Evidence for the cation-π interaction between Cu²⁺ and tryptophan

The cation-π interaction, a noncovalent interaction of electrostatic nature between a cation and an electron-rich π system, is increasingly recognized as an important force that influences the structures and functions of molecules including proteins. Unlike other metal cations, the transition metal cation Cu²⁺ is not regarded to take part in a cation-π interaction because Cu²⁺ tends to oxidize the π electron system, in particular that of Trp, and to introduce covalency in the metal-π electron interaction. This paper reports the first spectral evidence for the cation-π interaction between Cu²⁺ and Trp. The Cu²⁺ ion bound to the amino N-terminal Cu²⁺/Ni²⁺ binding motif composed of three amino acid residues interacts with the indole ring of the fourth Trp residue in a noncovalent manner. The Cu²⁺-Trp interaction produces a distinct negative band at 223 nm in circular dichroism (CD), which disappears upon mutation or depletion of the Trp residue or upon replacement of the Cu²⁺ ion by Ni²⁺. In UV absorption, a pair of negative/positive intensity changes is generated at 222/231 nm by the Cu²⁺-Trp interaction, being consistent with the previous observations on the indole ring interacting with K+ or a cationic His imidazole ring. The negative CD band around 223 nm is characteristic of the Cu²⁺-Trp pair and may be useful as a marker of the Cu²⁺-Trp cation-π interaction. Coordination of negatively charged ligands to Cu²⁺ is suggested to be important for the cation to be involved in a cation-π interaction.