We have already outlined that biomolecules like DNA, proteins (enzymes), and also lipids1 are composed of homochiral monomers that do not tolerate any pollution by the "wrong" enantiomer. In order to understand the decisive asymmetry of evolutionary processes at life's beginning, we have to think of an asymmetric element put into the vicinity of a racemic or prochiral chemical substrate. In this chapter, a crystallization nucleus spontaneously generated in solution will serve as such an asymmetric element. According to this model, the chiral information will be spread from the chiral crystallization nucleus to the whole crystal and subsequently transferred to an organic substrate which itself might evolve asymmetrically into a self-organizing living system.
In the context of mainly inorganic crystal chemistry, it is refreshing to remark that snail shells are spiral systems that - as a matter of principle - are chiral and can be right-turning or left-turning. Interestingly, most snails construct their chiral shells deliberately; within most species, snail shells are nearly homochiral. Usually, right-helical shells are stereoselectively preferred and dominant, as Brunner (1999) investigated and documented beautifully. In the case of typical edible snails (Helix pomatia), right-helical shells have the majority: only one out of 20 000 species is constructed in the atypical left-helical way.2 Other snail species show this particular stereoselectivity as well; in exceptional cases, as in the tower snail, left-helical shells are dominant. The Cuban tree snail is the exception, producing shells in a racemic right- to left-helical ratio.
At the University of Glasgow, biochemist Graham Cairns-Smith (1982, 1985) thought on the tricky question: "wherefrom does the shell, which is made up of the purely inorganic aragonite-modification of calcium carbonate, obtain the
1 The homochirality of lipids is, for example, manifested in the molecular side chains of vitamin E and chlorophyll, both containing two tertiary carbon atoms in their enantiopure ^-configuration (Spach and Brack 1983; Meierhenrich et al. 2001e).
2 In the French Bourgogne, where edible snails are thought to be delicious and processed on a tons-per-year scale, exact knowledge of the right- to left-helical shell ratio was obtained (Brunner 1999).
U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology 61
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information in which direction it has to turn?" The homochirality of the macroscopic inorganic calcite shell must have found a way into it. Cairns-Smith reasoned that organic proteins from the living and supple part of the snail might transfer the chiral information through the phase boundary to the inorganic calcite shell. This obvious transfer - and now the story becomes interesting - might be turned the other way around: would it be possible to transfer chiral information out of a miner-alogical crystal to organic molecules that are in immediate contact with its surface? Consequently, wouldn't it be possible that the origin of biomolecular asymmetry and, furthermore, the origin of life have inorganic roots? Cairns-Smith called this migration process the "genetic takeover" from crystals to the first living beings and founded therewith a respected and distinguished branch of research on the origin of life.3
Inorganic crystals were abundant on the early Earth and they have always been in contact with organic molecules as well. That is reason enough for us to take a closer look at the symmetry and chirality of crystals. Indeed, crystals can be chiral and they were proposed to have served as asymmetric elements during chemical evolution.
In general, solids are composed of molecules that are packed together tightly to create a rigid, stable structure. In almost every case, such an arrangement of molecules is patterned. This orderly pattern is the crystalline structure, also called the lattice structure, of the solid. A chiral lattice structure can be formed in three ways.
1. Enantiopure compounds crystallize and form a chiral crystal lattice. (The crystallization of chirally pure molecules necessarily results in a chiral lattice structure.)
2. Achiral compounds crystallize and form a lattice structure that is chiral; this asymmetric crystallization is unpredictable and is the topic of cutting-edge research.
3. Chiral molecules in the racemic state undergo spontaneous resolution in which the two enantiomers segregate into a conglomerate of enantiopure crystals.
Chiral crystals, like any other asymmetric object, exist in two enantiomorphous forms of equal energy (Sakamoto 2004). In this book we will discuss formation way number 2 (Sect. 4.1) and way number 3 (Sect. 4.2) in more detail, since only here an asymmetry is developed and enhanced by the system. Chiral crystals obtained by formation way number 1 require enantiopure compounds and do not increase any enantiomeric excess, making them uninteresting in the scope of this book.
Quartz, the most common mineral, belongs to the second group (way 2) and possesses a chiral lattice structure. Bear in mind that the monomer of quartz, silicon dioxide (SiO2, or silica) is achiral. Molten quartz is not optically active (Barron 2004). The quartz lattice, however, shows a helical structure twisting either to the right or to the left making two distinct enantiomorphous forms, which are represented in Fig. 4.1. Enantiomorphous quartz crystals are optically active; the optical
3 Advanced parts of Cairns-Smith's theory however are highly controversial, particularly when he argues on protein molecules that served as catalysts in the growth of "crystal genes" on clay surfaces, later on breaking away from the clay to become three-dimensional living cells.
rotation depends on the angle between optical axis and light, which means that the quartz crystal is anisotropic. We distinguish between (+)-d-quartz and (—)-l-quartz. Quartz is morphologically chiral (Herschel 1822; de Vries 1958).
Other chiral crystals exist in addition to quartz. Even if people believed that chiral crystallization is a very rare phenomenon, a survey of Koshima (2004) revealed that the statistical probability for the chiral crystallization of achiral compounds was around 8%. Sakamoto (2004) carefully surveyed 30 000 organic crystalline compounds and determined their chirality, showing that 19.4% of these compounds crystallized as chiral enantiomers. How can we identify them?
A chiral crystal must belong to a chiral space group. From a total of 230 space groups, 65 space groups are chiral, having translation and rotation symmetry elements only. Achiral crystals have symmetry elements as a mirror plane or a center of inversion. The most frequent chiral space groups are P21 and P212121. When the space group of the single crystal of a compound is chiral, the crystal is designated as chiral.
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