For decades, academic studies have tried to answer the logical question: do chiral crystals on Earth occur in racemic ratios or in an enantioenriched, if not enantiopure, form. Is there any enantioenrichment to be measured in crystals, at least locally,
10 The letter was originally published in Science News (June 17, 1989).
that might have caused a chiral bias in biomolecules today? The equal versus non-equal distribution of chiral crystals on Earth has been studied especially concerning d-(+)- and l-(—)-quartz enantiomorphs (Klabunovskii 2001). As we have outlined above, quartz is morphologically chiral and only one of the many chiral crystalline silicates to be found as minerals on the Earth's surface. Each of these minerals is assumed to exhibit, to differing degrees, analogous enantiomorphous composition (Tranter 1985a).
The asymmetric catalytic function of enantiomorph crystals on Earth was proposed as one of many scenarios to have introduced the first enantioenrichments into chiral organic molecules during chemical evolution (Klabunovskii 1963; Thiemann and Teutsch 1990; Wachtershauser 1991). The mechanisms of this process are known and have been reproduced in the laboratory (Bonner et al. 1974; Bonner et al. 1975a; Bonner and Kavasmaneck 1976; Bonner et al. 1981; MacDermott 1995; Bonner 1995b; Soai et al. 1999).
The distribution of d-(+)- and l-(—)-quartz crystals on the surface of Earth integrated over all locations are be expected to be equal (Klabunovskii and Thiemann 2000). Lemmlein (1973) detected an equal abundance of d-(+)- and l-(—)-quartz in 10 000 samples from 19 locations. It should be mentioned that this statement is still a subject of hot debate (Klabunovskii and Thiemann 2000), because other authors have detected a worldwide excess of 50.5% l-(—)-quartz in 16 807 samples (Palache et al. 1962) and 49.83% l-(—)-quartz in 27 053 samples (Frondel 1978).
In local deposits, however, the formation of an excess of d-(+)- or l-(—)-quartz is considered to be possible if the crystallization of quartz proceeds in the presence of fossil materials with (+)- or (—)-rotation or if a seeding effect gives rise locally to a large preponderance of one enantiomorph. In quartz samples from Plakas, Greece, the occurrence of 60.6% l-(—)-quartz in 549 samples was reported, whereas 300 crystals collected in Samshvildo, USSR, showed the occurrence of 56.0% d-(+)-quartz (Klabunovskii and Thiemann 2000). Based on these data, the conclusion was drawn that the distribution of d-(+)-quartz and l-(—)-quartz over the surface of the Earth is random and any enantioenrichments in chiral organic compounds being generated under the influence of optically active quartz crystals would occur locally and be randomly distributed.
In the context of enantiomorph crystals and an eventual global excess of l-(—)-quartz, it is interesting to note that d-(+)-quartz and l-(—)-quartz do not necessarily show exactly the same electronic energy. As we will discuss in more detail in Chap. 5, the weak nuclear interaction gives rise to small energy differences called parity non-conserving energy differences that can be calculated for a chiral molecule and its mirror image enantiomer. On this background, the energy of an enantiomorph crystal varies by a tiny amount compared with its mirror image enantiomer. Quantum mechanical ab initio calculations for quartz crystals showed that, per SiO2 unit, the l-(—)-quartz crystal is energetically stabilized by 2.24 • 10—20 atomic units (a.u.) versus its d-(+)-enantiomorphous form. Interestingly, an excess of 1.4% of l-(—)-quartz crystals over d-(+)-quartz crystals requires 1015 SiO2 units, which corresponds to a typical quartz crystal with a side length of 0.1 mm (MacDermott and Tranter 1989; MacDermott 1993). Nevertheless, the majority of involved scientists currently assumes that enantiomorphous forms of d-(+)-quartz and l-(-)-quartz are equally distributed on Earth.
Despite such discussions, chiral quartz crystals themselves are inert and not cata-lytically active, and therefore are unlikely to have been important prebiotic catalysts. Other chiral minerals might have a higher potential to serve as catalysts in prebiotic reactions as we will see in the next section.
Was this article helpful?