Biomimetic receptor system

Synthetic chemists have applied the concepts of multivalency and cooperativity to supramolecular chemistry to create, for instance, biomimetic receptors able to recognize small molecules (29,30), polysaccharides (31), and DNA (32). Most of these systems are based on calixa[n]arenes (33-35), cucurbituril (CB) (36,37), and cyclodextrine (CD) (38-40) (see Fig. 1), and they represent ideal guests for assembling photoactive species. 

New and highly efficient synthetic routes have generated many porphyrinlike compounds with unique characteristics for their uses in several other applications like oxidative catalysis [ 19,20] and as biomimetic model systems of the primary processes of photosynthesis [21,22]. Presently, the interest includes also the supramolecular units, including molecular recognition in chemical receptors and sensors [23-25], use as light-harvesting devices [26-29], and as materials for advanced technologies, mainly in nanosciences [30, 31]. 

Lyotropic liquid-crystalline nanostructures are abundant in living systems. Accordingly, lyotropic LC have been of much interest in such fields as biomimetic chemistry. In fact, biological membranes and cell membranes are a form of LC. Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface yet, the membrane is fluid and elastic. The constituent molecules can flow in plane quite easily but tend not to leave the membrane, and can flip from one side of the membrane to the other with some difficulty. These LC membrane phases can also host important proteins such as receptors freely floating inside, or partly outside, the membrane. 

In spite of all their advantages, sensitivity and selectivity, bio-sensors, however, do possess disadvantages connected with thermal and timely instability, high cost of bio-receptors and the need to add substrates in the solution under analysis as signal-generating substances. Some attempts to synthesize and use as receptors chemical organic catalytic systems, which will ensure the required selectivity and response rate, have become the basis for developing enzyme-free sensors [11], or biomimetic sensors. 

Abstract Calix[n]arenes represent a well-known family of macrocyclic molecules with a broad range of potential applications in many branches of supramolecular chemistry. Because of their preorganisation, calix[n] arenes are frequently used as building blocks and molecular scaffolds in the construction of more elaborate systems, such as artificial enzyme biomimetics and receptors. This review is focused on the recent development of calixarene-based anion receptors. 

Many artificial systems have been designed recently to imitate the function and behaviour of native enzymes - biomimetic chemistry [27]. Among them, calixarene-based receptors bearing one, two or three Zn(II) complexes on the upper rim were prepared as a model for phosphoesterases [28-31]. Dinuclear receptor 25 was reported to enhance the rate of transesterification of the RNA model substrate 2-hydroxypropyl-p-nitrophenyl phosphate more than 20,000 times compared with the non-catalysed reaction. The complexation mode for the phosphate anion can be described as cascade complexation where the anion is coordinated within the cavity formed by two zinc cations. 

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