Dimer cavity, inclusion complexes

Molecular Complexes. These species are formed by noncovalent interactions between the substrate and ligand. Among the kinds of complexspecies included in this class are small molecule-small molecule complexes, small molecule-macromolecule species, ion-pairs, dimers and other self-associated species, and inclusion complexes in which one ofthe molecules, the host, forms or possesses a cavity into which it can admit a guest molecule. 

As mentioned above, 3-p- BocCiNH-a-CD forms helical supramolecular polymers in aqueous solutions. In contrast, 6-p- BocCiNH-) -CD was synthesized and found to form an intramolecular complex using NMR measurements. When adamanta-necarboxylic acid (1-AdCA) was added as a competitive guest to an aqueous solution of 6-p- BocCiNH- -CD, NMR spectra showed that the Boc-cinnamamide part in its own fi-CD cavity was expelled into water. The 6-p- BocCiNH- -CD underwent a conformational change to form an inclusion complex with AdCx. Moreover, with an addition of an excess amount of a-CD, the cinnamoyl part was found to be included in a-CD to form a hetero dimer (Fig. 3.27). 

Photoswitchable supramolecular dendrimer-like structures were constructed starting from azobenzene-based and CD-based building blocks. For example a bis-azobenzene with a dipyridyl linker trans-Azo dimer) and a p-CD trimer formed an hyperbranched structure whose shape could be controlled by light (Fig. 11). AFM evidenced branched structures of several microns turning to disordered particles upon UV irradiation. Moreover a supramolecular dendrimer was recently built up in water starting from an hydrophilic hyperbranched polyglycerol with a-CD apex (CD-g-HPG) and an hydrophobic hyperbranched poly(3-ethyl-3-oxetane-methanol) with azobenzene apex (AZO-g-HPBO) (Fig. 12). The two components in 1 1 molar ratio selfassembled into a Janus-like dendrimer (JHBP, HPBO-6-HPG) of diameter ca. 5.1 nm, due to inclusion complexation of the azobenzene apex into the CD cavity of the partner... 

Abstract. A -ethyl-A -hexadecyl-4,4 -bipyridinium bromide (Cj6VBr2) and A -ethyl-A -octadecyl-4,4 -bipyridinium bromide (CigVBr2) were used as electroactive probes to assess the interactions between surfactants and cyclodextrins. Cyclic voltammetry, visible spectroscopy, fluorescence spectroscopy and surface tension techniques were used to detect the formation of complexes between the surfactant viologen probes and a- and -cyclodextrins. The voltammetric results suggest the formation of inclusion compounds in which the hydrophobic tail of the surfactant viologens penetrate the cyclodextrin cavity. The dimerization of the viologen cation radicals is essentially suppressed by the presence of a-cyclodex-trin (ACD) while no effects are observed in the presence of )5-cyclodextrin (BCD). The observed results are best explained by the relative solubility in aqueous media of each of the inclusion complexes in the several accessible viologen oxidation states. 

CB[n] nanoreactors have also been used to template the dimerization of peptides. Urbach et al. showed that CB[8] selectively recognizes N-temunal aromatic peptides, and that through the formation of 1 2 host-guest inclusion complexes, it templates the dimerization of these peptides [138]. This templating was shown via X-ray crystallography to occur via inclusion of the hydrophobic aromatic side chains inside the CB[8] cavity, while the proximal N-terminal ammonium groups are chelated by the portal carbonyls. 

Chlorhexidine forms an inclusion complex with para-sulphonatocalix[4]arene. I this complex chlorhexidine adopts V-shaped conformation (Fig. 38.8a). Two other derivatives of cahx[4]arene, i.e. the dimethoxycarboxylic acid and dihydroxyphosphonic acid which are derivatized on lower rim the calixarenes form inclusion dimers and their internal cavities are protected against inclusion of guest molecule (Fig. 38.8b, c). The co-crystals are formed instead and the chlorhexidine molecule adopts S and Z-shaped conformations, respectively. This may suggest that the conformation of guest molecule may be controlled by appropriate selection of host molecule. 

Fig. 38.17 (a) Dimer formation by a mutual inclusion of one methyl group of one para-methylcalix[5]arene in the cavity of another one [37] (b) self-inclusion of propenyloxy group in /)o ra-t-butyl-penta-propenyloxycalix[5]arene [38] (c) inclusion complex of carboxylcalix[5]arene with 1,12-dodecanediyldiammonium [39]... 

Ohashi and coworkers [12] have reported that MMP2 [20] calculations on the interactions of cyanine dyes with P- and y-cyclodextrin correctly reproduced the relative stability of the inclusion complexes, as well as predicting that in most cases a dye dimer would be preferentially bound within the cavity of the cyclodextrin. Electrostatic interactions between the dye molecules and the cyclodextrin played an important role, in addition to the VDW interactions, in stabilizing the complex. Menger and Sherrod [6,21], enroute to an exploration of the interactions of ferrocenylacrylate esters with P-cyclodextrin, have reported the calculated host-guest complexes between ferrocene and a-, p-, and y- cyclodextrin which are consistent with X-ray structures and spectroscopic data Ferrocene was found to bind in an equatorial manner with y-cyclodextrin, in an axial manner with P-cyclodextrin, and the predicted structure for the 2 1 complex between a-cyclodextrin and ferrocene was found to be precisely correct when the X-ray structure of the complex was published over one year later by Harada [22]. 

The [2+2]-photodimerizations of coumarin and its derivatives proceed selectively in soHd inclusion complexes with CDs. The yield and distribution of the photodimers are very different from those in the neat coumarin soHds. For the 4,7-dimethyl (70a) and the 4,6-dimethyl (70b) derivatives, for example, photodimers are obtained by irradiation of the CD complexes as shown in Table 73.1, whereas no dimer is formed from neat soHd coumarins. Depending on the substitution pattern of the coumarin molecules and the type of CD employed, complexes whose host/guest ratios are 1 1,1 2, and 2 2 are identified. The stereochemistry of the dimers depends on the cavity size of the CD and the relative orientations of coumarins within a complex. The photodimerization of acenaphthylene 41 is accelerated by the formation of 1 2 inclusion compound between y-CD and 41 to give cis (71a) and trans (71b) dimers (cis trans = 99 1). In contrast, the dimerization is inhibited by P-CD due to the formation of 1 1 inclusion complex. 

In order to determine the mechanism of complex-formation, however, kinetic methods must be used. Consider one host-two guests complex-ation. The two possible mechanisms are dimerization of the guest outside the cyclodextrin cavity followed by inclusion, and dimerization within the cyclodextrin cavity. Equilibrium measurements alone cannot distinguish between these two possibilities. The same is the case for 2 2 complex-formation, where a larger number of possible mechanisms exist. 

The rates of hydrolysis of the trifhioroacetates (201 X = H, Me) increase in a nonlinear fashion in the presence of jS-CD. Some differences in rate between the two substrates have been explained as being due to different modes of inclusion.173 The novel CDs (202) and (203) have been synthesized in 45% and 66% yields, respectively, and their complexation with various l/d amino acids have been examined. Importantly, (202) and (203) can be detected by fluorescence spectroscopy and they can recognize the size and shape but also the chirality of the amino acids.174 A /j-CD dimer with a linking bipyridyl group (204) has been synthesized and shown to bind both ends of potential substrates into two different cavities of the CD holding the substrate ester carbonyl group directly above a Cu(H) ion bound to die bipyridyl unit. This achieves... 

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