A review of conditions affecting the radiolysis due to40K on nucleic acid bases and their derivatives adsorbed on clay minerals: Implications in prebiotic chemistry

Abstract

This paper describes the possible effects of ionizing radiation arising from long-lived soluble radionuclides within clays, in particular 40K, at the epoch of the emergence of life on Earth. The free dispersion of soluble radionuclides constitutes an effective in situ irradiation mechanism that might have acted upon adsorbed nucleic bases and their derivatives on clays, inducing chemical changes on these organic molecules. Several types of well documented reactions for radiolysis of nucleic acid bases and their derivatives are known, even at low doses (i.e., 0.1 Gy). For example, estimates with a dose rate calculated from 40K from deep sea clays at 3.8 Ga ago, indicates that over a period of 1000 years the amount of organic material transformated is 1.8 X 10(-7) moles/kg-clay. Although ionizing radiation may also induce synthetic reactions with prebiological interest, all in all these considerations indicate that nucleic acid bases and their derivatives adsorbed on clays were exposed for long periods to degradation conditions. Such situation promotes decomposition of organic molecules rather than protection of them and enhancement of farther polymerization, as it has been usually taken for granted.

Sites of Adsorption of Adenine, Uracil, and Their Corresponding Derivatives on Sodium Montmorillonite

“…However, when we compare with other reports the scattering of results is large, and does not permit a clear ordering of their adsorption extent. We guess there is probably some extent of hydrolysis of nucleotides within clays that has not been quantified up to this date (Mosqueira et al, 1996). These factors altogether may explain the contradictions in data reported so far in the literature (Odom et al, 1979;Rao et al, 1980) with respect to the degree of adsorption of nucleotides.…”

“…Since Bernal (1951), and later on Cairns-Smith (1966), pointed out the relevance of mineral surfaces to the emergence of life on Earth, several studies have been published on the interaction between prebiotic molecules and clay surfaces (see for example Rao et al, 1980;Mosqueira et al, 1996;Negrón-Mendoza et al, 1996 and references therein). Special attention has been given to the adsorption of nucleic acid bases, and their corresponding derivatives on mineral surfaces, particularly on montmorillonite.…”

Chemical Evolution: An Approach From Radiation Chemistry

“…However, those radionuclides may have been quite important historically, acting at the time when chemical evolution occurred. In the case of the Earth's crust, it is possible to calculate the corresponding amount of energy available 3.8 x 10 9 years ago, in the form of ionizing radiation and heat, from the rates at which major long-lived radioactive species decay [18,19].…”

“…It has higher concentrations in acid igneous rocks and sedimentary alumino-silicates; 40 K has a concentration of 0.0117 %; despite its small isotopic concentration 40 K plays an essential part in heat production of the Earth. Its natural abundance on the primitive Earth 3.8 x 10 9 years ago was around eight times higher than it is today [18,19]. The homogeneous distribution of potassium salts in primitive oceans, lagoons, and tide pools may have been an effective internal source of energy and contributed in the transformation of the molecules present in solutions.…”

“…In this way, mineral surfaces could have provided templates and surfaces for sorption and may also have acted as catalysts for chemical reactions. A review of several experiments using a mineral surface to enhance the prebiotic synthesis induced by radiation and protection role of clays is shown in [18,20] and the references therein. The study of the behavior of bioorganic compounds in the presence of clay minerals is necessary to extend the knowledge of their role in the prebiotic epoch.…”