Host zwitterion

Diazoalkanes and related compounds are not suitable guests for the types of hosts discussed above. Very weak complexation was found with diazodicyanoimidazole (2.53 Sheppard et al., 1979) in which the mesomeric zwitterionic structure with a formal diazonio group (see Secs. 2.6 and 6.2) is dominant. However, no complexation was found for another compound with a formal diazonio group, the ben-zothiazol-azidinium salt 2.50 (Szele and Zollinger, 1982). 

The finding that thiamine, and even simple thiazolium ring derivatives, can perform many reactions in the absence of the host apoenzyme has allowed detailed analyses of its chemistry [33, 34]. In 1958 Breslow first proposed a mechanism for thiamine catalysis to this day, this mechanism remains as the generally accepted model [35]. NMR deuterium exchange experiments were enlisted to show that the thiazolium C2-proton of thiamine was exchangeable, suggesting that a carbanion zwitterion could be formed at that center. This nucleophilic carbanion was proposed to interact with sites in the substrates. The thiazolium thus acts as an electron sink to stabilize a carbonyl carbanion generated by deprotonation of an aldehydic carbon or decarboxylation of an a-keto acid. The nucleophilic carbonyl equivalent could then react with other electro-... 

Some authors based their approach to selective binding of the more lipophilic a-amino acids in water on hydrophobic effects using water-soluble, cavity-containing cyclophanes for the inclusion of only the apolar tail under renouncement of any attractive interaction of the hosts with the zwitterionic head . Kaifer and coworkers made use of the strong affinity of Stoddart s cyclobis(paraquat-p-phenylene) tetracation 33 for electron-rich aromatic substrates to achieve exclusive binding of some aromatic a-amino acids (Trp, Tyr) in acidic aqueous solution [48]. Aoyama et al. reported on selectivities of the calix[4]pyrogallolarene 34 with respect to chain length and t-basicity of aliphatic and aromatic amino acids, respectively [49]. Cyclodextrins are likewise water-soluble and provide a lipophilic interior. Tabushi modified )S-cyclodextrin with a 1-pyrrolidinyl and a carboxyphenyl substituent to counterbalance the... 

With a non-selfcomplementary, zwitterionic host for phosphorylcholine derivatives as the design goal, a bicycloguanidinium building block has been... 

In supramolecular chemistry, molecular recognition has evolved over the last 35 years and now much effort is directed towards the complexation of anionic [28], zwitterionic [29], ion-pairs [30] and neutral guests for various purposes, including catalysis [31[. Host molecules can be constructed covalently, or they can themselves also be assembled in a supramolecular fashion. This strategy, called receptor site self-assembly, has been exploited in recent years. Especially, dynamic host formation in the presence of a substrate is highly interesting [32]. 

Suggest experimental techniques for assessing the affinity of a host for a particular anion in solution. Which of these techniques might be appropriate for (a) podand (4.40), (b) diboryl hosts such as hydride sponge (4.69) and (c) zwitterions such as (4.35). 

We saw in Section 4.6.1 the use of zwitterionic hosts for anions in which the counter-cation is non-speciflcally bound on the outside of the molecule. We now turn to zwitterionic guests where the guest has both cationic and anionic parts. We have already described in Section 5.1.1 the zwitterionic structure of amino acids and amino acid binding has been a major part of the interest in the construction of hosts for zwitterions. An excellent example is compound 4.30 (Section 4.5) which is... 

Using the native cyclodextrin, the enantiomers of amino acid derivatives were enantioselectively complexed [21]. Further, for a more detailed analysis, zwitterionic tryptophan was employed [22]. For the complexation studies performed on this molecule the a-cyclodextrin was used, as its inner cavity is the smallest. The H NMR measurements showed that (R)-tryptophan formed a stronger complex with a-cyclodextrin compared with the (S) enantiomer. This is due to the number of hydrogen bonds which can be formed between each enantiomer and the host molecule. The NMR studies showed another very interesting fact the amino acid is very likely forming no intracavity complex. It has been suggested that it is coordinated near the rim of the cyclodextrin. 

A similar complexation study supported by thermodynamic parameters was performed by the same author for mono-(6-anilino-6-deoxy)- 3-cyclodextrin and mono-[ 6-( 1 -pyridinio)-6-deoxy]-P-cyclodextrin complexation with several amino acids in zwitterionic form [24]. The inclusion complexation was enthalpy driven for the former and entropy driven for the latter host. 

Zwitterionic cyclodextrins were designed and synthesized by Tabushi a long time ago as artificial receptors for amino acids in water [25]. Only a very low enantioselectivity was detected for Trp. Inoue also studied the complexation of two new P-cyclodextrin derivatives bearing m-toluidinyl and [(9-fluorenyl)-amino]alkylamino groups with various D/L-amino acids by fluorescence spectroscopy in buffered (pH=7.2) aqueous solution. An enantioselectivity as high as 33 was found for D/L-leucine and the former host [26]. 

Calixarene-based host 37 was utilized in the chiral recognition of various zwitterionic amino acids [59]. In competitive extraction experiments, a 9 1 enantioselectivity was observed for L-phenylalanine over the D-isomer with the bis(S,S-guanidinium) 37. 

As discussed in Sect. 2.5, the ability of the host and guest to self-associate must be established prior to any complexation studies. Carboxylic acids 33, 34, 47, and 49 showed no tendency to aggregate in chloroform [59]. In contrast, the lH NMR of carboxylic acid 31 contained several broad resonances that were consistent with a zwitterionic structure and/or an aggregate [56]. For this reason, complexation studies have not been carried out with the highly twisted molecular tweezers 31 or 32. For the guests used in these studies, dimerization constants were known and indicated that self-association would be negligible under the concentrations used. 

Di- and polytopic host-guest systems have provided a convenient starting point for the construction of larger assemblies and many systems of this type are now known. For example, in an early study Kimura et synthesised the catecholamine complex 17. The crown ether unit of this ditopic host was known to be an effective receptor for primary alkyl ammonium salts, whereas the partially protonated form of the hexamine ring had been documented to bind anionic substrates (such as car-boxy lates) or electron-donor substrates (such as catechols). Accordingly, this host forms stable 1 1 complexes with zwitterionic guests such as amino acids, pep-... 

The immobilization of photoisomerizable host molecules onto an electrode surface can be used for the construction of novel ion-selective electrodes. A photoisomerizable calix[4]arene derivative was incorporated into a polymeric membrane on an electrode surface and the two different isomeric states of the host molecule provided responses selective to Li" or Na" ions depending on the state.Another photoisomerizable host molecule has been constructed from spiropyran and crown-ether subunits. Different binding affinities for Li were found depending on the isomeric state of the photoisomerizable component (26). The researchers suggested that this effect was caused by the coordination of the Li ion to the 0 of the zwitterionic merocyanine-form (Figure 7.29). The photocontrolled host molecules were immobilized in a polymeric film onto an electrode surface, resulting in a photochemically switchable ion-selective electrode. 

Compound 8, which was synthesized by Schmidtchen (49). This receptor provides a fixed-binding site for anions that operates through a combination of electrostatic and hydrophobic forces (50). The absence of donor protons prevents any opportunity for hydrogen bonding, but the crystal structure of the iodide complex still indicated that the anion-binding site was in the center of the cavity (51). Zwitterionic hosts such as 9, have also been reported (52, 53). These net neutral hosts prevent the need for the substrate to compete against a... 

Fig. 42. Temporal variation of the second harmonic intensity from guest-host polymer doped with zwitterionic chromophore film and submitted to the illumination periods in the chromophore absorption band (541 nm) between t = L on and t = L off and during the corona poling [173,174]...

Very recently, it has been reported that a guest-host system, consisting of twisted 7r-zwitterionic chromophores TM and TMC in polyvinylphenol (PVP) provided very large EO responses (r33 as high as 330 pm/V at 1.31 pm 10 wt%), 3-5 times greater than that ever-reported [59]. 

Spiropyran molecules have also been utilized as reverse wettable surfaces, whereby, a hydrophilic surface may be photomodulated to a hydrophobic surface and vice versa. Lygeraki et al. utilized the polar (mercocyanine, zwitterionic open-form) and nonpolar (spiropyran closed-form) properties of the spiropyran-merocyanine photo-chromic transformation to photomodulate surface wettability. 6-N02 BIPS (10 wt %) was added to a poly(methy 1 methacrylate) host matrix and patterned to produce reverse wettable surfaces. However, reversibility is limited to 6-8 irradiation cycles because of degradative photooxidation [22], Reverse wettable surfaces are commercially important as they may enhance drug delivery or function as a self-cleaning surface. 

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