Bioinorganic and bio-organic studies of rhizobactin, a structurally novel siderophore related to the opines
The recent structure elucidation of rhizobactin, N 2-[2-[D-(l-carboxyethyl)-amino]ethyl]-N 6-L-(3-carboxy-3-)hydroxy-l-oxopropyl)-L-lysine, from Rhizobium meliloti DM4 disclosed a third chemical class of microbial siderophores (Gr. sidero = iron; phore = bearer; Smith et al., 1985); the ethylenediamine ligand also was unprecedented in a natural product. Yet determination of the absolute configuration and proof of the entire structure awaited total synthesis. This has been accomplished (Smith, 1989) and substantiates the conclusion that the siderophore is related in structure and activity to other N 2-substituted amino acids termed opines (Smith and Neilands, 1984a, b). As iminocarboxylic acids, opines, rhizobactin and phytosiderophores possess substantially reduced selectivities for iron(III). Such compounds could enhance the bioavailability of additional essential metal ions in the rhizosphere.