Copper ion complex formation of polypeptide containing side‐chain pyridyl groups

The complex formation behaviors of an α-helical polypeptide containing pyridyl ligands in the side chains, poly (Nω-2-pyridylmethyl L-glutamine) (P2PG), were investigated by absorption and CD spectroscopy using Cu2+ ion as a guest molecule in 2,2,2-trifiuoroethanol solution. In the low Cu2+ ion concentration, P2PG exhibited the predominant formation of Cu2+ and two pyridyl side-chain complexes, involving a regular arrangement of the pyridyl side chains. On the other hand, the complexes were converted to Cu2+ and one pyridyl side-chain species with increasing Cu2+ ion concentration. The conversion was accompanied by the disappearance of the side-chain ordered structure without any changes in the backbone conformation. Moreover, the still remaining coordination sites of Cu2+ were capable of complexing monomer pyridine (Py) added as a second guest to form ternary complexes, P2PG-Cu2+-Py, following the reconstitution of the ordered structure on the periphery of the α-helix backbone. The unique characteristics of P2PG can be explained in terms of the restriction of the pyridyl side chains to form intramolecular chelate complexes, depending on the rigid α-helix conformation. © 1994 John Wiley & Sons, Inc.