Crystal Structures of Two Modifications of [3,<i>O</i>‐didehydro‐mebmt¹, val²]‐cyclosporin and comparison of three different X‐ray data sets

The crystal structure of [3, O ‐didehydro‐MeBmt 1 , Val 2 ]cyclosporin (PSC‐833; 1 ) was investigated by X‐ray analysis. Data were collected from two different crystal modifications. Modification I crystallizes in P 3 1 21, a = b = 21.419 (2) Å, c = 32.101 (3) Å with one molecule in the asymmetric unit, modification II in P 3 2 21, a = b = 21.313 (2) Å, c = 62.053(3) Å with two molecules per asymmetric unit. This non‐immunosuppressive analogue of cyclosporin A adopts a similar backbone conformation to that found in the crystal structure of cyclosporin A and other analogues. Three different data sets of modification I were collected using an Enraf‐Nonius‐CAD4 diffractometer with Cu K α radiation at 20°, a Stoe‐Siemens four‐circle diffractometer with Mo K α radiation at − 120°, and an EMBL image‐plate scanner with synchrotron radiation at 12°. The quality of the data sets was evaluated by internal consistency, independent structure solution, and refinement. The structural parameters reported here for modification I are based on the synchrotron data.