COORDINATION CHEMISTRY OF MICROBIAL IRON TRANSPORT COMPOUNDS .19. STABILITY-CONSTANTS AND ELECTROCHEMICAL BEHAVIOR OF FERRIC ENTEROBACTIN AND MODEL COMPLEXES

Enterobactin, a microbial iron-transport compound with three catechol (o-dihydroxybenzene) groups, has been isolated from cultures of Klebsiella pneumoniae (formerly Aerobacter aerogenes) and studied by potentiometric, spectrophotometric, and electrochemical techniques. The enterobactin model compound N,N-dimethyl-2,3-dihydroxybenzamide (DMB) has also been examined, and the results of that examination have been employed in analysis of the enterobactin data. The stepwise equilibrium constants (log K) for successive addition of DMB dianion to a ferric ion are 17.77 (4), 13.96 (2), and 8.51 (3), respectively, for an overall formation constant of 40.2 (1). The proton-dependent stability constant K* for ferric enterobactin (H6ent) has been determined spectrophotometrically by competition vs. EDTA to be 10-9.7(3), where K* = ([Fe(ent)3-] [H+]6)/([Fe3+][H6ent]). We have approximated the protonation constants of enterobactin based on those of the model compound DMB, and thereby estimated a value for the conventional (proton-independent) formation constant (log kf) of 52, where Kf = [Fe(ent)3-]/([Fe3+][ent6-]). This is the largest formation constant reported for a ferric complex. The [Fe(ent)]6- complex undergoes a series of 1:1 protonations to yield sequentially [Fe(Hent)]2-, [Fe(H2ent)]-, and [Fe(H3ent)]. The chelate protonation constant for the first of these steps has been calculated to be log KMHL = 4.89 (6), where KMHL = [Fe-(Hent)]/([Fe(ent)][H+]). These protonation reactions correspond to a conversion from a catecholate to salicylate mode of bonding at low pH. Electrochemical study of the ferric enterobactin complex by cyclic voltammetry demonstrates that it is reversibly reduced at pH values above 10, with a formal potential of -986 mV vs. normal hydrogen electrode (NHE) (at pH 10). At lower pH values the reduction potential increases with decreasing pH, leading to an estimated value of -750 mV at pH 7. The physiological implications of these results are discussed. © 1979, American Chemical Society. All rights reserved.