Stereoselectivity of montmorillonite in the adsorption and deamination of some amino acids

Abstract

A B S T R A C T:Optical isomers deamination of L-and D-glutamic and aspartic amino acids and of their DL racemic mixtures has been achieved in the presence of Na-montmorillonite at pH = 6 and room temperature. The adsorption curves showed that the enantiomer adsorbed depends on the type of amino acid. Nevertheless, deamination reaction kinetics brought about a stereoselectivity of the clay mineral for the L-isomer and implicitly showed an unquestionable "structural chirality character" of the clay mineral.Since Bernal's hypothesis (1949) on the possible implication of clay minerals in the origin of life, several investigators (Paecht-Horowitz et al., 1970; Cairns-Smith, 1974, 1975 Lahav etal., 1978; Coyne etal., 1981; Coyne, 1985;Yamagishi, 1981 Yamagishi, , 1982Yamagishi, , 1983)have attempted to understand the action of clay minerals in the transformation and evolution of organic and bio-organic matter. Many investigators showed that the swelling clay minerals, especially montmorillonite, would be able to assume the role of a pseudo-enzyme (Mortland, 1984; Siffert & Naidja, 1987; Naidja & Siffert, 1989, 1990. The question which persists concerns the stereoselectivity of clay minerals in biochemical reactions, and in particular that raised by Degens et al. (1970) as to the cause of the L homochirality of amino acids in living organisms. It is necessary to note that organisms generally contain amino acids in their L-enantiomer form (Lavollay, 1980). Degens et al. (1970) reported a remarkable finding in connection with clay minerals, namely, that L,D-and D,L-aspartic acids in aqueous solution were polymerized at 90~ to differing extents by the catalytic action of kaolin. Thus the L-enantiomer was reported to polymerize to the extent of 25% and the D-isomer to the extent of only 3%. The authors concluded that an asymmetric synthesis of polypeptides had been accomplished on the clay surface and that polymers containing L-amino acids were formed preferentially to those containing D-enantiomers. A later report by Jackson (1971) purported to confirm these asymmetric polymerization findings, to show that the polymerization of aspartic acid failed to occur in the absence of kaolin as catalyst. Jackson also reported that L-phenylalanine was adsorbed by kaolinite to a higher extent than D-phenylalanine. In contrast to these observations, Bonner & Flores (1973) could find no evidence for the differential adsorption of D-versus L-phenylalanine by kaolin from either pH = 6 or pH = 2 solutions, using optical rotatory dispersion, gas chromatography and thin layer chromatography. However, optical rotatory dispersion measurements are unfortunately approximately within the experimental error of the observed rotations.9 1992 The Mineralogical Society

Enantioselective sorption of the chiral fungicide metalaxyl on soil from non-racemic aqueous solutions: Environmental implications

“…This is because the R-enantiomer would be preferentially sorbed over the S-enantiomer within a sorbed layer already dominated by other R-metalaxyl molecules. In other words, the enantioselectivity of the soil surfaces for Rmetalaxyl would have been self-induced by the sorbate [42]. A similar behavior was reported by Kühnle et al [43] for the adsorption of cysteine on a (110) gold surface, where adsorbed DL dimers were calculated to be energetically less stable than DD or LL homochiral dimers, which suppressed the formation of DL dimers at the gold surface.…”

First evidence for a stereoselective incorporation of nonylphenol diastereomers in soil-derived organo-clay complexes

“…For example, the enantioselective degradation of a-HCH by marine bacteria has been identified (Hühnerfuss et al 1992). Stereoselectivity has been reported also for adsorption of amino acids on clay minerals (e.g., Siffert and Naidja 1992), for which implications for the origin of life have been discussed (e.g., Zaia 2004). Further on, it is also well known that physiological effects of chiral organic compounds can often be related to only one enantiomer.…”

Catalysis in the Primordial World

“…92 L-glutamic acid is better adsorbed on Na + -montmorillonite (pH = 6.0) than its D-isomer, but quite opposite holds for aspartic acid, and both amino acids proved more reactive to deamination in their L-forms. 93 The problem is complex not only because of the different mechanisms of adsorption and polymerization of different amino acids, but also because different enantioselectivity could be observed even on different crystal faces of the same mineral. 94 At any rate, enantioselective adsorption and polymerization of amino acids on mineral surfaces cannot be denied, and it was even explained by "occasional chirality" of clay crystal lattice 70 or by stacking of the optically active ions in the interlayer space, 95,96 but the connection with biological homochirality remains obscure.…”