As we have seen in the above chapters, an enantiomeric enhancement can indeed be induced into racemic mixtures of organic molecules via internal or external asymmetric effects. We discussed random fluctuations of enantiomers, the spontaneous symmetry breaking via crystallization, the molecular parity-violating weak force, magnetochiral photochemical effects, and also the absolute asymmetric photochemical induction of an enantiomeric excess. Among all these attractive and topical models, a slight majority of scientists today favours asymmetric photoreac-tions to explain the origin of biomolecular asymmetry, because such reactions were successfully performed under realistic primordial conditions, as for example in simulated interstellar molecular clouds where chiral amino acid structures have been detected. We can envisage that future space missions such as the Rosetta spacecraft and others will provide us the required key information to ultimately decipher the origin of life's chirality. But attention: depending on these and other data, the scientific basis for other hypotheses mentioned above - if not entirely new - might also be strengthened. This is what makes science in general and the research field of life's molecular asymmetry in particular that attractive!
Anyhow, we have outlined that the total of the above models is capable of producing relatively small enantiomeric enhancements compared to the homochiral structures of the 20 L-amino acids and the D-ribofuranose sugar molecules embedded in biopolymers. Therefore, we have to reflect on possibly cumulative or magnifying ways to increase the enantiomeric excess from a few percent (or even much less) towards nearly homochirality. Do such models exist? This question is to be answered with a clear "yes". It seems that amplification processes able to increase small elementary enantiomer disparities to large enantiomer excesses present even a considerably lesser challenge for scientists than finding the enantiomer bias as such. A manifold of various amplification processes is abundant, at hand, and fully accepted by the scientific community (Thiemann and Bredehoft 2007).
In order to classify these amplification processes we will distinguish between (a) models that have been designed through mathematical approaches and (b) models that have been based on experimental work. I will try to treat the mathematical approaches as comprehensible as possible in order to provide a general access to the obtained highly intriguing results. Afterwards, experimental work that has been greatly stimulated by successful trials very recently will be presented. In a more
U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology 185
and Biogeophysics, © Springer-Verlag Berlin Heidelberg 2008
Amplification of a small enantiomeric enhancement r
Mathematical models r
Reaction sequences; bifurcating systems
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