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. 2001 May 8;98(10):5487-90.
doi: 10.1073/pnas.101085998. Epub 2001 May 1.

Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical homochirality

R M Hazen  1 , T R Filley, G A Goodfriend
Affiliations

Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical homochirality

R M Hazen et al. Proc Natl Acad Sci U S A. .

Abstract

The emergence of biochemical homochirality was a key step in the origin of life, yet prebiotic mechanisms for chiral separation are not well constrained. Here we demonstrate a geochemically plausible scenario for chiral separation of amino acids by adsorption on mineral surfaces. Crystals of the common rock-forming mineral calcite (CaCO(3)), when immersed in a racemic aspartic acid solution, display significant adsorption and chiral selectivity of d- and l-enantiomers on pairs of mirror-related crystal-growth surfaces. This selective adsorption is greater on crystals with terraced surface textures, which indicates that d- and l-aspartic acid concentrate along step-like linear growth features. Thus, selective adsorption of linear arrays of d- and l-amino acids on calcite, with subsequent condensation polymerization, represents a plausible geochemical mechanism for the production of homochiral polypeptides on the prebiotic Earth.

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Figures

Figure 1
Figure 1
(A) The common [21formula image1] trigonal scalenohedral (dog-tooth) form of calcite features adjacent crystal faces with enantiomorphic surface structures [after Dana (12)]. The markedly acentric surface structures of both the (3formula imageformula image1) face (B) and the (21formula image1) face (C) consist of corner-linked chains of CaO6 octahedra, cross-linked by planar CO3 groups, which are seen almost on edge. The l-aspartic acid is observed to adsorb preferentially on the (3formula imageformula image1) face, whereas d-aspartic acid adsorbs preferentially on the (21formula image1) face.
Figure 2
Figure 2
The d/l values for aspartic acid adsorbed on calcite crystals nos. 1 and 2, which display a finely terraced surface structure, differ for different faces. The R-type faces generally display d/l > 1, whereas L-type faces display d/l < 1. Rhombohedral (1formula image0formula image)-type cleavage faces (denoted C) show no significant d or l selectivity. The horizontal dashed line represents the observed 0.9948 ± 0.0015 d/l value for the aspartic acid experimental solution, based on four separate derivatizations of a 0.05 M solution.
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