Host-guest complexation process

Abstract. The inclusion behavior of the octopus cyclophane constructed with a rigid macrocyclic skeleton and eight hydrocarbon chains was studied in aqueous media by means of fluorescence and electronic absorption spectroscopy. Both hydrophobic and electrostatic interactions came into effect in the host-guest complexation process. The cyclophane acted as an effective apoenzyme model for constitution of an artificial vitamin B -dependent holoenzyme by simultaneous incorporation of pyridoxal-5 -phos-phate and a hydrophobic alkylammonium substrate into the host cavity to give the Schiff-base species, showing the substrate selectivity. 

A ratio of binding constants of corresponding complexations is usually treated as a measure of selectivity. Consequently, binding constants have been used as fundamental criteria for evaluation of host-guest complexation processes. In addition to the ratio of binding constants, the tanperature dependence of selectivity also provides important insights into the origins of supramolecular functions. 

The addition of the second methyl group on the phenol ring led to the observation of the consecutive inclusion process with a decrease in the dynamics for complex formation (Table 8, cf. 29 with 28 (R = CH3)). This result supports the previous suggestion190 that small guests can slip into the CD cavity and in one process form the stable host-guest complex. 

Actually, coordination complexes of different metal salts of DBTA with hydroxycarboxylic acid esters, hydroxycarboxylic acids and alcohols as well as host-guest complexes of DBTA with chiral phosphine oxides and racemic alcohols can be prepared and used for separation of optical isomers. In the next subchapters theoretical and practical aspects of these recent resolution processes are summarised. 

Efficiency of the above discussed resolutions changed in a wide range 0 < S < 0.433 (Table 5). Menthol (28) and 2-halogenocyclohexanol (35, 36, 37) enantiomers were the best ligands of DBTA for chiral recognition during host-guest complex formation, therefore these model compounds were used for further elaboration of the resolution processes (point 3.4.). 

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