Hydroxamate complexes

Another factor that will affect the complex redox potential is the architecture of the siderophore. A plot of ferri-siderophore redox potentials as a function of pFe for a series of hydroxamate complexes of differing denticity (shown in Fig. 19) exhibits a trend. The trend demonstrates that hydroxamate siderophores of higher denticity will form complexes with more negative E1/2 values than analogous siderophores of lower denticity. 

For hydroxamic acids, it is generally assumed that it is the Af-hydroxyamide/keto form, as opposed to the hydroximic/hydroxyoxime form, that predominates in acid medium, the environment usually required for most precipitates or colors to form . It is in general unknown what is the stoichiometry and structure of most metal hydroxamate complexes in solution. Nevertheless, the reaction of the majority of hydroxamic acids with metal ions can be written schematically as shown in equation 2. 

V Chemical Shifts for Selected Hydroxamate Complexes of Vanadate... 

Anderegg, G., F. L Eplattenier, and G. Schwarzenbach Hydroxamate Complex, III. Iron (III) exchange between sideramins and complexones. Helv. Chim. Acta 46, 1409 (1963). 

Schwarzenbach, G. and K. Schwarzenbach Hydroxamate complexes, I. The stability of the iron (III) complex of simple hydroxamic acids and ferrioxamine B. Helv. Chim. Acta 46, 1390 (1963). 

Hassib et al. [29] determined mefenamic acid by ferric hydroxamate complex formation and measurement of the absorbance of the colored complex at 530 nm. The method is applicable to mefenamic acid amounts varying from 0.5 to 74.5 mg/25 mL, and Ponstan capsules were successfully determined using this method. 

The structure of the simple hydroxamate complex tris(benzohy-droxamato) iron (III) (R = , R = H in Figure 1) has shown the most stable crystalline form of the solid to be the racemic cis isomer (11). (The convention for symbols of absolute configurations A and A are those of the IUPAC Proposal (12). The cis isomer is defined as the isomer... 

Model Hydroxamate Complexes. Attempts to prepare tris( hydroxamate) complexes of Co (III) with benzohydroxamic acid or its... 

The isomers of Cr(men)3 isomerize with half-lives (several hours) similar to the Cr(benz)3 complex. The rate of isomerization of the tris-(hydroxamate) complexes is therefore not particularly sensitive to the substituent of the hydroxamate nitrogen atom, since the men ligand contains an alkylated nitrogen atom, and the benz ligand contains an unsubstituted nitrogen atom. In the absence of an induced strain, the corresponding siderophore complexes must isomerize much more slowly because of the steric constraints of the ligand. 

Chromic Ferrichrome Complexes. The spectra for the model chromic hydroxamate complexes are reproduced in Figure 6. Since the visible and CD spectra of the isomers are wholly dominated by the metal complex chromophore, these data can be used to characterize and to identify coordination isomers of complexes formed by the siderophores. The preparation and characterization of the chromic complexes of des-ferriferrichrome and desferriferrichrysin have been reported (3). Although an examination of molecular models for both complexes shows two coordination isomers are possible (A-cis and A-cis), both chromic complexes consist exclusively of the A-cis isomer. These results agree with x-ray crystallographic investigations which have shown that both ferri-chrysin and ferrichrome A crystallize as only the A-cis isomer (14, 15). Both chromic complexes have identical CD spectra which are the same as the A-cis Cr(men)3 spectrum (Figure 6). 

Both the cis and trans geometrical isomers of chromic ferrioxamine B isomerize to equilibrium solutions with half-lives of several days at room temperature. This is considerably slower than that found for the simple tris hydroxamate complexes such as Cr(men)3 and is caused by the steric constraints of the ferrioxamine B ligand and its hexadentate chelation. 

The markedly negative redox potentials of tris-catecholate and tris-hydroxamate iron complexes (Figure 4) may be ascribed to the high stabilities of the iron(III) complexes and the rather low stabilities of their iron(II) analogues. Table 9 details the relevant data (interconnected by a thermochemical cycle earlier applied to amino acid pentacyanoferrate complexes ), and documents the remarkably higher stabilities of tris-catecholate than of tris-hydroxamate complexes of iron(III). 

Figure 6 The geometry and dimensions of the iron-coordination octahedron in a natural tris(hydroxamate) complex, triacetylfiisari-nine. (Reprinted with permission from Ref. 71. 1980 American Chemical Society)...

A comparison of the stability constants of the naturally occurring siderophores uncovers a difference of f7 orders of magnitude between enterobactin (K most stable hydroxamate complex, ferrioxamine E. Using the more comparable pM values, enterobactin remains stiU eight orders of magnitude more effective than ferrioxamine E. Enterobactin has the highest affinity for Fe ion of any biological iron chelator tested so far. 

A number of zinc compounds and complexes have been developed for therapeutic purposes. Several zinc complexes of cyclam-based ligands have proved potent inhibitors of HIV, while hydroxypyridinone complexes are being assessed as insulin mimics for the treatment of diabetes. There is some evidence that the administration of certain zinc chelates may help to ward off strokes, while a suberoylanihde hydroxamate complex has been in clinical trials as an anticancer agent since 2000. 

Holmen B. A., Tejedor-Tejedor M. I., and Casey W. H. (1997) Hydroxamate complexes in solution and at the goethite-water interface a cylindrical internal reflection Fourier transform infrared spectroscopy study. Langmuir 13, 2197-2206. 

Hydroxamate. Hydroxamate complexes of trivalent actinides can be prepared directly in aqueous solution and other polar solvents and extracted into organic solvents, but due to the high thermodynamic stability of the corresponding tetravalent actinide complexes they are rapidly oxidized. They can also be prepared in solution via electrochemical reduction of the tetravalent complexes. These complexes have been studied for their role in separating high and low valent actinides in nuclear fuel processing schemes. ... 

Hydroxamate. Similar to trivalent hydroxamates, pentavalent actinide hydroxamate complexes are generally unstable relative to tetravalent and/or hexavalent complexes. Pu or Pu hydroxamate complexes can be prepared however, at near-neutral and basic pH they rapidly reduce to Pu complexes. ... 

Simple complexes of hydroxamic acids with vanadium(V) have been reported, including ones with a variety of nuclearities.249,250 A wide range of TV-substituted mono- and dihydroxamic acids undergo oxygen abstraction upon reaction with V111 and Vlv to form Vv hydroxamate complexes and the respective amides and diamides.251... 

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