Artificial catalytic chemistry

Kauffmann defined a living system as a physical cell able to self reproduce and at least able to complete a single thermodynamic cycle that executes work. A minimal model of primitive self maintaining cells named chemoton was defined by Ganti It is composed of  

Since the production of a membrane costs energy, a reaction cycle is required that generates energy and allows its use in the production cycle. External resources are used as an energy source and as building materials of the protocell and partially converted to waste. The artificial catalytic cell would be selective in its waste production and produce instead 

The different reaction cycles often require different conditions. This generates a need for compartmentalization with communication between compartments.In modern cells, electrocatalytic processes with the consumption and generation of electrons and protons and their transport through membranes play an important role in this respect. The system operates far from equilibrimn, where cyclic behavior can be maintained. 

Kauffmann demonstrated that chemical reaction networks that exceed a minimum requirement of complexity convert to a state of self reproduction coupled to a production cycle. With increasing complexity, the system imdergoes a phase transition from a disordered, non-reproducing state to a self-organizing and self-reproducing state. It follows from the previous paragraph that there is also an upper limit to this complexity, beyond which a genetic apparatus becomes necessary for reproduction. 

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The understanding of the mechanistic principles underlying the catalytic chemistry of metalloproteases has been applied in the design and synthesis of a number of biomimetic metal complexes that ftinction as artificial proteases. These complexes include mononuclear wateractivating complexes, and multinuclear complexes that combine carboxyl terminal recognition, peptide carbonyl polarization, and water nucleophile activation fimctions. ... 

In the previous sections we have analyzed computational models of artificial chemistry that indicate that, in principle, the chemistry can be designed so as to create an artificial catalytic system, that optimizes its selectivity by evolutionary adaptation. 

We now can prepare, in principle, enzyme models by use of the concept of host design, where artificial enzymes are so designed as multiple recognition hosts schematically shown in Fig. 20. Although unsubstituted cyclodextrins are well known to catalyze some organic reactions such as ester hydrolysis, their catalytic activities are relatively small. Recent progress in cyclodextrin chemistry has shown that it is possible to enhance the catalytic... 

Simple dynamical systems have proved valuable as models of certain classes of physical systems in many branches of science and engineering. In mechanics and electrical engineering Duffing s and van der Pol s equations have played important roles and in physical chemistry and chemical engineering much has been learned from the study of simple, even artificially simple, systems. In calling them simple we mean to imply that their formulation is as elementary as possible their behaviour may be far from simple. Models should have the two characteristics of feasibility and actuality. By the first we mean that a favourable case can be made for the proposed reaction, perhaps by some further elaboration of mechanism but within the framework of accepted kinetic principles. Thus irreversible reactions are acceptable provided that they can be obtained as the limit of a consistent reversible set. By actuality we mean that they are set in an actual context, as taking place in a stirred tank, on a catalytic surface or in a porous medium. It is not usually necessary to assume the reaction to take place in a closed system with certain components held constant presumably by being in excess. 

In this review, however, only recent studies of the reactions of macromolecule-metal complexes will be reviewed. There have been some exellent reviews recently on macromolecule-metal complexes regarding syntheses, formation, characterization and catalytic activities l solar energy conversion 2), artificial oxygen carriers 3), and electrode processes 4). Furthermore, the preprints of the 1st International Conference on Macromolecule-Metal Complexes Tokyo Seminar on Macromolecule-Metal Complexes were published in 19875). These reviews and the preprints give useful information about the recent development of the basic and applied chemistry of macromolecule-metal complexes. 

Reymond, J.L. and Chen, Y.W. (1995) Catalytic, enantioselective aldol reaction with an artificial aldolase assembled from a primary amine and an antibody. Journal of Organic Chemistry, 60, 6970-6979. 

This method of selecting catalytic sites significantly depends on spontaneous processes, in contrast to the development of artificial enzymes and catalytic antibodies. The selection process is based on self-assembly, selforganization and self-optimization. Therefore, this selection approach bears the characteristics of supramolecular chemistry. A similar concept is used in natural evolution processes, resulting in the complicated life forms we see around us today. Therefore, it is clear that we can design the self-organizational processes used in supramolecular chemistry to proceed according to the concepts followed by this natural evolutionary process. 

Native enzymes, which can spatially and chemically recognize substrate molecules, are powerful catalytic systems in many biochemical processes under mild reaction conditions. The preparation of artificial enzymatic catalysts with the capability of molecular recognition capability, by a molecular-imprinting method, which creates cavities with a similar shape and size to the template molecule in polymer matrices has been developed [1-14]. The technique has been mainly established in the field of analytical chemistry - molecular receptors [15-23], chromatographic separations [24-28], fine chemical sensing [29-33]. All of the methods rely on the selective adsorption of target molecules on imprinted adsorption sites. The number of papers reported per year on molecular imprinting is summarized in Fig. 22.1. 

Of the methacrylics, methyl methacrylate (MMA) is by far the most important intermediate, with a production of approx. 2 X 10 t/a (1.6 X 10 t/a for Europe and USA [la]). It finds its main application in polymers as Plexiglass or Perspex, a crystal-clear artificial glass with high hardness, resistance to fracture, and chemical stability. About 80 % of the MMA is produced on the basis of the Acetone-Cyano//ydrin (ACH) process [lb]. This process involves stoichiometric chemistry and is characterized by an overall yield of MMA of about 80 % (based on acetone) with the stoichiometric production of about 2.4 t of ammonium bisul-fate/sulfuric acid waste per ton of MMA. This process is therefore faced with increasing environmental costs. As a consequence, several alternative catalytic processes for MMA production are - and have been - under development. 

The controversy on the existence of in vivo Diels-Alder reactions cannot be put to rest here, but the numerous examples of natural products containing cyclohexene groups and the catalytic effectivity of biological surroundings support the idea of in vivo Diels-Alder reactions. Apart from cell-free extracts, RNA-based mixtures of metals also show catalytic activity and it was demonstrated that this catalyst system can be quite effective as an artificial Diels-Alder-ase . We will show that water, the prime solvent of biosynthesis, also catalyses [4 -+- 2]-cycloadditions. Considering that biosyntheses are often of exceptional selectivity, it is clear that understanding biomimetic transfonna-tions in water as the solvent is an important goal of modem chemistry. The possibilities offered by and the reasons for Diels-Alder catalysis in water will be the main topic of this chapter. 

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