors laboratories. These include compound 9 that has two cytosine moieties appended to the sapphyrin macrocycle, as well as two guanine-bearing sapphyrins, 10 and 11. Here, as expected, the ditopic sapphyrin-guanine receptor 10 served to enhance the transport of cytosine-5 -monophosphate (but not GMP or AMP). By contrast, the two tritopic receptors, namely 9 and 11, were designed with the intention of allowing for the formation of the complexation-derived triple-helix-like C-G-C and G-C-G motifs in addition to phosphate chelation by sapphyrin. In other words, it was expected that a centrally... 

Development of ditopic receptors 58 for both an anion and a cation is a logical extension [19b]. It should be stressed that the recognition by cyclodextrins, calixarenes, hemicarcerands, and many other systems does not involve specific recognition sites, whereas tetrahedral recognition by 53 and 54, the adenine 56 selective binding, and multiple recognition like the one shown in formula 59 [20] require the presence of specific sites in receptor capable of directional interactions. 

The respective association constants of the latter anion to free 376 is only 10 M 1 while for the self-assembled complex it is equal to 2.5 104 M 1 [39]. Kubik and Goddard obtained a cyclic pseudopeptide ditopic receptor 378 for the simultaneous complexation of cations and anions [40]. Simultaneous encapsulation of N03 and PF6 ions by a highly charged (+12) anion receptor 379 was achieved by Schnebeck and coworkers [41]. By coordinating of 2,4,6-tris[(4-pyridyl)methylsulfa nyl]-l,3,5-triazine 380 with Ag, Hong and coworkers obtained nanosized tubes (Fig.7.8.1) that could host solvent and anionic molecules [42]. 

Tetrahedral tetraazonium compounds 5a and 5b, developed by Schmidt-chen, bind carboxylates, such as formiate, acetate, and benzoate, among a variety of other anions [8]. Although these receptors do not display hydrogen bond interactions, they are endowed with well localized polycationic centers, which are independent on the acidity of the medium. A ditopic receptor build up of both a 5a and a 5b subunits bridged by a p-xylylene spacer showed selectivity towards the largest members of a series of dianionic probes [9],... 

To come to a water-soluble, ditopic receptor model, Lehn etal. attached JV6-[24] crown-8 with an acridine-derived DNA-intercalator, so combining the anion binding strength of the first (in protonated state) [67] with the n-stacking capacity of the latter. Indeed, increased binding of ATP by 46 was observed as compared with the unsubstituted azacrown [68]. Additionally, the multifunctional host 46 catalyzes the hydrolysis of ATP [69] with a higher selectivity over ADP as compared with the parent crown but with a somewhat reduced eflectiv-ity. The reaction intermediate was shown to be the Af-phosphorylated crown. 

A fast and reversible copper translocation driven by the CuII/CuI couple was carried out within the flexible ditopic receptor 10.10 The translocation process, illustrated in Fig. 2.8, is fast and reversible and can be followed both visually and spectrophoto-metrically. In particular, a MeCN solution equimolar in both 10 and Cu11 is blue-violet (metal-centered absorption band = 548 nm,e = 120 M-1 cm-1), which indicates... 

Ditopic (or polytopic) receptors are those which possess two (or more) binding subunits. 

Thus, for both the terminal diammonium and dicarboxylate substrates, selective binding by the appropriate receptors describes a linear recognition process based on length complementarity in a ditopic binding mode. Important biological species, such as polyamines, amino acid and peptide diamines, and dicarboxylates [4.18] may also be bound selectively. Recognition is achieved by multiple coordination to metal ions in dinuclear bis-macrocyclic coreceptors that complex selectively complementary bis-imidazole substrates of compatible length [4.21]. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

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