Replication and Recombination in Minimal Synthetic Systems
Abstract
Examples of chemical systems capable of templating and catalysing their own synthesis - self-replicating systems - have begun to appear in the chemical literature over the last 10 years. Almost all of the successful systems have relied on the application of naturally-occurring recognition motifs based on either peptides or nucleic acids. Additionally, almost all of the well-characterised systems published to date have relied on chemical reactions such as the formation of amides and phosphate esters to construct the template. This renders the system trivial in an informational sense as the formation of the covalent bond between the replicator fragments and, hence, the template can have only one possible outcome - that is, mutation mediated by the covalent bond forming step is not possible in such systems.
Recently, we have designed and synthesised a system (Scheme 1) which exploits the non-linear kinetic behaviour of replicating systems to achieve the amplification of relative stereochemistry through a replication process. [Scheme 1] Cycloadduct 3 (Scheme 2) possesses the capability to replicate itself through the type of autocatalytic cycle shown in Scheme 1 , however, its diastereoisomer 4 is incapable of templating its own formation. Therefore, when the nitrone 1 and maleimide 2 are allowed to react in CDCI3 at 1DOC, both stereoisomers are formed initially. However, when the concentration of 3 rises to a sufficient level, the autocatalytic cycle amplifies the formation of 3, but not the formation of 4. This amplification process is responsible for the sigmoidal profile (Scheme 2) of the concentration-time curve for the formation of 3. Importantly, there is no crosscatalysis in this system. In other words, in addition to 4 being incapable of catalysing its own formation, it is also incapable of catalysing the formation of 3. [Scheme 2] Additionally, the kinetics of this replicating system do not exhibit a square root relationship to the concentration of template, but rather displays a direct response to the template concentration. Therefore, one might view the outcome of this process in Darwinian terms. Cycloadduct 3 is better adapted for replication and so is able to compete more effectively for the common building blocks 1 and 2 than cycloadduct 4. This fitness for replication results in an enhancement of the population of 3 relative to that of 4 (solid curves in the graph in Scheme 2) - in the absence of replication, 3 and 4 would be formed in an approximately 2:1 ratio (dashed curves in the graph in Scheme 2). This presentation will discuss the experiments required to elucidate the mechanism of replication in this system - including 1 H NMR studies and X-ray crystallography. Recombination experiments in which a number of replicator components compete for a common building blocks will also be discussed. The results obtained in this study will be contrasted with preliminary data obtained on two other related systems.- Publication:
-
Origins of Life and Evolution of the Biosphere
- Pub Date:
- August 2000
- DOI:
- 10.1023/A:1017300204711
- Bibcode:
- 2000OLEB...30..248A