Template-directed autocatalysis

In self-replicating systems employing three starting constituents competition between constituents can occur [9.205]. Such processes are on the way to systems displaying information transfer, whereas the two-components ones are non-infor-mational. A shift from parabolic kinetics to exponential growth of the template concentration is required for a selection process to take place [9.197]. The evidence for self-replication on the basis of template-directed autocatalysis as in 184 requires detailed mechanistic investigation on the origin of the catalytic effects observed [9.206]. 

First, asymmetric autocatalysis served to form chiral biomolecules. This reaction is a template-directed, self-replicating system, which successfully maintains ideal exponential growth kinetics and therefore high autocatalytic efficiency over many turnovers. 

But what is it about this system that makes us call it self-replicating How could we show that the system progresses through directed template catalysis rather than through simple chemical autocatalysis After all, (6) bristles with functional groups. The imide, amide, ribose and purine functionalities must all be considered as possible explanations for the autocatalysis observed. For example, imidazole is a well-known catalyst for acylation reactions, and the purine contains such a subunit. Could not this functionality be the cause The potentially catalytic functions of the product molecule had to be individually tested in the structural context of (6) and under the conditions where (6) acts as an autocatalyst. 

Direct asymmetric autocatalysis amplified the slight excess of one enantiomer, leading to the enantiopure compound by reaction with diisopropylzinc. It is widely accepted that enantiomerically enriched products must form from achiral precursors merely because of statistical fluctuations. Usually, however, enantiomeric enrichment by fluctuations is very low. Thus, an amplification process of enantiomeric enrichment is required. Detailed kinetic analysis revealed that autocatalysis and inhibition are the major players in asymmetric autocatalytic synthesis. It turned out that tetramers serve as catalyst in the Soai reaction. The transition state for the Soai reaction implicates two molecules of pyrimidine alcohols or alcoxides as the dimeric catalysts and one molecule of prochiral aldehyde substrate (Buono and Blackmond 2003). Further kinetic studies using different ratios of substrate and reagent showed that a tetramer template is used. 

We have recently suggested a general formal mechanism that accounts for the kinetics of networks that perform multiple autocatalysis and cross-catalysis using (7 and 8). In this description, the dimers T T are the only catalyt-ically active species. Each of the reactants E, combines directly with only one reactant of type N (like in our original network experiment) to form the templates T, either in the slow background reaction or in a catalyzed reaction. This analysis can easily be extended to include as many E, species as needed, as well as several reactants of type N. 

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