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(III)-petrobactin is estimated to be about 43. The pK(a) of the beta-hydroxyaspartyl hydroxyl group was determined to be 10.8 by H-1 NMR titration. C-13 NMR and IR spectroscopy were used to investigate Ga(III) coordination to the marinobactins. The coordination-induced shifts (CIS) in the C-13 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 beta-hydroxyamide group suggests this moiety is not coordinated in the Ga(III) complex. Differences in the IR spectrum of Ga(III)-marinobactin-C and Fe(III)-marinobactin-C in the 1600-1700 cm(-1) region also corroborates Fe(III) is coordinated to the beta-hydroxyamide moiety, whereas Ga(III) is not coordinated.