On the possible role of crystals in the origins of life. I. The adsorption of nucleosides, nucleotides and pyrophosphate by apatite crystals

Abstract

Ever since Oparin (1936) first formulated a scientific approach to the terrestrial origins of life, there has been increasing interest and research activity in the broad subject area (Fox, 1965). Beginning with Calvin's laboratory (Garrison et al., 1951) and Miller (1955), the successful “chance” synthesis of nearly all the biologically important monomeric substances (amino acids, sugars, fatty acids, even purincs and pyrimidincs) has been accomplished (Calvin and Calvin, 1964). Many laboratories have been involved and, in many cases, the starting materials and conditions employed can be reasonatly hypothesized to have occuired on a primitive prebiotic earth. In general, these syntheses involved reducing atmospheres (H 2, CH 4, H 2O, etc.) and energy sources sufficient to form free ladicals. Recombination of radicals is thought to give rise to HCN and H 2CO which spontaneously hydrolyze (Calvin and Calvin, 1964) to give a variety of monomers and, on some occasions, even peptides (Moser and Matthews, 1968; Fox, 1965a) The necessary second stage of prebiotic evolution has been less successfully explored experimentally. Here we are concerned with less energetic reactions involving dehydrations and phosphorylations: amino acids to polypeptides, purincs and pyrimidines to nuclcosides, nuclcosides to nucleotides and nucleotides to polynucteotides. The problem is obvious. Dehydration reactions are quite improbable in a dilute aqueous solution such as the primeval ocean. Most investigators (Fox, 1965) have followed the lead of Fox (1965a) in using anhydrous conditions and fairly high thermal energies (above 100 °C). Despite the success of this approach, the probability of such conditions occurring on a massive scale seems, intuitively, to be rather small. On the other hand, the alternating tidal wetting and drying of the oceanic shores is a massive process indeed, and tides date back to the beginnings of the oceans themselves. Moreover, the beaches, at low tide, reach temperatures uncomfortable to the touch on sunny days, even today. The problems with this system are principally two: a mechanism for concentrating solutes is needed and the relatively low temperatures available call for some catalytic process to drive dehydration reactions at appreciable rates. If the beaches themselves could be shown to have such properties, the second phase of prebiotic evolution could be fairly well explained. It seemed worthwhile to examine selected, naturally occurring solids for adsorptive and catalytic properties, particularly with respect to dehydration and phosphorylation reactions. Bernal (1951), among others, has suggested that clays or crystals may have participated importantly in prebiotic evolution. These suggestions were essentially speculative in character, however, and to our knowledge, no relevant laboratory investigation has been reported.