Discrimination cavity size

The M+ ions have only weak complexing tendencies, but these can be enhanced by suitably sized macrocyclic ligands (see Topic E3). Ligands with different cavity sizes can be used to discriminate between alkali ions. 

Because of limitation of CD cavity size, the formation of more than the quarternary inclusion complex may be difficult. Of course, if hydrogen-binding components by primary or secondary hydroxyl of CD rims are counted the inclusion complex of more than quarternary may be possible. However, the nomenclature herein especially counts the components included within the cavity with CD. In fact, it is very difficult to discriminate the third or fourth components. They may vary with the goals investigated. For example, / -CD/a-naphthanol/l,2-bromoethane or )8-CD// -naphthanol/l,2-bromoethane system displays strong phosphorescence, and the various alcohol molecules affect obviously the lifetime observed, resulting in the recognition of alcohol molecules, as shown in Table 5 [60]. The alcohol molecules here are named as the fourth component. 

A precise calculation of AGd as a function of the cationic radius would be very difficult because it would involve a complete conformational analysis of a large and complicated ligand system (82). Nevertheless, the dependency of the cation selectivity on steric interactions is capable of illustration. The term AGd can be estimated very crudely by using Hooke s law. As is shown in Fig. 16, ligands that are differentiated only by the radius of their equilibrium cavities can easily discriminate between cations of different size. This may explain why valinomycin and antamanide, two antibiotics with similar coordination spheres (54, 66), do not prefer the same cation (82). As it is no easy task to predict the exact dimensions of the cavity for a proposed ligand, the tailored synthesis of such ligands is conceivable yet problematic. 

Therefore, such results clearly demonstrate that the cavity formation process is the mechanism that controls the retention of the isomers of dendrimer 5 (G = 1), and their distinct sizes and shapes are the molecular characteristics responsible for their discrimination. 

The hydrophobic cavity provided by the octopus cyclophane is significantly flexible, and capable of performing molecular recognition toward hydrophobic guest molecules of various bulkiness through the induced-fit mechanism, not only in aqueous solution but in the bilayer membrane formed with a synthetic peptide lipid. However, such a flexible hydrophobic cavity seems to be unfavorable for size-sensitive molecular discrimination. 

The enantiomer selectivity and polymerization activity is greatly dependent on the stmcture of initiator complexes [232,240] and monomer conformation [236,241]. The complexes with 163,164, and 165 were nearly inactive in MBMA polymerization, whereas the complexes with 166 and 167 can polymerize MBMA with much lower enantiomer selectivity compared with the Sp-complex [4]. Sp and 163-167 have different shapes and sizes of the cavity of the ligand, which play an important role in chiral discrimination and influence the activity of the complex. 

---