Structure elucidation of azotobactin 87, isolated from Azotobacter vinelandii ATCC 12837

Chromopeptide siderophores (azotobactin 87-I and -II) were isolated from an iron deficient culture medium of Azotobacter vinelandii ATCC 12837 (= DSM 87). Their structures were elucidated by chemical degradation studies and spectroscopic methods, especially 2D-NMR-techniques. Total assignments of 1H-, 13C-, and 15N-resonances based on 2D-HOHAHA-, 1H/13C-HMQC-, 1H/13C-HMBC-, 1H/15N-HMQC/TOCSY-, and 1H/15N-HMBC-experiments are given as well as sequential information derived from 1H/1H-NOESY-, 1H/13C-HMBC- and 1H/15N-HMBC-experiments. Both Az 87-I and Az 87-II consist of a tetracyclic chromophore-- (1S)8,9-dihydroxy-4-oxo-2,3,4,5-tetrahydro-1H,10cH-3a,5,10b- triazaacephenantrylene-1-carboxylic acid--and a decapeptide chain linked with the N-terminus to the carboxy group of the chromophore containing also modified, non-proteinogenic amino acids. The sequence L-Ser-D-Ser-L-Hse-Gly-D-threo-OHAsp-Hse-Hse-Hse-D-N5OH-N5-R- Hbu-Orn-L-Hse was determined for Az 87-I, while Az 87-II contains a C-terminal L-Hse-lactone instead. Iron is chelated by the catecholic group of the chromophore, the beta-hydroxy aspartic acid, and the hydroxamate function formed by N5-hydroxyornithine and R-beta-hydroxybutyric acid.