An unexpected tetragonal unit cell for N-(L-2-aminobutyryl)-L-serine

The title dipeptide {systematic name: (S)-2-[(S)-2-azaniumylbutanamido]-3-hydroxypropanoate}, C₇H₁₄N₂O₄, was synthesized in the anticipation that it would form nanoporous crystals with hexagonal symmetry. Single-crystal X-ray diffraction analysis showed that it had instead adopted a unit cell in the space group I4, similar to L-alanyl-L-alanine [Fletterick, Tsai & Hughes (1970). J. Phys. Chem. 75, 918-922]. The resulting packing arrangement has a high density for a peptide (1.462 Mg m⁻³), which is rendered possible by extensive disorder over two positions for the ethyl side chain of the 2-aminobutyric acid fragment and over three positions for the serine side chain.