A phase transition from monoclinic C2 with Z′ = 1 to triclinic P1 with Z′ = 4 for the quasiracemate l-2-aminobutyric acid-d-methionine (1/1)

Racemates of hydrophobic amino acids with linear side chains are known to undergo a unique series of solid‐state phase transitions that involve sliding of molecular bilayers upon heating or cooling. Recently, this behaviour was shown to extend also to quasiracemates of two different amino acids with opposite handedness [Görbitz & Karen (2015). J. Phys. Chem. B, 119, 4975–4984]. Previous investigations are here extended to an l‐2‐aminobutyric acid–d‐methionine (1/1) co‐crystal, C4H9NO2·C5H11NO2S. The significant difference in size between the –CH2CH3 and –CH2CH2SCH3 side chains leads to extensive disorder at room temperature, which is essentially resolved after a phase transition at 229 K to an unprecedented triclinic form where all four d‐methionine molecules in the asymmetric unit have different side‐chain conformations and all three side‐chain rotamers are used for the four partner l‐2‐aminobutyric acid molecules. A phase transition taking place at 229 K upon cooling produces twinned crystals with triclinic symmetry. Massive side‐chain disorder due to steric conflict under ambient conditions is avoided at low temperature by adopting a high Z′ packing arrangement with a heterogeneous mix of side‐chain conformations.