Artificial supramolecular catalysis

Since this pioneering contribution, the development of artificial supramolecular catalysis within self-assembled capsules in organic solvents passed through the development of several new capsules characterized by different sizes, shapes. 

In this chapter we focus on supramolecular chemical reactivity. In particular this means predominantly the role supramolecular chemistry plays in accelerating or understanding chemical reactions. There are close parallels between artificial, abiotic supramolecular reactivity and biochemistry, for example in the study of enzymes, Nature s catalysts - described in Section 2.6. Synthetic catalysts can both model natural ones and allow the design of new, different kinds of reactions. Supramolecular catalysis sits somewhere between chemical catalysis (transition metal and organocatalysis) and biology. Some considerations within various kinds of catalysis are summed up in the chart shown in Figure 12.1. 

Processive catalysis of this kind involves extremely sophisticated molecular machinery including both RNA and protein components suggesting that it emerged relatively late in evolutionary terms. Processive catalysis is now a target for the design of artificial supramolecular machines and an interesting recent example is discussed in Section 10.7.3. 

Raynal M, et al. Supramolecular catalysis. Part 2 artificial enzyme mimics. Chem Soc Rev 2014 43 1734-87. 

Supramolecular catalysts using synthetic host molecules have been well researched. Early studies realized a hydrolysis reaction for ester derivatives using modified cyclodextrins (CDs). CDs are suitable for the study of two-substrate supramolecular catalysis and for the synthesis of artificial enzymes. 2-Benzyimidazoleacetic acid-modified a-cyclodextrin hydrolyzes m-tert-butylphenyl acetate at an accelerated rate. The imidazole has a benzoate group in a position that imitates the function of the aspartate ion with the catalytic triad characteristics of serine proteases such as chymotrypsin (Figure 12.1). ... 

As a novel concept for supramolecular catalysis, an artificial molecular clamp was attached to the activation site. Synthetic polymerases with artificial molecular clamps yielded high-molecular-weight polymers without solvents or co-catalysts. 

One of the essential impetuses to cyclic dye architectures is the development of artificial model systems of the light-harvesting complexes in purple bacteria, where cychc arrays of chromophores provide the fundamental structural feature for their functionahty [2], On the other hand, cavities of variable sizes and shapes are accessible upon the formation of macrocycles. The assemblies of this kind, thus, possess also potentials in substrate binding, molecular recognition, matter transportation and even supramolecular catalysis. 

Similar to calix[n]arenes, porphyrins have been known to be one of the support pillars of supramolecular chemistry attributing suitable photoactive and electroactive properties to the molecular structures designed around them for building artificial molecular devices. Thus, various metallated and free base porphyrin-calixarene assemblies could afford attractive scaffolds for application in the areas of multipoint molecular recognition, receptors, host-guest chemistry, catalysis and photoinduced electron transfers. 

Self-assembled structures are supramolecular assemblies of covalent backbones structured through intra- and interchain noncovalent interactions. These secondary structures arise from steric constraints and a network of weak interactions (i.e., hydrogen or Van der Waals bonding, dipole-dipole or amphiphilic interactions). Helical morphologies are stiU rarely represented in these artificial species but the control of the heHx sense, and a better knowledge of the chiral amplification mechanism, is highly desirable due to their potential use in many applications. For example, helically chiral polymers can be used as chiral stationary phases for HPLC or for catalysis. 

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