TWINNED CRYSTALS OF ENANTIOMORPHOUS MORPHOLOGY OF RACEMIC ALANINE INDUCED BY OPTICALLY RESOLVED ALPHA-AMINO-ACIDS - A STEREOCHEMICAL PROBE FOR THE EARLY STAGES OF CRYSTAL NUCLEATION

Crystallization of R,S-alanine in the presence of small amounts of oth er resolved alpha-amino acids results in the precipitation of the race mic compound as twinned crystals displaying a propeller-like morpholog y of 222 symmetry, in contrast to needlelike crystals of mm2 symmetry when grown from pure aqueous solution. This unusual twinning is explai ned by assuming that the supersaturated solutions contain ordered clus ters of structures akin to the mature crystals of (R,S)-, (R)-, and (S )-alanine. The resolved a-amino acid additive, e.g., R', stereospecifi cally inhibits growth of the R,S and R nuclei but not of the S nuclei. The latter serve as a template onto which the (R,S) crystals nucleate and grow. A model which takes into consideration the structural simil arity between the racemic and chiral resolved forms of alanine to expl ain the twinning is presented. An X-ray diffraction analysis of a twin ned crystal specimen is in agreement with a gradual change in structur e, from a chiral crystal of space group P2(1)2(1)2(1) to the racemic c ompound of space group Pna2(1). To substantiate the model, crystals of alanine were grown from optically enriched solutions containing exces s of one of the alanine enantiomers, which also induce the twinning ph enomenon and an overall reduction in crystal symmetry. The relevance o f these observations to the early stages of crystal nucleation is disc ussed.