Agrobactin, or N- [ 3-(2,3-dihydroxybenzamido)propyl]-4-[4-(2,3-dihydroxybenzamido)butyl]-2-(2,3-dihydroxyphenyl)-trans-5-methyl-2-oxazoline-4-carboxamide, is the new linear tricatechol-type slderophore of Agrobacterium tumefaciens. The ligand was crystallized as the ethyl acetate solvate (C32H3 OI0N4-2C4H8O2, 812.9 g/fw) in the orthorhombic space group P212121 (Z = 4), with unit cell dimensions a = 17.846 (18) A, b = 28.634 (19) A, and c = 7.835 (3) A at V = 4003.7 A' and at -135 (2) OC. The structure was refined by least-squares methods to an R factor of 0.052 (3749 observed data) and 0.071 for all 4627 unique reflections. The molecule assumes a conformation which is relatively flat (3.842-A thickness) and is efficiently stabilized by five relatively strong intramolecular hydrogen bonds. The formation of the oxazoline ring by the carbonyl group of one of the 2,3-dihydroxybenzoic acid moieties and the C"-C@-N" atoms of L-threonine is accompanied by inversion of the CB carbon atom. Three amide bonds link the ligating functional groups to the spermidine backbone. The results show that a large conformational change must occur on chelation with trivalent iron.