Synthesis of Glycine Oligomers Under Simulated Prebiotic

Conditions on the Surface of Layered Silicate Clay Minerals

    There are still many unanswered questions regarding the synthesis of the first complex biological molecules needed for the emergence of life.   From the simplest organic molecules, amino acids may be formed.  Peptides and polypeptides are then constructed from amino acid units.  These polypeptides then form the proteins necessary for living entities.  The simplest organic molecules were formed by natural means in many different ways.  Amino acids, the building blocks of polypeptides, are known to be synthesized from simple components through the action of heat, electrical discharge, UV radiation, and other mechanisms.  It is likely that many amino acids were found in abundance on the prebiotic Earth.  Amino acids have also been identified in extraterrestrial sources such as comets and meteorites.  How did these abundant amino acids come together to form polypeptides or proteins?

    In 1951, Bernal proposed that simple clay minerals may have played a role in the prebiotic formation of polypeptides.  Many clay minerals are known to adsorb or intercalate organic molecules.  Exchangeable metal cations in the inner regions or on the surface of the clays may then act to catalyze the reactions to form peptides and polypeptides from the adsorbed amino acids.  Indeed, many groups have shown that clay minerals in the presence of amino acids do produce peptide oligomers.

    We have used the technique of scanning force microscopy (SFM) to study the formation of peptide oligomers on the surface of the clay mineral hectorite.  Hectorite is a layered silicate mineral, composed of flat silicate layers sandwiching layers of Mg and Li cations.  Hectorite also contains exchangeable Na cations in its interlayer gallery regions.  For this study, we have exchanged the Na cations for Cu(II) cations, to promote the polymerization reactions.  In Fig. 1 below, an SFM image of a clean Cu(II)-exchanged hectorite surface is shown.  The dimensions are 1000 Å x 1000 Å..  The surface is generally very flat, except for numerous steps and faults (or cracks).  Several step edges of various heights are visible in this image.

    Fig. 1

    The amino acid glycine was applied to the surface, resulting in a coverage of approximately 0.05 monolayers.  Below we see a high resolution image of the hectorite surface after amino acid exposure (Fig. 2).  The dimensions are 500 Å x 500 Å.  No glycine monomer units are resolvable by the SFM instrument.  The step edge shown is 14 Å high as measured by SFM.  Organic material was extracted from this clay film with 0.1 m Calcium Chloride solution, and analyzed by HPLC.  No glycine peptides or higher oligomers were present.

   Fig. 2

    The experiment is then repeated, but now the glycine exposed hectorite film is subjected to alternate cycles of heating (to 90 degrees C) and wetting with ultra-pure water.  This cycling is performed in order to simulate possible prebiotic conditions on the early Earth.  An SFM image of such a cycled film is shown below in Fig. 3.  The dimensions are the same as in Fig. 2.  A step edge (of height 7 Å) is now highly decorated with small glycine peptides or oligomers.  HPLC analysis now indicates the presence of glycine oligomers up to 6 units in length.   The step edge has acted to not only adsorb and concentrate the glycine oligomer, but to facilitate the polymerization of the glycine peptides and polypeptides.  The polymerization is likely due to the availability of gallery Cu(II) cations at the exposed step edges.

   Fig. 3

    In addition to the glycine polymerization at step edge sites, oligomer formation was observed at small micro-pore sites on the clay surface.   An image of a surface site containing numerous micro-pores is shown below (Fig. 4).   The dimensions of this image are 750 Å x 750 Å.

   Fig. 4

The micro-pores function in much the same way as the step edges.   Concentration and constraint of the monomer, as well as providing access to interlayer Cu(II) to oxidize the monomer.  The availability of the Cu(II) cations at step edge and micro-pore sites is crucial to the formation of these peptides or oligomers.   Similar studies using non-exchanged hectorite (Na gallery cations) produce no peptides or polypeptides whatsoever.

    In conclusion, clay minerals may have indeed played a major role in the prebiotic formation of the first peptides or polypeptides.  We have directly observed the condensation of the amino acid glycine into glycine oligomers.   These reactions occur at surface step edges and micro-pore sites, where access to interlayer metal cations is made possible.  We are currently studying the site specific prebiotic polymerization of nucleosides on clay mineral surfaces, as well as the condensation of amino acids of varying sizes and reactivities at step edges and micro-pores.