Solution coordination chemistry of protohaemin IX peptide derivatives

As part of an investigation aimed at developing artificial models for mimic king haemoprotein activity, four C-aminoacyl protohaemin IX derivatives wit h one or two -L-Leu-L-HisOMe (complexes 3 and 1) or -beta-Ala-L-HisOMe (com plexes 4 and 2) peptide fragments have been studied by means of electronic and ESR spectroscopy. The bis-His complexes 1 and 2 are predominantly low-s pin (S = 1/2) species, while the mono-His complexes 3 and 4 are predominant ly high-spin (S=5/2) species. Products of the treatment of starting complex es 1-4 with trichloroacetic acid, acetic acid, methylimidazole, sodium cyan ide and sodium hydroxide have been analysed by electronic spectroscopy. Wea k-field ligands lead to predominantly high-spin hexacoordinated complexes, while strong-field ligands result in predominantly low-spin ones. Copyright (C) 1999 John Wiley & Sons, Ltd.