Peptide axial coordination

Axial Coordination to Copper (II)—Peptide Complexes. Although inplane coordination dominates the thermodynamic, kinetic, and spectral properties of the Cu( II)-peptides, axial coordination also is important. While the carboxylate groups in Cu(H.3G4)2" and Cu(H 2GGhis)"... 

Structural studies95-97,101 103 on cytochromes of the c and c2 types show that the heme group provides a core around which the peptide chain is wound. The 104 residues of mitochondrial cytochrome c are enough to do little more than envelope the heme. In both the oxidized and reduced forms of the protein, methionine 80 (to the left in Fig. 16-8A) and histidine 18 (to the right) fill the axial coordination positions of the iron. The heme is nearly "buried" and inaccessible to the surrounding solvent. 

In nickel(III) peptide complexes, there is a strong in-plane field provided by the deprotonated peptide linkages (117, 118). Two axially coordinated water molecules are present in the tetragonally distorted complexes which exchange much more rapidly than for the [14]aneN4 species with a substitution rate of constant >106 M l sec-1 for the formation of the imidazole complex (141). However, except for the terminal peptide group, equatorial substitution is very slow. Substitution and rearrangement (125) reactions of these species reveal acid-... 

The development of synthetic enzymes and proteins has also been achieved through the preparation of structurally defined peptide nanostructures. A nice example, reported by DeGrado and co workers [67], is the construction (Fig. 27) of a four a-helix bundle system (72) that was shown to complex four metalloporphyrins by their axial coordination with the imidazole of the properly oriented histidines. This type of structure could be used as an artificial photosynthetic center. Along the same lines, Benson and co-workers [68] recently prepared a miniature hemoprotein, 73, by linking two units of a 13-amino acid peptide to a porphyrin. UV-visible and CD studies confirmed that the metalloporphyrin is indeed sandwiched between the a-helical peptides, as depicted in 73. 

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