Chromic ferrichrome complexes

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). 

Ferrichrome iron transport in the enteric bacteria affords an experimentally feasible model for mechanistic studies. The ligand is rugged and can be labeled to high specific activity by microwave discharge activation of tritium gas. The labile ferric ion can be replaced with chromium to yield the kinetically stable isostructural chromic complex, chromichrome (61). Assuming this analog has the transport properties of the iron complex, its 3H counts would be an index of the behavior of... 

Ferrichrome transport in E. coli adheres to mechanism 2, since the nondissociable chromic complex enters the cell at the same rate as the iron. It is somewhat mysterious that the ligand is so efficiently rejected in the more rapid phase of siderophore iron assimilation. The ligand— again as the chelate compound—enters the cell at a substantially slower rate. 

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

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