Journal of Biological Physics 22: 87-100, 1996. @ 1996 Kluwer Academic Publishers. Printed in the Netherlands. An Inquiry into the Selective Protection of Glycine during the Radiolysis of Glycine-Alanine Mixtures in Aqueous Solutions and its Implications to the Preservation of Optically Active Amino Acids in the Early Earth RAFAEL NAVARRO-GONZkLEZ’j*, MITSUHIKO AKABOSH12, ALFRED0 ROMERO’ and CYRIL PONNAMPERUMAt3 ‘Institute de Ciencias Nucleares, Universidad National Autonoma de Mexico, Circuit0 Exterior, CiuaW Universitaria, Apartado Postal 70-543, Mexico D.F. 04510, Mexico; ’ Research Reactor Institute, Kyoto University, Kumatori, Osaka-590, Japan; ‘Laboratory of Chemical Evolution, Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, U.S.A. (*author for correspondence) (Received: 12October 1995; accepted in final form: 22 March 1996) Abstract. Akaboshi et al. (1990)hasfound an unexpected protectionof the achiralamino acid, glycine, towards ionizing radiation at the expense of the selective destruction of the chiral amino acids, alanine andaspartic acid. Thepresent work examines the mechanism of this protection for the case of alanine. Wehave developed a computer model for the radiolysis of glycine, alanine and glycine-alanine mixtures in aqueous solution. It is established that thisprotection is due in part to the reaction of the o-radicalof glycine with alanine to regenerate a more stable a-radical,according to the followingreaction, CH(NH,+)CO; + CH3CH(NH;)CO; + CH,(NH,+)CO; + CH&(NH;)CO; Therate constant of this reaction was estimated to be6 104MM-’ s-r. Theimplications for this selective protection of glycineare. considered for ahypothetical case in whichthere wouldbeanenrichment of about10% of L&mine in the primitive ocean and taking the glycine/alanine ratios obtained in CHs- andCOZ- dominated atmospheres using electricdischarge experiments. It is predicted that alanine would berapidly destroyed and radioracemized in spite of thefactthat theconcentration of alanine is equal or significantly lowerthan thatof glycine.Assuming that chiral amino acids were aprerequisite for the origin of life, it canbe deduced that life couldhave appeared in a relatively shortperiod of timeunless there was a constant supply of optical amino acids fromextraterrestrial sources. Key words: Alanine, Amino acids, Chirality, Glycine, Ionizing radiation, Kinetics, Primitive hydrosphere, Racemization, Radiolysis, Radioracemization 1. Introduction The effect of ionizing radiation on aqueous solutions of amino acids was extensively studied in the 1950s and 1960s because of a dual interest. On the one hand, these investigations were significant to acquire a basic understanding of the mechanism of