THE MOLECULAR AND CRYSTAL-STRUCTURE OF THE GLYCOPEPTIDE A-40926 AGLYCONE

The crystal structure of a glycopeptide antibiotic A-40926 aglycone wa s investigated by X-ray analysis at -120 degrees. A-40926 crystallises in the orthorhombic space group P2(1)2(1)2(1) with two monomers in th e asymmetric unit, a = 21.774(4), b = 28.603(7), c = 29.757(4) Angstro m. 'Conventional' direct methods approach failed to solve the structur e, but a novel iterative real/reciprocal space procedure was successfu l. Refinement against 11248 F-2 data led to R1 = 13.3% for 6770 F > 4 sigma(F). The two monomers of A-40926 have similar conformations and a re bound by antiparallel II-bonds to form a 'chain' structure of conne cting dimers. The antibiotic molecule possesses a 'binding pocket' for the C-terminal carboxy group of the cell-wall protein, which is consi stent with suggestions based on NMR data and the recently reported cry stal structure of ureido-balhimycin. In A-40926 the monomers are polym erically linked by II-bonds, quite unlike the tight dimer formation ob served in ureido-balhimycin.