Kinetics and mechanism of iron(III) dissociation from the dihydroxamate siderophores alcaligin and rhodotorulic acid

The kinetics and mechanism of siderophore ligand dissociation from their fu lly chelated Fe(III) complexes is described for the highly preorganized cyc lic tetradentate alcaligin and random linear tetradentate rhodotorulic acid in aqueous solution at 25 degreesC (Fe2L3 + 6H(+) reversible arrow 2 Fe-aq (3+) + 3 H2L). At siderophore:Fe(III) ratios where Fe(mj is hexacoordinated , kinetic data for the H+-driven ligand dissociation from the Fe2L3 species is consistent with a singly ligand bridged structure for both the alcaligi n and rhodotorulic acid complexes. Proton-driven ligand dissociation is fou nd to proceed via parallel reaction paths for rhodotorulic acid, in contras t with the single path previously observed for the linear trihydroxamate si derophore ferrioxamine B. Parallel paths are also available for ligand diss ociation from Fe-2(alcaligin)3, although the efficiency of one path is grea tly diminished and dissociation of the bis coordinated complex Fe(alcaligin )(OH2)(2)(+) is extremely slow (k = 10(-5) M-1 s(-1)) due to the high degre e of preorganization in the alcaligin siderophore. Mechanistic interpretati ons were further confirmed by investigating the kinetics of ligand dissocia tion from the ternary complexes Fe(alcaligin)(L) in aqueous acid where L = N-methylacetohydroxamic acid and glycine hydroxamic acid. The existence of multiple ligand dissociation paths is discussed in the context of sideropho re mediated microbial iron transport.