Mirror symmetry breaking at the molecular level.

Abstract

Reasoning from two basic principles of molecular physics, P invariance of electromagnetic interaction and the second law of thermodynamics, one would conclude that mirror symmetry is retained in the world of chiral molecules. This inference is fully consistent with what is observed in inorganic nature. However, in the bioorganic world, the reverse is true. Mirror symmetry there is definitely broken. Is it possible to account for this phenomenon without going beyond conventional concepts of the kinetics of enantioselective processes? This study is an attempt to survey all existing hypotheses concerning this phenomenon.Operation of mirror reflection, or space inversion P, enables one to classify any molecular structure under either of two groups. One group involves molecules having neither symmetry planes nor symmetry centers, i.e., noninvariant with respect to P. These molecules occur in the form of two mirror antipodes (L and D enantiomers), possess optical activity, and are called chiral (from the Greek word XsLp, meaning hand). Among members of the other group are achiral molecules having either symmetry planes or symmetry centers. These molecules are invariant with respect to P and are optically inactive.The main characteristic of the chemistry of chiral compounds is associated with P invariance of electromagnetic interaction. This type of interaction as a rule dominates coupling of intramolecular electrons and nuclei, and therefore, the states of a chiral molecule are described by a symmetric double-well potential with minima corresponding to the L and D configurations (1).In compounds with an asymmetry center, e. However, in living nature, the situation changes dramatically. Broken mirror symmetry of bioorganic objects was first noticed by Louis Pasteur and led him to the conclusion that the molecular substrate of life was not only chiral but also asymmetric (4).What can be said about it now that relatively simple organisms have been studied in so much detail that apparently the only question that remains unanswered is how all these organisms could arise?It is common knowledge that polymer constituents of the double-stranded DNA structure may involve millions of nucleotide links, that similar RNA chains incorporate hundreds and even thousands of nucleotide monomers, and that polymer chains of enzymes usually consist of several hundred amino acid links. DNA and enzymes play essentially different roles; DNA macromolecules are informational carriers, whereas macromolecules of enzymes are functional carriers. RNA plays a part of an intermediary between DNA and enzymes and occasionally takes on the duties of either of the sides (5, 6).However, from the "chiral" viewpoint, all these biopolymers feature a remarkable trait, namely, nucleotide links of RNA and DNA incorporate exclusively D-ribose and D-deoxyribose, respectively, whereas enzymes involve solely L enantiomers of amino acids. In other words, the primary structures of DNA, RNA, and enzymes are homochiral. This property is inherent in all informat...

Spontaneous mirror symmetry breaking and origin of biological homochirality

“…The limited enantioselectivity (LES) model was proposed for SMSB as an alternative to avoid the mutual inhibition between enantiomers/catalysts [35]. The minimal set of the LES model is composed of enantioselective autocatalysis (2.3) and non-enantioselective autocatalysis (2.6).…”

Mirror symmetry breaking as a problem in dynamic critical phenomena

“…This requires that the system be far from criticality. The bifurcation equation [1] is then η(1 − η 2 ) = 0 and there are three stationary solutions:…”

“…Otherwise, the reverse reaction must be taken into account. Furthermore, as pointed out by Avetisov and Goldanskii [1], reversibility in the mutual inhibition reactions will account for the limited enantioselectivity of chiral catalysts, so that the catalytic effect of each enantiomer leads to the formation of both L and D products. To include these effects, the following reactions would have to be added to the above scheme Eqs.…”

“…An outstanding problem associated with the origin of life is to explain chiral symmetry breaking in nature, why for example, it came to be that the nucleotide links of RNA and DNA incorporate exclusively righthanded sugars while the enzymes involve only the lefthanded amino acids. A recent survey of hypotheses concerning this phenomenon, experimental realizations and additional pertinent bibliography can be found in the references [1,2,3,4,5].…”

“…This scheme is a partial hybrid between the Frank model and the Avetisov and Goldanskii (AG) reaction (see e.g., Eq. (13) of [1]). Whereas Frank's original model gives rise to pure homoquiral states in which only one enantiomer is present, the complete AG model leads to chiral symmetric broken final states where both enantiomers are present in unequal proportions.…”