Adsorption of Chiral Organic Molecules on Enantiomorphous Crystals

We have seen that crystals can be chiral and that such chiral crystals can originate from the crystallization of a racemic solution or melt in laboratory experiments and also in deposits of enantiomorphous crystals on Earth. Locally, high excesses of enantiomorphous crystals have been recorded. The next question will be whether organic molecules such as amino acids can enantioselectively interact on mineral surfaces of enantiomorphous crystals via adsorption or similar processes in order to transfer the chiral information from the crystal lattice to the organics on its surface.

The preferential adsorption of one amino acid enantiomer out of a racemate onto one of the morphologically d-(+)- or /-(-)-crystal surfaces of quartz minerals was first demonstrated to be feasible by Bonner et al. (1974) at Stanford University. The asymmetric adsorption phenomenon, however, required that non-aqueous solvents were employed. Experimental conditions rigorously excluded moisture, making this particular reaction therefore implausible in any realistic prebiotic environment.

Particular attention should be given to an experiment that was recently performed at the Carnegie Institution at Washington, DC. Robert Hazen and his team (2001) studied a geochemically plausible scenario for the origin of biomolecular asymmetry by selecting the common rock- and sediment-forming mineral calcite (CaCÜ3) with its rhombohedral space group R3c to study its individual interaction with D- and L-amino acids. It was known before that particular enantiomers of amino acids can be selectively incorporated into opposite enantiotopic faces of centrosym-metric crystals (Weissbuch et al. 1984). A peculiarity of the calcite crystal is that it is not chiral at all but its scalenohedral faces possess surface structures that have the potential for chiral selectivity. The surfaces themselves are not chiral.

After rigorous surface cleaning, calcite crystals with surface areas of 6-36 cm2 were dipped into a 0.05 molar racemic aspartic acid solution. The adsorbed aspar-tic acid was removed after 24 h from the calcite crystal by pipetting and subjected to enantioselective gas chromatography on a Chirasil-Val phase. As illustrated in Fig. 4.7, a chiral selectivity in the adsorption of D- and L-enantiomers was observed to be of 10% difference for the two enantiomers (partly up to 40%).

Accompanying computational work of Hazen and Asthagiri (2007) was performed recently on ab initio calculations of alanine and aspartate adsorption on calcite 214 surfaces. They found, in coherence with the above experimental data, that alanine has a 2-point interaction and, therefore, no chiral selection, whereas

Fig. 4.7 Selective adsorption of l-aspartic acid (left) and d-aspartic acid (right) enantiomers on different scalenohedral calcite faces. Such a sorting process could explain why life selected l-amino acids for the construction of proteins (Hazen 2001)

D-aspartate has a strong 3-point binding to right-handed 214-type faces, but only 2-point binding to left-handed faces, hence the strong selective adsorption.

Hazen (2001) proposed that the results might be extrapolated to other amino acids and sugars, and concluded that polymerization reactions of amino acids on calcite surfaces represent a plausible geochemical mechanism for the production of homochiral polypeptides on the prebiotic Earth.

Besides calcite crystals, other minerals have been proposed to have violated molecular parity in primordial organics. In particular, pyrite crystals attracted our interest, since pyrite (FeS2) has been linked to prebiological carbon fixation by providing both the energy source and the mineral surface for bonding the organic products of carbon fixation. Pyrite crystals are of crucial importance for surface metabolism in the frame of the popular "iron-sulphur world" proposed by the landmark theory of Giinter Wachtershauser (1992) for a chemoautotrophic origin of life on Earth. Gunter Wachtershauser is Honorary-Professor at the University of Regensburg and European Patent and Trademark Attorney in Munich. Interestingly, and hitherto ignored by the scientific community, pyrite crystals occur not only in the achiral cubic space group Pa3, but also in a triclinic space group P1. The triclinic space group was attributed to the low temperature formation of pyrite in sedimentary rocks or hydrothermal ore deposits. Triclinic crystals of space group P1 are enan-tiomorphous; they occur as l-pyrite and d-pyrite crystals (Wachtershauser 1991). The achiral space group Pca2i classifies a third orthorhombic modification of pyrite crystals.

Wachtershauser (1991) suggested that in a pyrite-pulled chemoautotrophic origin of life,11 the organic constituents are seen as being produced in intimate contact with the pyrite surface and, in fact, concomitantly with the growth of the asymmetric pyrite crystal. This means that the homochirality of a given pyrite crystal was expected by Wachtershauser to be transferred with a high enantioselectivity into the organic constituents formed on its surface.

But what have been the concrete experimental results of iron-sulphur world simulations in the laboratory up to today? The formation of a carbon-carbon bond was successfully proven to be possible in simulated iron-sulphur world conditions using activated acetic acid (Huber and Wachtershauser 1997). However, hitherto no chiral molecule had been produced under such conditions even in the racemic state, not to mention any enantiomeric excesses induced by pyrite crystals in their chiral triclinic modification. We have thus no experimental proof yet that the chirality from the interior of a pyrite crystal might have been transferred to any prochiral or racemic molecule on its surface.12 This beautiful suggestion remains a hypothesis only.

In addition to calcite and pyrite minerals, organophilic silica surfaces in the form of zeolites were proposed as host structures for the enantioselective interaction with prebiotic organic guest molecules. Particularly dealuminated zeolites with laevo/dextro-oriented channels were suggested as mineral surfaces involved in the asymmetric origin of life (Smith 1998). Experimental data on this proposal are, however, not yet available.

In summary, the supramolecular induction of an enantiomeric enhancement using natural and synthetic chiral crystals as hosts provides us with a method for transferring chirality to a prochiral substrate through host-guest interactions. Even though the factors and mechanisms that govern the product chirality and enantiomeric enhancement are not sufficiently clear, and the optical yields obtained are not satisfactorily high at present, it should be emphasized that the supramolecular induction of an enantiomeric enhancement exhibits several phenomena that are completely different from conventional chirogenesis performed in isotropic media and are thus worthy of future research (Wada and Inoue 2004).

11 Chemoautotroph organisms chemically fix all their carbon constituents from carbon dioxide or other Ci -units in contrast to heterotroph organisms that are dependent on taking up organic carbon compounds as food.

12 In November 1999, during a personal communication at the University of Bremen, G. Wachtershauser himself did not attribute a causal function to chiral pyrite crystals to biomole-cular homochirality.

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