Clays and the Origin of Life: The Experiments

Abstract

There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolism here on Earth? First, there is the recent finding of a chiral amino acid (isovaline) that formed on the surface of a clay mineral on several carbonaceous chondrites. This points to the formation of amino acids on the surface of clay minerals on carbonaceous chondrites from simpler molecules, e.g., CO₂, NH₃, and HCN. Additionally, there is the catalytic role of small organic molecules, such as dicarboxylic acids and amino acids found on carbonaceous chondrites, in the formation of Fe-clays themselves. Amino acids and nucleotides adsorb on clay surfaces on Earth and subsequently polymerize. All of these observations and more must be subjected to strict experimental analysis. This review provides an overview of what has happened and is now happening in the experimental clay world related to the origin of life. The emphasis is on smectite-group clay minerals, such as montmorillonite and nontronite.

Keywords: Earth; Mars; catalysis; clay; montmorillonite; nontronite; organic reactions; origin of life; saponite; smectite.

Figure 1

Two examples of typical crystal structures of common clay minerals: (a) montmorillonite, a 2:1 type smectite with layers consisting of two tetrahedral sheets and one octahedral sheet with layers separated by the interlayer space with hydrated cations; (b) kaolinite, a 1:1 type clay mineral with layers consisting of one tetrahedral and one octahedral sheet.

Scheme 1

An example of an acid-catalyzed reaction of 2-methyl pent-2-ene with alcohol to 2-methyl-2-alkoxy pentane over Al-montmorillonite catalyst.

Scheme 2

Clay-catalyzed reaction of acetals with enol ethers to produce precursors of α,β-unsaturated aldehydes. R¹ = C₆H₅, CH₃, C₂H₅, n-C₆H₁₃, or -(CH₂)₅-; R² = H or -(CH₂)₅-.

Scheme 3

Esterification of alcohol with montmorillonite catalyst. R = alkyl or aryl or substituted aryl; R” = primary or secondary alcohol.

Scheme 4

Synthesis of N¹,N²-dibenzylideneethane-1,2-diamine using montmorillonite as catalyst.

Scheme 5

An early Diels–Alder reaction is the dimerization of oleic acid on montmorillonite. (It is presumed that only the first step of dehydrogenation is catalyzed by montmorillonite, but not the Diels–Alder addition part itself.)

Scheme 6

Montmorillonite-supported transition-metal salts (zinc and nickel chlorides) catalysis of Friedel–Crafts alkylation.

Scheme 7

Oxidation of benzidine to benzidine blue catalyzed by nontronite. Benzidine blue is a radical cation, and the blue color indicates that both aromatic rings are involved in the resonance structure.

Scheme 8

Decarboxylation and deamination of glycine to serine.

Scheme 9

Essential reactions in the catalytic breakdown of a dye such as Rhodamine B with H₂O₂ and nontronite as solid catalyst.

Scheme 10

Formation of formic acid from CO and H₂O.

Scheme 11

Reaction pathway for the formation of uracil from β-alanine and urea.

Scheme 12

Reaction of serine to form pyruvate using a coenzyme and clay as catalysts.

Scheme 13

Reaction of serine to form pyruvate using clay and coenzyme as catalysts.

Scheme 14

Reaction of glutamic acid and pyridoxal phosphate to form α-ketoglutaric acid with Cu²⁺-smectite as the catalyst.

Scheme 15

Reaction of isocitric acid conversion to α-ketoglutaric acid with Na⁺-montmorillonite as the catalyst.