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could have been one of the causes of the L-homochirality of the protein amino acids.

Nevertheless, quoting Bonner (1991): ‘‘it should be emphasized that none of Julg’s

speculations and conjectures is supporte d by a single shred of experimental

evidence’’.

It must be noted that the stimulating hypotheses of Cairns-Smith (1982) empha-

sizing the role of clays in the origin of life do not account for the crucial one-

handedness of biopolymers required to ensure the survival of self-replicating organic

systems (Avetisov et al., 1991).

Obviously, the possibility that clays can discriminate between optica l isom ers of

amino acids attracted a great deal of interest and hopes but at the same tim e gen-

erated much controversy about false positives. No doubt is attached to results

obtained when clay minerals are coated with chiral adducts, such as phenanthroline

tris-chelated nickel ions (Yamagishi, 1985). For non-coated clay minerals, there is

always a serious risk of biological contamination or imprinting by bacteria or bio-

ﬁlms. Even if the puriﬁcation is pushed to the extreme, biological imprinting of the

clay during its geochemical formation is still possible. Repeating the experiments

with artiﬁcial clay minerals appears to be the best way to legitimate the data.

7.4.3. CLAYS AS PREBIOTIC CATALYSTS

By analogy with contemporary living systems, primitive life is often considered to

have emerged as a cellular species, requiring boundary molecules able to isolate the

system from the aqueous environment (membrane). Also needed would have been

catalytic molecules to provide the basic chemical work of the cell (enzymes) and

information-retaining molecules that allow the storage and the transfer of the

information needed for replication (nucleic acids).

Formally, the syn thesis of polymers of nucleotide and amino acids appears sim-

ple. The condensation reaction consists of eliminating water molecules between

monomer units, thus linking them together. However, the formation of either pro-

teins or nucleic acids from their monomers in water is not energetically favoured.

For example, the peptide bond of proteins is thermodynamically unstable in water,

so energy is required to link two amino acids together in an aqueous media; and for

example, the free energy needed for two amino acids to form the dipeptide in water is

about 4 kcal/mol at 37 1C and pH 7. The equilibrium constant of the reaction is only

about 10

–3

, and the equilibrium concentration of the dipeptide for 1 M solution of

the free amino acids is only slightly above 10

–3

M. The thermodynami c barrier is very

large for the formation of a long-chain polypeptide. Dixon and Webb (1958) pointed

out that 1 M solution in each of the 20 proteinaceous amino acids would yield at

equilibrium a 10

–99

M concentration for a 12,000 Dalton protein. The volume of this

solution would have to be 10

times the volume of the Earth to yield one molecule

of protein at equilibrium. So energy input, such as heat or chemical activation,

is necessary to have made nucleic acids and polypeptides on the primitive Earth.

7.4.3. Clays as Prebiotic Catalysts 383

Polymers can be prepared in the presence of water if chemical condensing agents are

added or if activating groups are bound to the reacting monomers to provide the

requisite free energy for bond formation. In both cases, it is important to limit the

competing hydrolysis of the activating groups. Polymer formation on clays is a

solution to this problem.

7.4.4. CLAY-CATALYSED SYNTHESIS OF RNA

There is general consensus that RNA is the most important biopolymer in early

life on Earth (the RNA world), since even modern peptide bond formation (protein

biosynthesis) in the ribosome was found to be catalysed by RNA and not by protein

enzymes (Ban et al., 2000). DNA is believed to have appeared after RNA and to

have later derived from RNA for the following reasons:

(i) The ribose structural unit present in RNA monomers is probably formed

from formaldehyde or an oligomer of formaldehyde. The prebiotic synthesis of

the deoxyribose of DNA requires the reaction of a mix of starting materials.

(ii) The modern biosynthesis of DNA triphosphates proceeds from RNA

triphosphates using an unusual enzyme that apparently evolved after RNA

formed.

(iii) RNA, in contrast to DNA, exhibits catalytic activity as well as information

storage, and thus could have served both as a catalyst and storehouse of genetic

information in the ﬁrst life (Zaug and Cech, 1986).

Initial attempts to form RNA by heating monomers or by using condensing agents

does not yield long oligomers. Polymerization studies, which use activated mon-

omers, were the most successful and these will be reviewed he re, keeping in mind

that, in most instances, potential prebiotic sources of the activated monomers

remains to be discovered.

One of the successful applications of Bernal’s (1949) proposal is the observation

that oligomers containing 6 to 14 monomer units are formed when RNA monomers

ImpA (Scheme I) activated on the phosphate with an imidazole group condense in

the presence of montmorillonite (Ferris and Ertem, 1993).

The formation of oligomers was generalized to the other nucleotide bases, cyto-

sine, guanine, inosine and uracil. Catalysis has the potential for limiting the number

of isomers formed. For example, the natural 3

-linked phosphodiester bond is

favoured in the reaction of purine nucleotides on montmorillonite. The 2

-link is

favoured in the absence of catalysis and in the clay- catalysed reactions of pyrimidine

nucleotides. Analysis of the dimers formed in the reaction of mixtures of two or more

activated nucleotides demonstrate strong sequence selectivity of the dimers formed

(Ertem and Ferris, 2000). The 5

-purine-pyrimidine sequence is favoured over the

-pyrimidine-purine sequence at the end of the polymer chain by a factor of about

20. In addition, ﬁve 5

-sequences (A–C, A–U, G–C, A–A and GA) are formed in

significantly larger amounts (73% total yiel d) than the eleven others in the reaction

Chapter 7.4: Clay Minerals and the Origin of Life384

of mixtures of the four activated monomers of the nucleotides A, C, G and U. The

formation of short RNA oligomers by montmorillonite catalysis is the ﬁrst step in

the preparation of the RNAs needed for the initiation of the RNA world. In the

second step, it was shown that it is possible to generate longer RNAs by elongation

of short oligomers (Scheme II) using the ‘‘feeding’’ protocol (Ferris et al., 1996;

Ferris, 2002).

The decanucleotide (pdA)

pA bound to Na

-montmorillonite is fed with ImpA

and the reaction mixture is allowed to stand for one day at 25 1C. The suspension is

then centrifuged to remove the aqueous phase from the montmorillonite. Fresh

ImpA solution is added to the clay–oligomer complex and the reaction allowed to

proceed for another day. Polyadenylates containing more than 20-mers are formed

after feeding twice with ImpA, with the main products being 11–14-mers. Polynuc-

leotides containing more than 50-mers are formed after 14 feedings, with the prin-

cipal oligomeric products containing 20–40 monomer units. 25–30-mers are obtained

with the corresponding uridine activated monomer (Ferris, 2002).

The reaction of

D,L-ImpA shows partial inhibition by the monomer unit of the

opposite handedness. The longest oligomer formed is an 8-mer, while the longest

oligomer formed from one enantiomer is a 10-mer (Joshi et al., 2000; Urata et al.,

2001). The linear dimers formed from racemic mixtures of ImpA and ImpU exhibit a

60:40 ratio of the

D–D and L–L dimers to D–L and L–D dimers formed (Joshi et al.,

2000).

Adenine

3' 2'

Scheme I. RNA activated monomer ImpA with the 2

and 5

positions for the phos-

phodiester links.

(pdA)

HO OH

Adenine

3' 2'

ImpA

(pdA)

pApA

etc.

ImH

Scheme II. Elongation of a decanucleotide by reaction with ImpA.

7.4.4. Clay-Catalysed Synthesis of RNA 385

7.4.5. POLYPEPTIDE FORMATION ON CLAY MINERALS

Clays can also be used to condense amino acids in water using chemical activation,

temperature/moisture cycles or both of them. A model for the prebiotic formation of

polypeptides is based on the contemporary biosynthesis of proteins, which proceeds

via the chemically activated monomer aminoacyladenylate (Scheme III).

The aminoacylphosphate derivatives of 5

-AMP condense to polypeptides on

montmorillonite (Paecht-Horowitz et al., 1970; Paecht-Horowitz and Eirich, 1988).

The products are polypeptides as long as 56-mers in which the C-terminal group of

the peptide is attached to the 2

- and/or 3

-hydroxyl group of 5

-AMP. In a homo-

geneous aqueou s solution, mixed anhydride alanyladenylate condense partially up to

heptaalanine but deactivation via hydrolysis remains the main pathway. It was

initially reported that aminoacyladenylates are formed by the reaction of amino

acids with ATP in the presence of a zeolite, but it is not possible to reproduce this

synthesis (Warden et al., 1974).

Glutamic acid, an acidic amino acid containing an additional carboxylic acid

group on the side-chain, was subjected to the feeding protocol using the condensing

agent, carbonyldiimidazole (Scheme IV ). The activation proceeds via the interme-

diary formation of an N-carboxyanhydride (Ehler and Orgel, 1976; Brack, 1987).

Illite was incubated with the amino acid and carbonyl diimidazole until short

oligomers are formed. The solid is separated by centrifugation and fresh monomer

and activating agent are added. The process is repeated as often as necessary for the

HO OH

Adenine

OCHR

Scheme III. Aminoacyladenylate.

HOOC

-CH(CH

-CH

-COOH)-COOH ++2 ImH

Scheme IV. Formation of the N-carboxyanhydride of glutamic acid with carbonyl

diimidazole.

Chapter 7.4: Clay Minerals and the Origin of Life386

production of oligomers of lengths of 40-mers or more. In the absence of clay,

oligomers up to the 10-mer are detected, but the majority of the products are shorter

than the 5-mer. In the presence of illite, the shorter oligomers remain in the supe-

rnatant while the longer oligomers adsorb to the clay mineral. After 50 feedings,

oligomers up to at least 55-mer are detected, the bulk of the adsorbed product being

in the 30–50 size range (Ferris et al., 1996; Hill et al., 1998). Illite failed to produce

substantial amounts of aspartic acid, the other acidic amino acid (Hill et al., 1998).

The feeding protocol is not restricted to negatively charged amino acids. Oligoargi-

nines are accumulated on the surface of illite using carbon yl diimidazole. In the

absence of a mineral, the longest detectable oligome r is the 6-mer. In the presence of

illite, oligomers up to the 12-mer were detected after 10 cycles (Liu and Orgel, 1998).

The formation of long polypeptide chains from neutral amino acids (ones that are

neither acidi c nor ba sic) was not yet accomplished.

Fluctuating moisture and temperature cycles are used to polymerize amino

acids in the presence of clays. Lahav et al. (1978) subjected mixtures of glycine and

-kaolinite or Na

-bentonite to wet-dry and temperature ﬂuctuations

(25–94 1C) and observes the formation of oligopeptides up to ﬁve glycine residues

in length. Only trace amounts of diglycine form without clays. White and Erickson

(1980) studied the effects of the dipeptide histidyl–histidine in the polymerization of

glycine during ﬂuctuating moisture and temperature cycles on kaolini te. A turnover

of 52 is observed, i.e. each molecule of dipeptide helps the polymerization of

52 molecules of glycine. The drying/wetting cycles at 80 1C are extended to alanine in

the presence of smectites (montmorillonite and hectorite) (Bujda

k and Rode, 1997).

Only 0.1% of alanine dimerizes on hectorite and no reaction proceeds on mont-

morillonite. Clay minerals more efﬁciently catalyse peptide chain elongation than

amino acid dimerization. The reaction yields of tripeptides from diglycine and cyclic

diglycine reached about 0.3% on montmorillonite and 1% on hectorite.

Different amino acids thioesters were polymerized as shown in the following

equation, in the presence of montmorillonite (Bertrand et al., 2001)

2CHR2CO2S2C

! H2ðNH2CHR2COÞ

2S2C

þ n HS2C

Thioesters represent a moderate, prebiotically plausible, amino acid activation

(Weber and Orgel, 1979; Weber, 1998) and have been shown to adsorb onto mont-

morillonite. They easily form in warm, acid and sulphur-rich environments. They

play a key role in modern metabolisms, e.g., acetyl-coenzyme A, and could be the

origin of a prebiotic energy transfer (De Duve, 1998 ). In the presence of clay, the

formation of the cyclic dipeptide, the main product formed in the control reaction, is

totally inhibited. However, no oligomer longer than the tetrapeptide could be ob-

tained. It is likely that thioesters remain inserted in the clay miner al layers since they

could not be detected in the supernatant. Leu–S et subjected to wetting-drying and

temperature (25–80 1 C) cycles, polymerizes up to the heptapeptide in the presence of

montmorillonite.

7.4.5. Polypeptide Formation on Clay Minerals 387

7.4.6. UNRESOLVED QUESTIONS AND CONCLUSIONS

Since Bernal’s suggestion that clays could have participated in the processes

leading to primordial life, mineral surfaces were endowed with exceptional virtues

such as the possibility of hosting a primitive mineral life or of having generated the

biological one-handedness. So far, these exceptional virtues were not legitimized by

any experimental data.

On the contrary, clay minerals were shown to be excellent catalysts promoting the

formation of biopolymers of the ﬁrst life in an aqueous environment, as demon-

strated by the remarkable experiments run by the group of Orgel at the Salk Institute

in San Diego, California (Hill et al., 1998; Liu and Orgel, 1998; Joyce and Orgel,

1999) and by the group of Ferris at the Rensselaer Polytechnic Institute in Troy,

New York (Ferris and Ertem, 1993; Ferris et al., 1996; Ferris, 2002). The elongation

of RNA to chains longer than 40-mers could have provided the RNAs that initiated

the RNA world. It was proposed that such RNAs with chain lengths greater than

40-mers would have been able to replicate by template-directed synthesis with suf-

ﬁcient ﬁdelity to maintain the core information content of their sequences (Joyce and

Orgel, 1999). In addition, it was postulated that a 40-mer is the minimum chain

length required for RNA to catalyse the reactions of other RNA molec ules (Joyce

and Orgel, 1999; Szostak and Ellington, 1993). The longest chains of both poly-

nucleotides and polypeptides are obtained by using the feeding protocol that

represents a plausible model for pr ebiotic polymerization, simulating rocks that are

in constant contact with low concentrations of activated monomers, or that are

episodically washed with higher concentrations.

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Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

393

Chapter 7.5

PILLARED CLAYS AND CLAY MINERALS

F. BERGAYA, A. AOUAD AND T. MANDALIA

CRMD, CNRS-Universite

d’Orle

ans, F-45071 Orle

ans Cedex 2, France

7.5.1. PILLARING CONCEPT

The term ‘pillaring’ is often associated with the formation and preparation of

catalytically active microporous materials. Thus, many reviews devoted to ‘pillared

interlayered clays’ (PILC

) are published in catalysis journals (Pinnavaia, 1984;

Burch, 1988; Figueras, 1988; Fripiat, 1997; Gil et al., 2000a) although some recent

summaries appeared in journals on porous materials (Kloprogge, 1998; Gil et al.,

2000b; Ding et al., 2001), and as chapters or subchapters in books on clay minerals

(Bergaya, 1990; Konta, 2001). Most of these publications deal with the history,

synthesis, properties, and applications of the PILC.

The pillaring concept was extended to other layered materials (Vaughan, 1988a;

Mitchell, 1990), such as manganese oxide (Wong and Cheng, 1993) and LDH (see

Chapter 13.1). This chapter is an attempt to summarize the vast volume of literature

on (cationic) clay minerals that has accumulated over the past 30 years.

7.5.2. PILLARED CLAY MINERALS AND CATALYSIS

That clays can function as catalysts was known for a very long time (Robertson,

1986). For example, Gurvich (1915) used atta pulgite (palygorskite) and mont-

morillonites to polymerize pinene. The most widely known application of clay ca-

talysis is the French Houdry cracking process developed in the 1930s (Houdry et al.,

1938). By the 1960s, clays were replaced by synthetic zeolites (Breck, 1980) because

the latter show better activity and selectivity for cracking. However, PILC (which

may be co nsidered as two-dimensional zeolite-like materials) began to compete with

zeolites in the 1970s. Intensive research into the synthesis of suitable cracking cat-

alysts for heavy petroleum was stimulated by the ‘oil shock’ of 1973. This is because

DOI: 10.1016/S1572-4352(05)01012-3

The term ‘PILC’ is not strictly correct. Since it is the clay mineral that is pillared, the term ‘PILCM’

(for pillared interlayered clay mineral) should be used. However, by analogy with using ‘clay’ for ‘clay

mineral’ (see Chapter 1), PILC will continue to be used on the basis of past usage.