Supramolecular recognition sites

Receptors prepared by organic synthesis which are used as sensor coating with defined molecular recognition properties, may increase the sensitivity of a sensor in the gaseous and in the liquid phases. Such sensors can be considered as quick and simple assay systems to detect supramolecular interactions between the synthesized receptors and analytes at the solid/liquid or solid/gaseous interfaces (Fig. 10.1) [26], 

Those classes of substances which form cavities have been found to be the most specific hosts. These cavities can be entered only by guests with appropriate morphology, and 

Calix[n]arenes are cyclic condensation products of para-substituted phenol derivatives and formaldehyde [29], They are highly interesting for the development of sensitive coatings due to their conformational flexibility and the ease by which they may be modified chemically. Chemical modification can be done either in the meta position, or by reactions at the hydroxy group. In this way, bulky substituents [30], chelating substituents [31], aromatic residues [32], crown ethers [33,34], peptides [35,36], etc. can be introduced. A first approach to combinatorial synthesis of calix[4]arene receptors has been published by Reinhoudt and co-workers [37,38], who prepared calixarenes with different substituents. In solution, these calixarenes lead to formation of hetero-oligomers with barbiturates, and these hetero-oligomers were detected by MALDI-TOF mass spectrometry and H-NMR spectroscopy. 

Cyclophanes are cyclic macromolecules with aromatic units connected via short aliphatic chains. The shape and the properties of their cavities can be varied by modification 

Pedersen [49] discovered the crown ethers and their complexation behavior. The first crown ethers were by-products and have been identified as a new class of substances because of their extraordinary complexation behavior [50]. The synthesis typically starts from brenzcatechin, and one or two aromatic rings and five to ten ether units are included in the macrocyclus. 

---

The binding of guest species to supramolecular recognition sites incorporated into elastic polymers can lead to dimensional changes in macroscopic dimensions that can be used for different applications such as in actuators, sensors, process control, and, in particular, drug release systems. Schneider and coworkers have carried out extensive work in this field. A few years ago, they synthesized... 

D. G. and Pikramenou Z. (1997) Chemosensing of Monocyclic and Bicyclic Aromatic Hydrocarbons by Supramolecular Active Sites, in Desvergne J.-P. and Czarnik A. W. (Eds), Chemosensors of Ion and Molecule Recognition, NATO ASI series, Kluwer Academic Publishers, Dordrecht, pp. 159-76. 

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. ... 

The design of supramolecular catalysts may make use of biological materials and processes for tailoring appropriate recognition sites and achieving high rates and selectivities of reactions. Modified enzymes obtained by chemical mutation [5.70] or by protein engineering [5.71] represent biochemical approaches to artificial catalysts. 

Supermolecules built from small molecules mainly involve intermolecular interactions. On the other hand, with macromolecules the supramolecular association may be either intermolecular, occurring between the large molecules, or intramolecular involving recognition sites located either in the main chain or in side-chain appendages, thus leading to chain folding and structuration of the macromolecular... 

Rotaxanes and catenanes are supramolecular (multicomponent) species [1, 15] strictly related to, but also very different from, pseudorotaxanes (Fig. 2). Whereas pseudorotaxanes can undergo dissociation into their wirelike and macro-cyclic components, rotaxanes and catenanes are interlocked species, whose dissociation requires breaking of a covalent bond. The general strategy to prepare rotaxanes and catenanes with high yields is based on the template effect [44], which relies on the presence of molecular recognition sites in the components to be assembled. 

---