The Early Earth Atmosphere and Early Life Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 11

Although complete homochirality in the prebiotic chiral monomers may not be
necessary for the initiation of prebiotic homochiral polymer chemistry, a small
initial enantiomeric excess may be required. This small enantiomeric excess could
be produced by some physical chiral influence. On a cosmic scale, enantioselective
mechanisms depending on parity violation or circularly polarized light, are the only
ones that could predetermine a particular handedness. In all other mechanisms, like
a local chiral environment induced by a mineral surface, the ultimate choice would
arise purely by chance [ 22 ]. That life evolved to use L-amino acids in proteins, and
D-sugars in nucleic acids after it began remains a possibility, but there is consid-
erable evidence that the formation of these biopolymers from homochiral material
is necessary for them to function. Usually, in laboratory synthesis, unless special
experimental conditions are used, a racemic (50:50) mixture of the D- and its
L-enantiomer will be obtained when chiral molecules are formed from achiral
starting materials. When a particular reagent or catalyst is added, an excess of
one of the enantiomers can be used to direct the formation of more homochiral
molecules. However, by which mechanism and when the first one-handed building
block for life arose, still remains an open question [ 26 ].
Clay minerals have long been postulated to play a role in concentrating the
products of non-biological organic processes leading to the Huxley-Darwin-
Oparin-Haldane model for the origin of living systems. Because of their large
surface areas and adsorption characteristics, clays can act as catalysts for the
formation of RNA oligomers from activated nucleotide precursors. Furthermore,
montmorillonite clay and alumina both catalyze polymerization of amino acids in
peptide formation reactions [ 27 ]. Chiral selectivity has been demonstrated in
experimental and theoretical studies. The layered structure of clays could also
allow them to play a key role in concentrating, shielding and catalyzing the
assembly of some of the most important molecules necessary for the development
of living systems.
Clay minerals such as montmorillonite, kaolinite and illite (Table 1 ) represent
a naturally abundant class of layered aluminum-silicates that are important in the
context of prebiotic chemistry due to their good biocompatibility, string adsorption,
ion exchange ability, and expansibility [ 28 ], ancient origin and wide distribution.
Different studies have demonstrated that they can absorb a variety of biomolecules
including proteins, DNA, RNA, lipids, purines, pyrimidines [ 29 - 31 ].
Miller-Urey type experiments have been performed in the presence of montmo-
rillonite, resulting in an increased yield of alkylated amino acids [ 32 ], or in the
preferred synthesis of some specific compounds such as glycine, alanine, or aspartic
acid [ 33 ]. Greenland and coworkers [ 34 , 35 ] investigated different adsorption
mechanisms of amino acids on H + -, Na + -, and Ca 2+ -montmorillonite. They noticed
that basic amino acids, such as arginine, histidine, and lysine (Table 2 )were
preferentially adsorbed on Na + -andCa 2+ -montmorillonite by cation exchange,
whereas non-polar amino acids such as alanine, serine, leucine, or phenylalanine,
and acidic amino acids like aspartic acid and glutamic acid were adsorbed on H + -
montmorillonite by proton transfer. They also noticed that the adsorption of glycine
and its oligopeptides onto Ca 2+ -montmorillonite and Ca 2+ -illite increased with the
degree of oligomerization. Hedges and Hare [ 36 ] suggested that the amino and