Crystal structure of ferrioxamine B: a comparative analysis and implications for molecular recognition

Ferrioxamine B was successfully co-crystallized with ethanolpentaaquomagnes ium(II) and perchlorate ions as counter ions, C27H62Cl3FeMgN6O26, and the c rystal structure has been determined by single-crystal X-ray diffraction. T he crystals are monoclinic, space group P2(1)/n, four molecules per unit ce ll with dimensions a = 21.1945(7) Angstrom, b = 10.0034(3) Angstrom, c = 10 6.560(1) Angstrom, and beta = 106.560(1)degrees. The crystal structure cont ains a racemic mixture of A-N-cis,cis and Delta -N-cis,cis coordination iso mers. The structural parameters and the conformational features of ferrioxa mine B compare very well with those of ferrioxamines D-1 and E, with an exc eption of the orientation of the pendant protonated amine, which is pointin g away from the connecting amide chains and towards the carbonyl face of th e inner coordination shell distorted octahedron. This pendant protonated am ine, in conjunction with the carbonyl face of the Fe(III) coordination shel l, is proposed to play an important role in the recognition and membrane tr ansport processes.