2.5 Multiple Recognition and its Applications

2.5 Multiple Recognition and its Applications

Once recognition units for specific groups have been identified, one may consider combining several of them within the same structure. This leads to polytopic (manysite) coreceptor molecules containing several discrete binding subunits that may cooperate for the simultaneous binding of several substrates or of a multiple-bound substrate. Both homotopic and heterotopic receptors are possible, depending on whether the binding subunits are identical or not. Aside from the standard effects of binding on individual discrete substrates, the close proximity between bound substrates often allows for increased interactions. In addition, coreceptors provide a link to higher forms of molecular behavior, which includes cooperativity and allostery. Allostery refers to the binding at a site remote from the catalytic site. As a result of the binding, the structure of the catalytic site may change just enough so that it becomes markedly more active (or inactive). Other areas falling under this are regulation, as well as communication or signal transfer if a molecular/atomic species is released or taken up.

One may distinguish cosystems, for which the binding of several substrates is commutative, and cascade systems, for which the substrate binding steps must follow a given sequence to be effective. An example would be when a polyammonium macrocycle (an example of a homotopic substrate) first binds Cu ions, which in turn serve as interaction sites for attracting imidazole and pyrimidine into the complex (figure 2.8) (Coughlin et al. 1979).

Figure 2.8: Cascade-type dinuclear copper (II) cryptate formed with a macrocyclic polyamine as the ligand. The copper ions bind first, followed by the imidazole groups. (With permission from Supramolecular Chemistry, J.-M. Lehn. 1995 VCH Verlagsgesellschaft.)