SPONTANEOUS BREAKING OF THE L, D SYMMETRY IN PHOTOLYTIC PRODUCTION AND DEGRADATION OF AMINO ACIDS K. TENNAKONE a' b, * a Institute of Fundamental Studies, Hantana, Kandy, Sri Lanka b Department of Physics, University ofRuhuna, Matara, Sri Lanka (Received May 29, 1990) AbstraeL The radiolysis experiments of amino acids have revealed the presence of bimolecular interaction between like enantiomers which suppress their photodegradation and between opposite enantiomers that enhance the photodegradation. Based on a mathematical model, it is suggested that this phenomenon could have given rise to chiral stereoselection in biochemical evolution. 1. Introduction The possibility that chiral stereoselection in biochemical evolution is a result of spontaneous breaking of the L, D symmetry in parallel chemical reactions, have received much attention (Frank, 1956; Decker, 1974; Kondepudi and Nelson, 1983; Tennakone, 1984). All models of this type proposed, assumes autocatalytic pro- duction of L, D enantiomers, e.g. (Kondepudi and Nelson, 1983). A +B ~ X L [I] A B - x D [2] A +n+XL ~- 2XL [3] A + B + X R ~ 2XD. [4] Followed by an inhibitary interaction between L and D species, e.g., XL + X D ~" C, [5] where the concentrations of the reactants A, B and C are assumed to be maintained constant. The rate equations for [1] - [5] have an unstable symmetric solution and a stable asymmetric solution. In the presence of even a minute, but universal, right-left asymmetric influence, the selection occurs in the same sense with a high probability (Kondepudi and Nelson, 1983; Mason, 1984). Thus a global symmetry breaking becomes possible. The external chiral factor needed for biasing the symmetry breaking is believed to be weak neutral currents (Letokhov, 1975; Hegstrom, 1985) or polarized radiation from betadecaying radio active materials (Garay, 1968; * Sumanasekara Chair in Natural science. Origins of Life and Evolution of the Biosphere 20:515-519, t 990-1991. © 1990-1991 Ktuwer Academic Publishers. Printed in the Netherlands'.