Host-guest type interactions

Research continues into other agents, apart from surfactants (which are discussed in Chapter 6), which can enhance the solubility of dmgs. The calixarenes are another type of host, existing in a cup-shape in a rigid conformation. The 4-sulfonic calix[n]arenes can form host-guest type interactions with drugs such as nifedipine, a poorly water soluble agent, seen in Fig. 5.9. 

Fig. 2. Classification/nomenclature of host—guest type inclusion compounds, definitions and relations (/) coordinative interaction, (2) lattice barrier interaction, (J) monomolecular shielding interaction (I) coordination-type inclusion compound (inclusion complex), (II) lattice-type inclusion compound (multimolecular/extramolecular inclusion compound, clathrate), (III) cavitate-type inclusion compound (monomolecular/intramolecular inclusion...

In view of the earlier work of Lawton and Powell on unit cell dimensions and contents of channel-type clathrates, the guest molecules are expected to be severely disordered within the cylindrical cavities. First, they may be packed head-to-head or head-to-tail second, the length of the chain-like molecules is unlikely to match the unit translation imposed by the ordered host lattice. As a consequence, host-guest functional interactions will differ within successive unit cells and a dramatic decrea of chiral discrimination is bound to ensue for lack of unique stereospedfic complementarity. 

Type I(chiral), Type II(RRSS racemic), or Type in(RS racemic) is mainly determined by the host-guest pairwise interaction. The interaction energy in a single host-guest pair is probably very small. However, it can be amplified or accumulated, in the crystal phase, to a magnitude large enough to alter the intercolumn interaction. 

Fig. 1. Schematic representation of a receptor—substrate (host—guest) complex involving cavity inclusion of the substrate and the formation of different types of weak supramolecular interactions between receptor (hatched) and substrate (dotted).

Fig. 6. Diagrammatic (two-dimensional) representation of different modes of lattice inclusions involving coordinative (H-bond) interactions (indicated by broken lines) (a) cross-linked matrix type of inclusion (host-host interaction, true clathrate) (b) coordinatoclathrate type of inclusion (coordinative host-guest interaction, coordination-assisted clathrate)...

Noncovalent interactions play a special role in synthetic procedures used to assemble various types of supramolecular species. These syntheses rely on the stabilization provided by non-covalent interactions between recognition sites incorporated within precursors. Various types of non-covalent interactions can be used as a recognition motif utilized to guide the synthesis.Targeted synthesis of macro- and supramolecular structures of various sizes, shapes, and functionality has now become possible. Supramolecular chemistry offers incredible applications in various fields such as medicinal chemistry (drug delivery systems),host-guest chemistry,catalysis,and molecular electronics. ... 

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