The 31P resonance of the trans form of 2 appears to show line broadening effects even when the cis isomer is in the slow-exchange limit (Figure 2). It is probable that the energy minimum for the trans molecule corresponds to the distorted structure indicated in Figure 3a and that the inequivalent phosphorus ligands are undergoing rapid intramolecular exchange.The passage of the cis and trans resonances in 2 to a simple quintet with increasing temperature shows that cis-trans isomerization via a polytopal1-4 rearrangement is operative here. The analysis of the hydride spectrum for compound 1 is the same as for 2 except that only the cis isomer was detected.The distortions in the cis and trans isomers indicated in Figure 3 suggest a polytopal rearrangement alternative to the trigonal or "Bailar" twist.5 The distortion modes could, in highly excited vibrational states, approach a tetrahedral disposition of phosphorus nuclei about the iron nucleus; hydrogen traverse (tunneling or classical) of trigonal faces would then complete the nuclear permutation process. This alternative is more attractive on steric grounds because of the obvious steric relief in the transition state. We cannot now rigorously distinguish between these two possible rearrangements, but detailed analysis of nmr transitional line shapes using the density matrix approach and analogous studies of other L4MH2 complexes now in progress may provide a definitive answer.Full details of spectral assignments and mechanistic studies will be published shortly.

The crystal structure of ferrioxamine E, Caa^sNsOgFe-l.S^O, has been determined with single-crystal x-ray diffraction. The crystals are triclinic, space group P\, two molecules per unit cell with dimensions a = 19.77 (1) A, b = 10.512 (2) k,c = 8.029 (2) A, a = 106.84 (2)°, ß = 93.91 (2)°, and y = 97.74 (3)°. The intensity data (3963) were collected on an automatic diffractometer. The structure, determined by the heavy-atom method, was refined by least-squares methods. The final R was 0.076 for all data. The Fe3+ ion is coordinated in a cis configuration to three hydroxamate groups. The molecule is quite flat with a total thickness of 3.6 A. The peptide carbonyl bonds are approximately perpendicular to the plane of the molecule and all point in the same direction.Most aerobic microorganisms contain compounds which solubilize and transport iron(III). These low molecular weight compounds are called siderochromes. The subject has recently been reviewed by Neilands1 and Emery.2 The stability constants of deferrisiderochromes, the ligands, with iron(III) are of the order of 1030, while they have little or no affinity for iron(II). The compounds are separated into families in which the iron binding atoms may be furnished by hydroxamic acid groups, carboxylic acid and hydroxyl group, or catechol residues. All siderochromes contain peptide linkages which are not coordinated to the iron-(III) atom. The families are ferrioxamines, ferrichromes, enterobactins, fusarinines, rhodotoluric acids, and aerobactins. Many of the compounds show either growth-factor or antibiotic activity.The first isolation and characterization of a siderchrome was performed by Neilands for ferrichrome.3 The compound showed potent growth-factor activity. An important discovery was also the fact that deferrisiderochromes are produced in high yield in the medium when the organisms are grown under conditions of low iron stress.4,5 This observation together with the high stability constants of siderochromes and other evidence led Neilands to suggest that these compounds act as cellular transport agents for iron in aerobic microorganisms.6 At the present time there is also substantial evidence for a process of "active" transport of siderochromes across the membrane into the cell.1,7The ferrioxamines are metabolic products of actinomycetes and were originally isolated and characterized as ferrioxamines A, B, C, Di, D2, E, F, and G by a group of workers in Switzerland.8,9 All ferrioxamines are trihydroxamates and the deferrioxamines are either cyclic (D2 and E) or linear. The chemical structures of the ferrioxamines are known. The structures were established by degradation methods and chemical syntheses by Keller-Schierlein,

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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 33-membered 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.

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