Macroscopic chiral crystals can self-segregate

The molecular building blocks of life are nearly all chiral: They can’t be superimposed on their mirror images. What’s more, biomolecules all have the same chirality, or “handedness,” even though their left-handed and right-handed isomers are energetically identical. Motivated by the mystery of biohomochirality, Cristóbal Viedma of Complutense University in Madrid, Spain, started experimenting with chiral crystals—achiral ions arranged into right- or left-handed lattices. In 2005 he found that a mix of the two chiral isomers of crystalline sodium chlorate, suspended in aqueous solution, could be transformed into a single chiral form by grinding the crystals (see Physics Today, April 2005, page 21 ) or by boiling the solution. Now, Viedma and a team of collaborators have discovered another surprising effect. Shaking a suspension of millimeter-sized crystals of mixed chirality grew them into centimeter-sized aggregates, each of which was almost entirely homochiral. Could some crystals be changing their chirality within the aggregates, perhaps by way of an amorphous intermediate? That possibility was ruled out by a further experiment: One of the chiral forms in the initial mix was dyed purple, and the other was left colorless. If individual crystals changed chirality, the dye would be distributed between both types of chiral aggregate. But as shown in the figure, the purple crystals all ended up in the same aggregates. The researchers conclude that the crystals attach preferentially to those of the same chirality. The detailed mechanism for that microscopic recognition between macroscopic crystals remains unknown. (C. Viedma et al., Angew. Chem. Int. Ed., doi:10.1002/anie.201303915, 2013 .)—Johanna Miller