Specific guests

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

A strategy has been developed for the design, synthesis, and testing of new dathrate hosts that possess relationship complementarity to specific guest-compounds. 

Carbohydrates not only act as ligands, but they can also provide scaffolds for molecular recognition processes. It is well known that cyclodextrins (CDs) are able to form an inclusion complex with specific guest molecules. In the last years, NMR experiments combined with other techniques have been used to highlight different recognition events. 

Fig. 2. Schematic representation of the use of unnatural side chain interactions to alter the conformational equilibrium towards a more ordered structure. The binding of a specific guest displaces the equilibrium towards a conformation that orients the binding side chains complementary to the geometry of the guest...

Fig. 2 Free enthalpy diagram for the complexation of the specific guest A to host W in relation to the competitive substrates B and 2B.

There is vast amount of literature involving studies of rare earth complexes with specific guests under the umbrella of supramolecular chemistry. The ligands involved in this area of rare earth complexes are not usually available commercially. In fact the ligands are designed and synthesized and later the rare earth complexes of the synthesized ligands are prepared. An example is cited here to illustrate the scheme involved in the preparation of lanthanide supramolecular complexes. 

As mentioned above, some naturally occurring cyclic hosts that possess molecular recognition capabilities were known before crown ethers (the first artificial host molecules) were discovered. For example, the cyclic oligopeptide valinomycin and the cyclic oligosaccharide cyclodextrin were found to bind to specific guest molecules. The chemical modification of cyclodextrin was particularly well-researched, and artificially modified cyclodextrins became one of the most important compoimds used in host-guest chemistry. 

The ability of a host molecule to bind specific guests is called molecular recognition. 

Defining the maximum difference in chemical shifts for a specific guest proton in the uncomplexed state and the host-guest complex as A<5max, and the difference between the uncomplexed shift of the same guest proton, as the host is added, as AS the following is found ... 

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