Interestingly, intramolecular photocrosslinking of the helical conformer was demonstrated by Hecht and co-workers as an approach to organic nanotubes . More recently, photoirradiation was employed by the same group to switch the helical conformation of mPE foldamers with a central azobenzene unit . This concept has potential applications in smart molecular delivery vehicles.
Upon folding, 23n forms an internal cavity with a diameter estimated to be 8.7 A according to molecular modeling. (See Chapter 7 for more detailed discussions on the usage of foldamers for molecular recognition.) In a polar solvent mixture (i.e. 40% water in acetonitrile), the cavity is filled with small solvent molecules that are eagerly waiting to be displaced. Thus, 23i2 could bind hydrophobic molecules such as monoterpenes with binding energies (—AG) in the range of 45 kcal/mol . If the chain length is increased, the cavity takes a tubular shape and should prefer a rod-like guest such as 25 . Since burial of poorly solvated surfaces is the driving force for solvophobically driven molecular association, the natural expectation is that tightest binding would occur when the binding cavity of the host molecule and the guest match in terms of size and shape, so that minimum solvophobic surface is exposed. This selectivity in binding derives from geometrical matching of the host and the guest, and is a manifestation of geometrical manipulation mentioned earlier.
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