Structure Elucidation of Azotobactin 87, Isolated from Azotobacter vinelandii ATCC 12837*, **

Abstract

Chromopeptide siderophores (azotobactin 87-1 and -II) were isolated from an iron deficient culture medium of Azotobacter vinelandii ATCC 12837 (=DSM 87). Their structures were elu­ cidated by chemical degradation studies and spectroscopic methods, especially 2D-NMR-tech-niques. 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/ ¹³N-HMBC-experiments. Both Az 87-1 and Az 87-11 consist of a tetracyclic chromophore -(1S)8,9-dihydroxy-4-oxo-2,3,4,5-tetrahydro-1H,10cH-3a,5,10b-triazaacephenantrylene-1-carb-oxylic 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-Om-L-Hse was determined for Az 87-1, while Az 87-11 contains a C-terminal L-Hse-lactone instead. Iron is chelated by the catecholic group of the chromophore, the β-hydroxy aspartic acid, and the hydroxamate function formed by N⁵-hydroxyornithine and R-β-hydroxybutyric acid.