Random Mechanisms

Random mechanisms interpret the appearance of biomolecular asymmetry by a mere chance process inside the system. This process is undirected towards the chiral configuration of the produced or affected chiral molecules. Experimental studies on random mechanisms were often realized in the laboratory.

Examples include the asymmetric precipitation from super-saturated solutions: an aqueous solution of the amino acid asparagine containing its two mirror-image forms called enantiomers in a 50/50 racemic ratio was crystallized at various temperatures resulting in an enantiomeric excess in the precipitate and the opposite excess in the remaining aqueous solution (Thiemann 1974; Thiemann and Darge 1974). Here, a small asymmetry in an elementary nucleation effect was amplified in macroscopic crystals where it could be observed experimentally (see also Thiemann and Wagener 1970). The obtained enantiomeric excess is non-predictable

Kondepudi et al.1990 Yamagata 1966 Ulbricht & Kuhn & Braun 1929

Wächtershäuser 1991 MacDermott & Vester 1962 Rikken &

Tranter 1989 Merwitz et al. 1998 Raupach 2000

Kondepudi et al.1990 Yamagata 1966 Ulbricht & Kuhn & Braun 1929

Wächtershäuser 1991 MacDermott & Vester 1962 Rikken &

Tranter 1989 Merwitz et al. 1998 Raupach 2000

Amplification of the enantiomeric enhancement

Fig. 1.1 Theories for an abiogenic origin of life's chiral bias. One distinguishes between random processes (left branch) and deterministic processes (right branch). Enantiomeric excesses produced by these models are considered generally to be small and insufficient for the evolutionary generation of the first biomolecules; experimental approaches for their amplification substantiated by mathematical models have been developed and random. The asymmetry-creating crystallization was recently reproduced, adapted, and actualised for the amino acid tyrosine (Shinitzky et al. 2002), leaving higher concentrations of L-tyrosine in solution.

The spontaneous chiral symmetry breaking in a similar autocatalytic crystallization system caused a sensation in the scientific community (Kondepudi et al. 1990; Kondepudi and Asakura 2001) that will be systematically presented in Chap. 4. In these and similar scenarios the amplification of relatively small enantiomeric excesses by suitable mechanisms is of high importance. Stereospecific autocata-lysis was proposed as such a process of amplifying capacity: the presence of one enantiomer encourages the generation of the same one but inhibits automatically the synthesis of its corresponding mirror image, leading ultimately to a state of homochirality (Frank 1953). Also, this process is to be classified as a random mechanism, since the initial small asymmetry tipping the balance of the whole system towards homochirality occurred just by chance. Promising mathematical formalisms of reaction sequences including an autocatalytic step (Kondepudi 1987), their experimental models (Buhse et al. 1993a,b), and successful autocatalytic amplification experiments observed in chemical reactions (Soai et al. 1995) were proposed and tested and will be outlined in the appropriate chapters (4 and 10) later in this book.

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