Spontaneous generation and amplification of optical activity in α-amino acids by enantioselective occlusion into centrosymmetric crystals of glycine

Abstract

Recent studies of crystal growth in the presence of specific additives have suggested a new approach to the generation and amplification of optical activity in initially racemic mixtures^1–4. This approach is based on the observation that particular enantiomers can be selectively incorporated not only into chiral crystals^5–7 but also into the opposite enantiotopic faces of centrosymmetric crystals^3,4,8. If one such face of a centrosymmetric crystal is blocked, therefore, enantioselective occlusion into the opposite face will remove one enantiomer preferentially from the racemic mixture. We report here that this process can take place with centrosymmetric crystals of glycine, in a way that may be of relevance to the generation of optical activity during prebiotic evolution. Glycine crystals float in solution, such that only one face is available for growth. The addition of a particular enantiomer of a hydro-phobic amino acid orientates the floating crystals in such a way that the face that occludes amino acids of the opposite chirality is exposed to the solution. Thus, a statistical fluctuation in the orientation of crystals floating in an initially racemic solution could lead to the generation and amplification of optical activity.