Siderophores of fluorescent pseudomonads.

Though iron is fourth in abundance in the earth crust it is not readily available to many m icroorga­ nisms: In the early days of earth life developed in a reductive atm osphere where iron necessary for many physiological processes was abundant in its divalent form. Fe2+-salts are sufficiently w ater soluble to provide an adequate supply of this elem ent to the cells. But as a consequence of the photolytic cleavage of water, oxygen was set free and soon with rare exceptions only trivalent iron abounded which is present in the soil mainly in the form of its oxide hydrates [Fe20 3 . n H zO ]x. D ue to their low dissociation constants the con­ centration of free Fe3+ is at best 10-17 mol/1 at physiological pH-values while 10-6 mol/1 would be needed to provide the necessary supply. To cir­ cum vent this problem soil bacteria as well as the ones infecting higher organisms (where the avail­ ability of iron is limited for o ther reasons such as com plexation by peptides as, e.g., transferrins) produce a variety of com pounds which can form w ater soluble Fe3+-complexes, so-called sidero­ phores. Fe 3+ forms octahedral d5 high spin complexes which can accomodate three b identate ligands with oxygen or occasionally nitrogen and sulfur as binding sites. Most frequently encountered are catecholate and hydroxam ate units. M ixed systems are no t uncommon and other structural types as a-hydroxy-carboxylates are observed occasion-