A solution to the observed Z' = 2 preference in the crystal structures of hydrophobic amino acids

Chiral amino acids without functional groups in their side chains (hydrophobic amino acids) systematically form crystals with two molecules in the asymmetric unit. In contrast, racemates of the same compounds form crystals with Z' = 1. The present investigation addresses the origin of this important difference between enantiomeric and racemic crystals. Through a series of ab initio calculations on infinite two-dimensional slabs, derived from crystal structures, as well as calculations on full crystal structures it is shown that it is indeed possible to explain the observed behaviour. Additionally, the (not unexpected) observation that amino acids usually form racemates in the solid phase rather than undergoing racemic separation upon crystallization is rationalized on the basis of energy calculations.