Enantiomeric crystallization from DL-aspartic and DL-glutamic acids: implications for biomolecular chirality in the origin of life.
Amino acids in living systems consist almost exclusively of the L-enantiomer. How and when this homochiral characteristic of life came to be has been a matter of intense investigation for many years. Among the hypotheses proposed to explain the appearance of chiral homogeneity, the spontaneous resolution of conglomerates seems one of the most plausible. Racemic solids may crystallize from solution either as racemic compounds (both enantiomeric molecules in the same crystal), or less commonly as conglomerates (each enantiomer molecule separate in different enantiomeric crystals). Only conglomerates can develop a spontaneous resolution (one of the enantiomeric molecule crystallizes preferentially, the other one remains in solution). Most of natural amino acids are racemic compounds at moderate temperatures. How can we expect a hypothetical spontaneous resolution of these amino acids if they are not conglomerates? In this paper we show how DL-aspartic and DL-glutamic amino acids (racemic compounds), crystallize at ambient conditions as true conglomerates. The experimental conditions here described, that allows this 'anomalous' behaviour, could be also found in natural sedimentary environments. We suggest that these experimental procedures and its natural equivalents, have a potential interest for the investigation of the spontaneous resolution of racemic compounds comprising molecules associated with the origin of life.