Crystals, optically active

Okada and coworkers [66] discovered that the crystallization of this molecule from methanol yielded optically active crystals that were sometimes dextrorotatory and sometimes levorotatory, and that underwent rapid racemization upon redissol-ving in methanol. They went on to study the kinetics of this process as well as the nature of the different crystalline forms. [67, 69] Bonner [50, 65, 70] has extended and confirmed these observations and conclusions, with special attention to the possibility that the crystallization might provide the means for studying the efficacy of... 

G. B. Kauffman and I. Bernal, Alfred Werner s awareness of spontaneous resolutions and the meaning of hemihedral faces in optically active crystals , Structural Chem., 1993, 4 (2), 131-138. 

Since Pasteur s discovery this problem of the origin of homochirality in life has attracted the attention of many scientists in relation to the origin of life itself [3,5-7]. Japp expected a directive force when fife first arose [3], and various directive forces have been proposed later, such as different intensities of circularly polarized light in a primordial era, adsorption on optically active crystals, or parity breaking in the weak interaction. However, the expected degree of chiral asymmetry or the value of the enantiomeric excess (ee) turns out to be very small [6], and one needs a mechanism to amplify ee enormously to a level of homochirality. 

A hydrated hydrobromide salt obtained from P. ormondiu apparently as a racemate, yielded optically active crystals that were shown by X-ray analysis to be (6-epz -podopetaline)2-6HBr-5H20. This work confirmed the absolute configuration of the alkaloid (14).16... 

Thus there are two classes of optically active crystals. One, like quartz and sodium chlorate, only shows the effect in the crystalline state. Here the ability to rotate the plane of polarized light is related to the fact that the atoms in the crystal are arranged as right-handed or left-handed spirals or other asymmetric shapes. The second class of optically active crystals contains molecules or ions, such as certain tartrates, that are themselves asymmetric the effect persists even when the crystal melts or is dissolved. More details on this subject are given in Chapter 14. 

Although many optically active crystals contain optically active molecules, this is not mandatory. Crystals of quartz, one variety of silicon dioxide, SiC>2, occur in left or right-handed forms although no molecules are present. The structure of this material is composed of helices of corner-linked Si04 tetrahedra. The direction of the helix determines the left or right-hand nature of the crystals. 

The exclmer emission originates from the racemic crystals and the monomer emission from the optically active crystals. These two emissions are readily distinguished because of their... 

Ferroni and Cini tried to resolve the racemic chelate complex Be[(PhCOCH)2CO]2 using optically active crystals of NaClOs as chiral adsorbents, but results were uncertain (see Table 1.7.). 

Kawashiro K., Morimoto S., Yoshida H. and Sugiura K. (1977) The synthesis and chromatography of peptide nitriles. Origin Life, 8, 347-353. Gillard R.D. and da Luz de Jesus J.D.P. (1979) Studies on the origin of as57mmetry. Part I. Optically active crystals as selective adsorbents, J. Chem. Soc., Dalton Trans. 1779 -1782. 

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