Structure Elucidation of the Gallium–Ornibactin Complex by 2D‐NMR Spectroscopy

From a Pseudomonas strain novel hydroxamate/carboxylate siderophores were isolated which were named ornibactins representing the modified tetrapeptide L ‐Orn 1 ( N δ‐OH, N δ‐acyl)‐ D ‐ threo ‐Asp(β‐OH)‐ L ‐Ser‐ L ‐Orn 4 ( N δ‐OH, N δ‐formyl)‐1,4‐diaminobutane. The side‐chain amino function of Orn 1 ( N δ‐OH, N δ‐acyl) is acylated by 3‐hydroxyoctanoic acid in the most lipophilic component of the microheterogeneous siderophore mixture. The structure elucidation was based on GC amino acid analysis, GC‐MS and electrospray mass spectrometry of the iron complex. The 3‐hydroxyoctanoic acid and the primary sequence were established on the basis of 1 H and inverse detected 13 C two‐dimensional NMR spectra (COSY, TOCSY, NOESY, HMQC, HMBC) of the gallium‐ornibactin‐F complex which allowed the complete assignment of all 1 H and 13 C signals.