Acetobacter turbidans α-amino acid ester hydrolase: merohedral twinning in P2₁ obscured by pseudo-translational NCS

The structure elucidation of the α-amino acid ester hydrolase from Acetobacter turbidans by molecular replacement is described. In the monoclinic crystal, the molecules are related by both rotational and pseudo-crystallographic translational NCS (non-crystallographic symmetry). Refinement of the structure converged at unacceptably high R factors. After re-evaluation of the data, it was found that the crystal was merohedrally twinned, with a high twinning fraction. It is shown that the pseudo-crystallographic NCS causes aberrant behaviour of conventional twinning indicators, which explains why the twinning was only realized at the refinement stage.