Isomer-Selective Adsorption of Amino Acids by Components of Natural Sediments

Abstract

We present evidence that under circumstances of low pH and organic-free surfaces an ordinary estuarine sediment can exhibit strong optical isomer selectivity in its absorption of a number of amino acids. This selectivity can also be seen to a lesser degree in the minerals quartz, montmorillonite, and kaolin. Adsorption reactions were performed with racemic amino acid mixtures, and after equilibrium, deviations from a D/L ratio of 1 were measured and in many cases were found to be significant. This was particularly pronounced at pH 4.0, where selective removal of the L isomers by adsorption onto sedimentfractions was almosttotal. Changes in both the nature and degree of selectivity were also observable in different sediment size fractions. While we are at this stage unable to identify the mode of primary selectivity, adsorption experiments with these candidate sediment components, quartz, kaolin, and montmorillonite do exhibit some selective behavior. We believe that the existence of natural chirally selective components in sediment may indicate a new approach to the development of chiral catalysis and synthesis.

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

“…Thus, questions remain as to whether soil components assemblages are net "isomer-blind" at the macro-scale in their sorption of chiral compounds or whether chiral constituents, e.g. L-amino acids forming proteins, D-ribose and D-deoxyribose units forming ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), or even chiral minerals [20,22], could lead to the enantioselectivity that is evident in biological systems [23].…”

“…In this regard, although several authors have reported results of batch sorption experiments where different chiral compounds seemed to exhibit strong enantioselectivity in their sorption on natural soil and sediment materials [23][24][25], others have failed to identify sorption enantioselectivity [21,[26][27][28]. Since sorption is a key process determining the amount of pesticide available for other transport and degradation processes, enantioselective sorption to chiral soil, sediment, or aquatic components would greatly affect the fate of chiral pesticides in the environment by making one of the enantiomers more available than the other [2,9,17].…”

“…2) are in contradiction with this possibility, since greater affinity of the soil surfaces for the R-enantiomer should have resulted in different sorption of the individual enantiomers also from racemic initial solutions. For example, after equilibrating racemic amino acid solutions with mineral sorbents and sediment samples, Wedyan and Preston [23] found solution D/L ratios significantly different from 1 and proposed the existence of natural chirally selective components in sediment. This behavior, however, is not seen in Figure 2a, because, as observed in earlier studies [18,21,27,28], the differences in sorption of the R-and Senantiomers from initial racemic metalaxyl solutions were not statistically significant (p> 0.05).…”

Probing the Enantioselectivity of Chiral Pesticides

“…For example, enantioselective sorption to chiral clays or chiral soil, sediment, or aquatic organic matter could affect the fate of chiral compounds in the environment (29 ).…”

Environmental behavior of the enantiomers of the chiral fungicide metalaxyl in Mediterranean agricultural soils

“…Even though the ability of certain soil components with chiral crystalline structures (quartz) or chiral surface properties (calcite) to enantioselectively adsorb different chiral compounds was reported long ago (Bonner and Kavasmaneck, 1976;Hazen et al, 2001), the enantioselectivity of pesticide sorption in soil is difficult to identify and predict due to the heterogeneity and complexity of organomineral soil colloids and also to the fact that the different enantiomeric morphologies of soil components can be equally abundant (Frondel, 1978, Matallo et al, 1998. Thus, questions remain as to whether soil components assemblages are net "isomer-blind" at the macro-scale or whether inherent selectivity could lead to the homochirality that is evident in biological systems (Wedyan and Preston, 2005). In this regard, many biological structures are chiral and, consequently, are good candidates to display a chiral preference.…”