Biomolecular asymmetry

Meierhenrich UJ, Thiemann WHP (2004) Photochemical concepts on the origin of biomolecular asymmetry. Orig Life Evol Biosph 34 111-121... 

Meierhenrich, U. J., Thiemann, W. H. P., Photochemical Concepts on the Origin of Biomolecular Asymmetry, Orig. Life Evol. Biosph. 2004, 34, 111 121. 

NONEQUILIBRIUM SYMMETRY BREAKING AND THE ORIGIN OF BIOMOLECULAR ASYMMETRY... 

Mossbauer spectroscopy should also be mentioned here as a very promising method for combining the structural and dynamic studies of biomolecular systems. The asymmetry of Mossbauer spectra caused by the anisotropy of vibrations of Mossbauer atoms allowed—for example, to find that the mean square amplitude of vibrations of Fe atoms normal to the plane of porphyrin ring (which are responsible for many important biological functions of hemoproteins) is about five times larger than in the... 

How chiral asymmetry has been propagated through the diverse levels of biomolecular systems... 

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. 

---