Models for the bacterial iron-transport chelate enterochelin.

Abstract

MICROBIAL iron-transport compounds, or siderochromes are of two general structural types, the phenolates and the hydroxa-mates1,2. X-ray studies of several of the latter, for example, ferrichrome A (ref. 3), ferrioxamine E (ref. 4) and myco-bactin P (ref. 5), establish the anion of hydroxamic acid (−N(O−)CO−) as the dominant metal-binding moiety, with discrete, neutral [FeO6] (refs 3 and 4) or [FeO5N] (ref. 5) units being involved. No similar structural data are at present available, however, for any member of the phenolate class and there is some ambiguity about the metal-binding sites. This is demonstrated for the most widely studied member of the group, enterochelin (or enterobactin), in Fig. 1. Both modes of attachment have parallels in the well known colour reactions of iron(III) with phenols, polyphenols and catechols and in the strong coordination of deprotonated amide nitrogen in simple peptide complexes of the transition elements7. Furthermore, molecular models (Drieding or CPK) of iron (III) enterochelin can readily be constructed with either bonding combination. Here we report our studies of two catechol (1,2-dihydroxy-benzene) complexes containing the coordination type given by Fig. 1a and compare some of their properties with those of iron(III)-enterochelin (part of this work was presented in ref. 8).