Séminaire Institut

Friday 26 February 2010 à 14h00.

Homochirality in Chemical Reaction and in Crystal Growth

''If an object cannot be overlapped to its mirror image by translation and rotation, it is said chiral or has a chirality. Your left hand is an example of a chiral, and its enantiomer is your right hand. Another example is quarz which has two enantiomorphous crystalline shapes. They are different in optical activity to polarized light as well. According to the difference in crystal shape and in optical activity, Pasteur found that even a molecule like tartaric axcid has chirality. Except optical activity, physical properties of two enantiomers are the same, and in the normal chemical reaction, two enantiomers are produced with the same amount: The product is a racemic mixture. In life, however, nature is one-handed: L-amino acids and D-sugars dominate. The origin of this chiral symmetry breaking, or homochirality, in life is a long mystery. After the first theory by Frank in 1953, it took forty years for Soai to find a first chemical system which shows an amplification of enantiomeric excess (ee). The Soai reaction contains a nonlinear autocatalytic process but irreversible, and could not produce enantiomerically pure products. We proposed a theory that by adding a recycling process the chiral symmetry breaks completely [1]. A similar chirality selection problem exists in crystal growth. In 1898 Kipping and Pope evaporated water from NaClO3 solution, and obtained racemic mixture of two types of enantiomorphous crystallites. In 2005, Viedma vigorously ground a solution with d- and l-crystallites with glass balls. By starting from a solution with a small ee value, he obtained a homochiral state in a short time. Last year the same grinding is found to convert even a molecular chirality of conglomerating organic molecules. We proposed a lattice gas model equivalent to Blume-Emery-Griffith model, and by means of kinetic Monte Carlo simulations we studied how grinding acts as a recycling process and leads to homochirality [2]. [1 ] Y. Saito and H. Hyuga: J. Phys. Soc. Jpn 73(2004)33; 74(2005)1629. [2 ] Y. Saito and H. Hyuga: J.Phys.Soc.Jpn 77(2008)113001; 78(2009)104001.''