Biomolecular handedness

Ever since the beginning of life on primitive Earth, biopolymers and biomolecules have essentially comprised optically active constituents because of the natural selection of Z-amino acids and tZ-sugars. Although the origin of this biomolecular handedness is a long debated issue among biologists, chemists, physicists, and astronomers,1 5 it is accepted that our life is a consequence of the chemistry of homochiral biosubstances. Deoxyribonucleic acid (DNA) is a classic example of a chiral biopolymer. Its chirality is essentially characterized... 

Tranter, G. E., MacDermott, A. J., Overill, R. E., and Speers, P. (1992). Computational studies of the electroweak origin of biomolecular handedness in natural sugars. Proc. Royal Soc., London A, 436, 603-15. 

Weissbuch, L, Zepik, H., Bolbach, G., et al. (2003). Homochiral oligopeptides by chiral amplification within two-dimensional crystalline self-assemblies at the air-water interface relevance to biomolecular handedness. Chemistry, 9 (8), 1782-94. 

Keywords Asymmetric autocatalysis Biomolecular handedness Chiral clusters ... 

Deterministic and stochastic processes should be taken into account. The deterministic scenario invokes transfer of the intrinsic chirality of the universe to the biopolymers of life [6,7]. Ab initio theoretical estimates that take into account the chiral electroweak forces indicate that the L-amino acids and the D-sugars are more stable than their corresponding enantiomers [8,9]. The minute energy differences between these enantiomeric pairs, under Darwinian reaction kinetics in a flow reactor, were invoked to account for the biomolecular handedness that arose when fife began [7]. Several reports describing deterministic mechanisms that could have induced mirror sym-... 

S. Mason, G. Tranter, The electroweak origin of biomolecular handedness, Proc. Roy. Soc. Lond. A 397 (1985) 45-65. 

Mason S.F. (1984) Origin of biomolecular handedness, Aot., 311, 19-23. Tranter G.E. (1985) Parity-violation energy differences of chiral minerals and the origin of biomolecular homochirahty, Nat., 318, 172 -173. MacDermott A. J. (1993) The weak force and the origin of life, in Chemical evolution. Origin of life, (Potmamperuma C. and Chela-Flores J. (eds.), Deepak Hampton, pp. 85-99. 

MacDermott, A. J. and G. E. Tranter, 1990, Biomolecular handedness and the weak interaction, in Symmetries in Science IV Biological and Biophysical Systems, Plenum Press, New York, pp. 67-124. 

The above example shows how a far-from-equilibrium chemical system can generate and maintain chiral asymmetry, but it only provides a general framework in which we must seek the origins of biomolecular handedness. The origin of biomolecular handedness, or life s homochirality, remains to be explained [11, 12]. Here we shall confine our discussion to how the theory of nonequilibrium symmetry breaking contributes to this important topic. We cannot yet say with confidence whether chiral asymmetry arose in a prebiotic (i.e. before life) process and facilitated the evolution of life, or whether some primitive form of life that incorporated both L- and D-amino acids arose first and subsequent evolution of this life form led to the homochirality of L-amino acids and D-sugars. Both views have their proponents. 

Although derived in the context of biomolecular handedness, this formula is generally valid for any system that breaks a two-fold symmetry, such as mirror inversion. Using this formula, it is possible to understand the extraordinary sensitivity of bifurcation to small systematic biases that favor one enantiomer by increasing its production rate. For example, it can be estimated that the chiral asymmetry of the electroweak interaction can create differences of the order of one part in 10 between the enantiomers. Application of the above theory shows that if the autocatalytic production rate of the chiral molecules is faster than the racemization rates, then for a period in the range 10" to 10 years, the enantiomer favored by the electroweak force will dominate [16]. For such a scenario, there is currently no experimental evidence to show us how chiral autocatalysis with the required properties can originate in prebiotic chiral molecules. 

Many different scenarios have been suggested for the possible origins of biomolecular handedness. An extensive review can be found in the literature [19]. Note that, even if one is considering a process of chiral asymmetry generation after life arose, equations of the type (19.3.17) can still be used to describe the symmetry-breaking process, but this time the model will contain as reactants the self-replicating unit of life. 

Chiral molecules are characterized by three-dimensional handedness and can exist in two enantiomeric forms of opposite absolute configuration (AC). Most natural products and biologically active compounds are chiral and their biological and molecular functions are closely related to their chirality, that is, AC and conformation. Furthermore, many drugs derived from natural products or of purely synthetic origin are currently used in enantiopure form. Therefore, the unambiguous determination of the AC of chiral compounds is critical for the studies of natural products and biomolecular systems.1... 

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