The structure of deferriferrioxamine E (nocardamin), a cyclic trihydroxamate

The structure of deferriferrioxamine E, a cyclic trihydroxamate type of siderophore, has been determined as part of a study of conformational differences between the siderophores and deferrisiderophores. Deferriferrioxamine E hexahydrate, C27H48N609.6H20, Mr = 708.8, crystallizes in the trigonal space group P3, with a = 24.346 (6), c = 5.392 (1) A, Z = 3, V = 2767.8A 3 at 138 K; a = 24.610(9), c = 5.410 (2) A, V = 2837.6A 3, D x = 1.244, D m = 1.237 Mg m -3 at 293 K. The structure was determined by direct methods from 3825 diffractometer data (Cu K6 radiation at 138 K) and refined to a final R factor of 0.067. The molecule consists of a 33membered macrocycle formed by condensation of three groups of 1-amino-5-pentylhydroxylamine and three succinyl groups through peptide and hydroxamate linkages. The macrocyclic ring is essentially flat with an average thickness of about 4.0 ]k. There are major conformational differences between the three equivalent segments of the molecule, particularly in the pentane chain and in the orientation of the peptide linkages, leading to substantial asymmetry for the potentially symmetric molecule. The three hydroxamate groups are all trans with all the oxime O atoms lying outwards while all the carbonyl O atoms are directed inwards of the macrocyclic ring. The three peptide bonds are all trans. All six water molecules in the asymmetric unit lie inside the macrocyclic ring and form extensive hydrogen bonding.