Guest zwitterionic

These heteroditopic receptors display high affinities for eiflier neutral guests, zwitterions, anions, contact ion-pairs and metal ions. However, despites this versatility, remarkable selectivity is obtained within each class of guests ... 

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). 

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]. 

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... 

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-... 

That crosslinking has indeed occurred is confirmed by the very existence of aggregates at 25 °C, as in its absence the diblock copolymers are completely soluble at this temperature. Tunability of the solubility of the PMEMA block in water arises from the fact that its lower critical solution temperature (LCST) lies between 25 and 60 °C. This reversible hydration of the core could be a very useful feature to trigger release of occluded guest molecules from the core interior. More recently, utilizing a similar methodology, zwitterionic shell-crosslinked systems have also been prepared wherein the core and shell domains contain amine and carboxylic acid groups, respectively, or vice versa. Such systems exhibit an isoelectric point, at a pH wherein the crosslinked micelles ( 40 nm) become electrically neutral and precipitate out in water addition of acid or base causes complete redissolution of these nanospheres [58]. 

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]. 

Selectivity Thermodynamic and Kinetic, p. 1225 Self-Assembly Definition and Kinetic and Thermodynamic Considemtions, p. 1248 Solvent Effects in Guest Binding, p. 1322 Spherands, p. 1344 Zwitterion Receptors, p. 1639... 

The first attempts at receptors for carboxylate concentrated on protonated polyazamacrocycles. These compounds bind their guests via a combination of electrostatic interactions and hydrogen bonds. An early example of a ditopic receptor composed of a macrocyclic polyamine and crown ether subunit was reported by Kimura. " The triprotonated form of 13 forms 1 1 complexes with zwitterionic compounds, such as glycine. 

Several points may be garnered from these results (1) the Ha and Ha side of L-Trp, which can be viewed as the head of this guest molecule, deeply penetrated the cavity of 1 and experienced the largest 7t-7t inhuence (2) the hydrophilic zwitterion end of L-Trp, which can be viewed as the tail of this molecule, was left outside the cavity in contact with H2O and (3) the cavity of 1 appears to be very shallow. These three points can be easily rationalized, since the head of has the... 

Figure 8 (a) Complexation of R- and -tryptophan zwitterion, 20, to form diastereomeric host-guest complexes 21 characterized by... 

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