Specific binding of the tyrosine residue in copper(II) complexes of Tyr-Pro-Gly-Tyr and Tyr-Gly-Pro-Tyr

Abstract

The syntheses of the tetrapeptides Tyr-Pro-Gly-Tyr and Tyr-Gly-Pro-Tyr (H₃L) are reported, together with the results of a potentiometric and spectrophotometric study of their H⁺ and Cu²⁺ complexes. Proline acts as a break-point to metal-ion co-ordination when inserted into a peptide chain and in Tyr(1)-Pro(2)-Gly(3)-Tyr(4)[Pro(2)] the Pro residue enforces a bent conformation and the formation of an unusually stable [CuHL] complex with co-ordination through the terminal amine-N of Tyr(1), the neighbouring peptide CO, and the O_Tyr^– of Tyr(4). This necessitates a 17-membered chelate ring. With Tyr-Gly-Pro-Tyr [Pro(3)] there is also O_Tyr^––C bonding but this is a result of dimer formation in [(CuL)₂]^2–