Iron amide linkages

Carbon monoxide reacts with [Fe(TPP)] to form a five-coordinate complex [Fe(TPP)CO], which can be reduced electrochemically to the corresponding iron(I) species from which, however,245 CO spontaneously dissociates. The Fe—CO interaction is stabilized by the,presence of hydrocarbon chains bound by amide linkages to the ortho position of the TPP phenyl rings. Carbon monoxide adducts of iron(I) complexes of a number of these superstructured porphyrins have been reported.245 The chemistry of these highly reduced species is of relevance to understanding240 the reactions of cytochrome P-450 and the peroxidases. 

N, 6-N-di(2,3-dihydroxybenzoyl)-L-lysine (58) is a siderophore produced by Azotobacter vinelandii which has only two catechol groups. However, of the catecholate siderophores by far the best studied is enterobactin. A major difference between hydroxamate and catecholate siderophores occurs in their utilization as transport agents. For the former, the iron complex is taken up by the bacterial cell, the iron released, and the hydroxamate siderophore re-secreted for additional iron chelation. In contrast, enterobactin is destroyed by enzymatic hydrolysis within the cell and therefore the ligand is not recycled. This hydrolysis of the amide linkages of the iron(III) enterobactin lowers the redox potential of the chelate complex sufficiently to allow iron reduction — and thus uptake of iron into the cell metabolism (59, 60). 

Momenteau s route to doubly-strapped porphyrins was easily adapted to produce compounds in which an axial base was incorporated into one of the straps Condensation of tetra(o-hydroxyphenyl)porphyrin (250) (mbcture of four isomers) with one equivalent of 1,12-dibromododecane gave a mixture of two singly-linked porphyrins, depending on whether adjacent (255), or opposite (254) meso-phenyl groups were linked. This mixture was reacted with 3,5-bis(3-bromopropyl)pyridine 256 and the desired cross trans-linked isomer 257 isolated by preparative tic (5% overall yield) (Scheme 77). A similar porphyrin 259 was prepared from a,p,a,p-tetra(o-aminophenyl)porphyrin (142) in this case the straps were tied to the porphyrin skeleton by amide linkages (Scheme 78). Following iron insertion and reduction, visible absorption and H-NMR spectra of both compounds were consistent with a five-coordinate high spin (S = 2) iron(II) complex. 

Yu et al. have covalently linked myoglobin and horseradish peroxidase to an aligned SWCNT electrode through the amide linkage. Quasi-reversible Ee +/Ee + voltammetry was observed for the iron heme enzymes, myoglobin, and horseradish peroxidase. The results demonstrated that the trees in the nanotube forest are potent current collectors that conduct electrons from the external circuit to the redox sites of the enzymes. 

Hydroxamic acids are in general colorless, somewhat low-melting solids. However, most of them can be crystallized and the potassium salt of benzhydroxamic acid forms beautiful plates. In terms of stability to acid, the hydroxamic acid bond is intermediate between that of amides and esters. By use of carefully controlled conditions it is possible to split the hydroxamic bond in the presence of the peptide linkage. In the absence of iron, hydrolysis is effected cleanly with hot mineral acid in base extensive degradation occurs even without a contaminating metal ion. Once it has been liberated from the hydroxamic acid linkage the hydroxylamino moiety should always be maintained at a pH below the pKa ( 5). 

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