The Molecular and Crystal Structure of the Glycopeptide A‐40926 Aglycone

The crystal structure of a glycopeptide antibiotic A–40926 aglycone was investigated by X‐ray analysis at −120°. A‐40926 crystallises in the orthorhombic space group P 2 1 2 1 2 1 with two monomers in the asymmetric unit, a = 21.774(4), b = 28.603(7), c = 29.757(4) Å. ‘Conventional’ direct methods approach failed to solve the structure, but a novel iterative real/reciprocal space procedure was successful. Refinement against 11248 F 2 data led to R 1 = 13.3% for 6770 F > 4σ ( F ). The two monomers of A‐40926 have similar conformations and are bound by antiparallel H‐bonds to form a ‘chain’ structure of connecting dimers. The antibiotic molecule possesses a ‘binding pocket’ for the C‐terminal carboxy group of the cell‐wall protein, which is consisten with suggestions based on NMR data and the recently reported crystal structure of ureido‐balhimycin. In A‐40926 the monomers are polymerically linked by H‐bonds, quite unlike the tight dimer formation observed in ureido‐balhimycin.