The Early Earth Atmosphere and Early Life Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 12

carboxyl terminal groups of the amino acids were involved in their adsorption to
kaolinite. Dashman and Stotzky [ 37 , 38 ] reported that kaolinite was less efficient in
the adsorption of amino acids and peptides than montmorillonite. Other clay min-
erals, such as serpentine, can adsorb measurable quantities of aspartic and glutamic
acid [ 39 ], whereas allophane can take up appreciable amounts of alanine [ 40 ].
The adsorption of nucleic acid bases on clay minerals has also been investigated.
Lawless et al. and Banin et al. [ 41 , 42 ] described the adsorption of adenosine
monophosphate (5'-AMP) by montmorillonite in the presence of different metal
cations and variation of the medium acidity. They found a maximum adsorption on
Zn 2+ -montmorillonite at a pH value close to neutrality. Winter and Zubay [ 43 ]
found that montmorillonite adsorbed more adenine than adenosine, 5'-AMP,
5'-ADP, or 5'-ATP, while hydroxyapatite preferably adsorbed 5'-AMP compared
to adenine or adenosine. They also found an adsorption dependence on the acidity
of the medium. Other types of clay minerals have also been examined. Hashizume
and Theng [ 44 ] found that allophane exhibited a greater affinity for 5'-AMP over
adenine, adenosine or ribose.
The transformation of formaldehyde, the simplest aldehyde (HCOH), into
complex sugars like ribose and from there to nucleic acids (e.g., RNA) in experi-
ments simulating the early Earth conditions was proposed by Butlerov in 1861
under the name of formose reaction [ 45 ]. Clay minerals, such as montmorillonite
and brucite can catalyze the self-condensation of formaldehyde and even more
important, montmorillonite can stabilize several of the formed oligomers [ 46 , 47 ].
Fraser and colleagues [ 27 ] have shown that vermiculite clay gels with large
interlayer spacing act as amplifiers that sequentially change the D/L ratios of
solutions containing the amino acids alanine, lysine, and histidine. A proposed
natural ancient scenario for their results could be a clay-rich lagoon containing
simple prebiotic molecules. The continuous repetition of D/L fractionation like the
one they found, over geological periods of time, could lead to clay/solution
nanofilm multilayers with amino acids having a chiral excess. Alternatively, chiral
excess could have originated under the physicochemical conditions of interstellar
space, or could have been produced in a modified type of Miller-Urey synthesis in
the presence of polarized light or dust particles with chiral centers. Such activated
clay films have the capacity to concentrate and to induce significant chiral separa-
tion of amino acids. They propose that such a mechanism could be the reason for the
chiral selectivity observed for the rare amino acid isovaline in the Murchinson or
Orgueil meteorites [ 27 ]. The discovery of isovaline (2-amino-2-methylbutanoic
acid) in the biosphere suggests an extraterrestrial origin of amino acids and has
been linked to the homochirality of life on Earth [ 50 ].
4 General Conclusions
Clay minerals formed since early geological times on Earth and were widely
distributed. Due to their catalytic and adsorption properties, they have played a
significant role in the search for an explanation of a biology dominated by L-amino