Invariom refinement of a new monoclinic solvate of thiostrepton at 0.64 Å resolution.

A new monoclinic solvate containing two molecules of the thiopeptide antibiotic thiostrepton in the asymmetric unit has been crystallized in space group P2(1). Single-crystal diffraction data to a resolution of 0.64 Å were collected at the SLS synchrotron, allowing structure solution by direct methods and resolution of the disorder present. Valence electron density can be observed in the Fourier residual density from refinement with the independent-atom model, which is a prerequisite for successful application of more sophisticated aspherical-atom scattering factors such as the invariom model when aiming to improve the structural model. Invariom refinement improves quality indicators such as R1(F) for thiostrepton, as previously demonstrated for small molecules. The nonspherical electron-density model also allows the direct derivation of a dipole moment and an electrostatic potential for the whole molecule, which is discussed in the context of antibiotic activity and molecular recognition.