Quartz crystal asymmetry

Why Nature uses only one enantiomer of most important biochemicals is an easier question to answer than how this asymmetry came about in the first place, or why L-amino acids and D-sugars were the favoured enantiomers, since, for example, proteins made out of racemic samples of amino acids would be complicated by the possibility of enormous numbers of diastereomers. Some have suggested that life arose on the surface of single chiral quartz crystals, which provided the asymmetric environment needed to make life s molecules enantiomerically pure. Or perhaps the asymmetry present in the spin of electrons released as gamma rays acted as a source of molecular asymmetry. Given that enantiomerically pure living systems should be simpler than racemic ones, maybe it was just chance that the L-amino acids and the D-sugars won out. 

In general the optical activity exhibited by a crystal will persist in other states of aggregation only if it is due to the asymmetry of the finite molecule or complex ion. In this case it is also necessary that the energy of activation for racemization (change d 1) must exceed a certain value ( 80 klmol" at room temperature). The optical activity cannot, of course, be demonstrated unless there is a means of resolution (separation of d and / forms) or at least of altering the relative amounts of the two isomers. The optical activity of many crystals (e.g. quartz, cinnabar, NaC103) arises from the way in which the atoms are linked in the crystal it is then a property of the crystalline material only. 

An x-ray beam was reflected from the (1011) plane of quartz or, in some cases, from a cleavage plane of mica (fifth-order reflection). The radii of curvature of the bent crystals were 1002 and 1217 mm. The dispersion achieved using these crystals was 2.33 XU/mm (i.e., 4.01 eV/mm) and 3.59 XU/mm (6.01 eV/mm). This enabled us to resolve completely the doublet whose components were separated by 3.65 XU. A crystal was bent and the aperture was reduced until we achieved the lowest value of the half-width of the line of metallic titanium (AA = 0.93 XU). The asymmetry index of this line was a = 1.13. 

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