Cobalt(III) complexes of L-histidylglycylglycinate: Preparation, characterization, and conformation

The preparations of three cobalt(III) complexes of L-histidylglycylglycinate are reported. In one complex, [Co(NH3)(H-2HisGG)], the peptide is coordinated as a quinquedentate chelate through the terminal NH2, two deprotonated peptide nitrogens, the carboxylate, and the imidazole group. In another, [Co(NH3)2(H-2HisGG)], the carboxylate is replaced by a second NH3. The third complex has properties consistent with the superoxo binuclear complex NH4[(H-2HisGG)Co(O2 -)Co(H -2HisGG)]. The structures of the first two complexes were derived from the complexes' electronic absorption, circular dichroism, and 1H and 13C NMR spectra. The coordinated imidazole's NH-1 has acid dissociation constants at 298 K of 10.73 ± 0.04 and 10.69 ± 0.04 for [Co(NH3)(H-2HisGG)] and [Co(NH3)2(H-2HisGG)], respectively. In the diammine complex, the free carboxylate group has a pKa, value of 4.34 ± 0.04 at 298 K