Abstract
Mechanisms leading to a molecular evolution and the formation of homochirality in nature are interconnected and a key to the underlying principles that led to the emergence of life. So far proposed mechanisms leading to a non-linear reaction behavior are based mainly on the formation of homochiral and heterochiral dimers. Since homochiral and heterochiral dimers are diastereomers of each other, the minor enantiomer is shifted out of equilibrium with the major enantiomer by dimer formation and thus a reaction or catalysis can be dominated by the remaining molecules of the major enantiomer. In this article a mechanism is shown that leads to homochirality by the formation of a highly catalytically active transient intermediate in a stereodynamically controlled reaction. This is demonstrated by Soai's asymmetric autocatalysis, in which aldehydes are transformed into the corresponding alcohols by addition of dialkylzinc reagents. The mechanism of chirogenesis proposed here shows that an apparently inefficient reaction is the best prerequisite for a selection mechanism. In addition, stereodynamic control offers the advantage that the minor diastereomeric intermediate can be interconverted into the major diastereomer and thus be stereoeconomically efficient. This is supported by computer simulation of reaction kinetics.
Highlights
The single-handedness of molecular building blocks, such as amino acids and sugars, in biologically relevant metabolisms and structures is considered the signature of life and an important prerequisite for the emergence of life (Hegstrom, 1984; Blackmond, 2004, 2011; Kawasaki et al, 2008; Hawbaker and Blackmond, 2019; Karunakaran et al, 2019; Teichert et al, 2019)
It could be shown that the formation process of the hemiacetal is endergonic and that there is a rapid conversion equilibrium between the hemiacetals
Simulations with the kinetic model of the Soai reaction under variation of the kinetics and thermodynamics of the hemiacetal formation allowed the prediction of the amplification of the enantiomeric excess depending on the addition of the alcohol as additive
Summary
The single-handedness of molecular building blocks, such as amino acids and sugars, in biologically relevant metabolisms and (polymeric) structures is considered the signature of life and an important prerequisite for the emergence of life (Hegstrom, 1984; Blackmond, 2004, 2011; Kawasaki et al, 2008; Hawbaker and Blackmond, 2019; Karunakaran et al, 2019; Teichert et al, 2019). One of the most exciting questions is how biological homochirality developed from a predominantly achiral environment. Several theoretical approaches to this question have been investigated experimentally over the last decades and many findings show how enantiomer enrichment may have occurred by physical processes or chemical reactions. Some of the most promising theoretical proposals for asymmetric amplification of initial small imbalances are autocatalytic reactions (cf Figure 1) (Alberts and Wynberg, 1989; Soai and Kawasaki, 2008; Tsogoeva, 2010; Bissette and Fletcher, 2013). A comprehensive review of such processes was recently compiled by Blackmond (2020)
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