Interaction between Side-Chain Pyridyl Groups of α-Helical Polypeptide and Guest Trp in the Ternary Cu2+ Ion Complex

Abstract

Abstract Interaction between the side-chain pyridyl ligands of an α-helical poly(Nω-2-pyridylmethyl-l-glutamine) (P2PG) and guest ligands such as tryptamine and d- and l-tryptophan (d- and l-Trp) within ternary Cu2+ complexes was investigated by absorption and circular dichroism (CD) spectroscopy in 2,2,2-trifluoroethanol solutions. The host complex species, Cu2+-one pyridyl-substituted side-chain (s-2PG) complexes (1 : 1 Cu2+-P2PG, [Cu2+]/[s–2PG] > 1.0) and Cu2+- two s-2PGs complexes (1 : 2 Cu2+-P2PG, [Cu2+]/[s–2PG] < 1.0), were prepared, using Cu2+ and P2PG depending on the Cu2+ ion concentration. All guest molecules could bind noncovalently to the 1 : 1 Cu2+-P2PG system, having the ligand sites of Cu2+ partly unfilled, to yield their ternary Cu2+ complexes. The respective ternary complexes had induced optical activities in the CD spectra and a charge-transfer band indicating ligand–ligand stacking interactions involving charge transfer between the coordinated s-2PG and the guest indole moieties around Cu2+. The CD spectral intensity was in the order tryptamine < d-Trp, l-Trp, which suggests that the coordination of both amino and carboxyl groups of Trp to Cu2+ is important for the regular arrangement of the ligands on the periphery of the α-helix backbone of P2PG. The differences in the CD spectra suggest that the indole rings of d- and l-Trp face in a direction opposite to that of the pyridyl ring of P2PG. d- and l-Trp could also bind the 1 : 2 Cu2+-P2PG host system, whose Cu2+ coordination sites are filled with s-2PGs, but the 1 : 2 Cu2+-P2PG did not yield the ternary complexes with tryptamine.