A counter double-current distribution procedure is described which permits the convenient preparation of larger amounts of bacitracin A from commercial bacitracin. Since the procedure requires a significantly smaller number of transfers than regular countercurrent distribution to effect a satisfactory resolution of the bacitracin mixture, an analytical countercurrent distribution carried out on the 2nd International Standard for Bacitracin in 1963 for the World Health Organization is described for comparison. Structure-stability relationships for bacitracin A have been investigated further. The role of ionic zinc in complex formation with bacitracin A was investigated in relation to increased stability of high antibiotic activity. Zinc and bacitracin A combine as a 1:1 complex, whose association constant was determined spectrophotometrically to be 2.5 × 103 at pH 6.34. From data presented, ionic zinc appears to be bound by coordinate bonds to four positions of bacitracin A molecule in the zinc-bacitracin A complex, two on the histidine residue and two on the aminoterminal thiazoline residue. Such binding indicates close spatial proximity of the histidine and terminal thiazoline residue and thus supports the previously postulated conformation of bacitracin A. Optical rotatory dispersion studies of bacitracin A and its derivatives in the far-ultraviolet region has been further investigated with respect to the conformation of bacitracin A in solution. © 1969, American Chemical Society. All rights reserved.
