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_-2XNO₂)₃], were synthesized and their structure was determined by NMR spectroscopy.