Metachelins, Mannosylated and N‑Oxidized Coprogen-Type Siderophores from <i>Metarhizium robertsii</i>

Abstract

Under iron-depleted culture conditions, the entomopathogenic fungus <i>Metarhizium robertsii</i> (Bischoff, Humber, and Rehner) (= <i>M. anisopliae</i>) produces a complex of extracellular siderophores including novel O-glycosylated and N-oxidized coprogen-type compounds as well as the known fungal siderophores <i>N</i>^α-dimethylcoprogen (NADC) and dimerumic acid (DA). Metachelin A (<b>1</b>), the most abundant component in the <i>M. robertsii</i> siderophore mixture, was characterized as a 1094 Da analogue of NADC that is O-glycosylated by β-mannose at both terminal hydroxyl groups and N-oxidized at the dimethylated α-nitrogen. The mixture also contained a 1078 Da analogue, metachelin B (<b>2</b>), which lacks the <i>N</i>-oxide modification. Also characterized were the aglycone of <b>1</b>, i.e., the <i>N</i>-oxide of NADC (<b>3</b>), and the monomannoside of DA (<b>6</b>). <i>N</i>-Oxide and <i>O</i>-glycosyl substituents are unprecedented among microbial siderophores. At high ESIMS source energy and at room temperature in DMSO, <b>1</b> underwent Cope elimination, resulting in loss of the <i>N</i>^α-dimethyl group and dehydration of the α–β bond. High-resolution ESIMS data confirmed that all tri- and dihydroxamate siderophores (<b>1</b>–<b>6</b>) complex with trivalent Fe, Al, and Ga. In a chrome azurol S assay, all of the <i>M. robertsii</i> siderophores showed iron-binding activity roughly equivalent to that of desferrioxamine B