Nitrogen-15 NMR chemical shifts in oligopeptides coordinated to cobalt(III)

Dipeptide, tripeptide, and tetrapeptide complexes with cobalt(III) ions were studied as model compounds for evaluation of 15N NMR chemical shifts induced in proteins upon binding transition metal ions. Coordination of oligopeptides to cobalt(III) resulted in large negative 15N NMR shifts for amine nitrogens (-76 to -32 ppm) and deprotonated amide nitrogens (-47 to -10). Coordination-induced shifts were affected by the nature of moiety at the trans position; the shifts were always larger with a carboxylato oxygen than with an amine nitrogen in the trans position. Thus, coordination-induced 15N NMR shifts provided direct and specific information on the stereochemistry of peptide coordination. Two new complexes, [Co(Gly-gly-gly-glyH(-3))(NH3)2] and Ba[Co(Gly-L-hisH(-2))(NO2)3], were synthesized and their structure was determined by NMR spectroscopy.