Abstract
Recent progress in mirror symmetry breaking and chirality amplification in isotropic liquids and liquid crystalline cubic phases of achiral molecule is reviewed and discussed with respect to its implications for the hypothesis of emergence of biological chirality. It is shown that mirror symmetry breaking takes place in fluid systems where homochiral interactions are preferred over heterochiral and a dynamic network structure leads to chirality synchronization if the enantiomerization barrier is sufficiently low, i.e., that racemization drives the development of uniform chirality. Local mirror symmetry breaking leads to conglomerate formation. Total mirror symmetry breaking requires either a proper phase transitions kinetics or minor chiral fields, leading to stochastic and deterministic homochirality, respectively, associated with an extreme chirality amplification power close to the bifurcation point. These mirror symmetry broken liquids are thermodynamically stable states and considered as possible systems in which uniform biochirality could have emerged. A model is hypothesized, which assumes the emergence of uniform chirality by chirality synchronization in dynamic “helical network fluids” followed by polymerization, fixing the chirality and leading to proto-RNA formation in a single process.
Highlights
Homochirality and LifeEver since Pasteur revealed the molecular asymmetry of organic compounds in 1848 [1], the origin of the homochirality of biologically relevant molecules has attracted considerable attention.The chirality of organic compounds is due to the tetrahedral structure of carbon with a valence of four bondings to adjacent atoms
The achiral Ia3d phase remains uniformly dark after rotating one of the polarizers in either direction, confirming that it is optically inactive (Figure 8a,b). This shows that the network formation in the liquid crystalline (LC) Cubbi phases, transmitting chirality and disfavouring helix reversals [136], is a powerful tool for achieving mirror symmetry breaking under thermodynamic equilibrium conditions and for the long-term stabilization of the chirality synchronized helices in the LC network phases with cubic [26,88,89,128,129,130,131,132,133] or non-cubic symmetry [137,138]
The first experimental proof of a spontaneous mirror symmetry breaking in the liquid state, occurring even in the absence of any applied pressure and at temperatures compatible with the occurring even in the absence of any applied pressure and at temperatures compatible with the conditions of abiogenesis, was observed for liquid phases occurring in the vicinity of the Cubbi phases conditions of abiogenesis, was observed for liquid phases occurring in the vicinity of the Cubbi of polycatenar molecules, like compounds 3 [25]
Summary
Ever since Pasteur revealed the molecular asymmetry of organic compounds in 1848 [1], the origin of the homochirality of biologically relevant molecules has attracted considerable attention. It is well known that in all existing organisms the carbohydrates exist in the d-form and the amino acids in the l-form (see Figure 1; there are l-sugars and d-amino acids in biological systems, but if they are involved, they fulfill specific functions, see [2]) This homochirality is considered as a signature of life. Already by 3.95 Ga, the first signatures of life appear as carbon isotope signatures In this astonishing narrow window of only about 200–300 million years (0.2–0.3 Ga), the first cells came to existence, meaning that mirror symmetry must have been broken and the genetic code developed during this, on a geological time scale, amazingly short period.
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