Ferric stability constants of representative marine siderophores: Marinobactins, aquachelins, and petrobactin

The coordination of iron(III) to the marine amphiphilic marinobactin and aquachelin siderophores, as well as to petrobactin, an unusual 3,4-dihydroxybenzoyl siderophore is reported. Potentiometric titrations were performed on the apo siderophore to determine the ligand pK _a values, as well as the complex formed with addition of 1 equiv of Fe(III). The log K _ML values for Fe(III)-marinobactin-E and Fe(III)-aquachelin-C are 31.80 and 31.4, respectively, consistent with the similar coordination environment in each complex, while log K _ML for Fe(lll)-petrobactin is estimated to be about 43. The pK _a of the β-hydroxyaspartyl hydroxyl group was determined to be 10.8 by ¹H NMR titration. ¹³C NMR and IR spectroscopy were used to investigate Ga(III) coordination to the marinobactins. The coordination-induced shifts (CIS) in the ¹³C NMR spectrum of Ga(III)-marinobactin-C compared to apo-marinobactin-C indicates that the hydroxamate groups are coordinated to Ga(III); however, the lack of CISs for the carbons of the β-hydroxyamide group suggests this moiety is not coordinated in the Ga(III) complex. Differences in the IR spectrum of Ga(lll)-marinobactin-C and Fe(III)-marinobactin-C in the 1600-1700 cm ^-1 region also corroborates Fe(III) is coordinated to the β-hydroxyamide moiety, whereas Ga(III) is not coordinated.