Nancy S. Rowan

Properties of metal-ion coordinated imidazoles: nmr and C-2 H Exchange in Co(III) Complexes

Abstract The 1 H and 13 C nmr spectra of Co(NH 3 ) 5 ImH 3+ and the 1 H nmr spectra of αCotrien(ImH) 2 3+ and βCotrien(ImH) 2 3+ are reported. The p K a values determined from the dependence of the chemical shift on pH are 10.0, 9.6, and 10.1, respectively. The range of the chemical shift between the acid and base forms is unusually small in the 1 H nmr, 0.5–0.7 ppm for the C-2 H and about 0.25 ppm for the C-4 H and C-5 H. In the 13 C nmr, C-2 and C-4 have large shifts to low field and C-5 a small shift to high field on deprotonation. The C-2 proton is not exchanged with solvent 2 H under acidic or basic conditions, in marked contrast to the corresponding proton in both imidazole and 1-methylimidazole. These spectroscopic and chemical properties should be useful for the direct identification of metal-ion coordinated histidines in proteins.