Dissociations of copper(ii)-containing complexes of aromatic amino acids: radical cations of tryptophan, tyrosine, and phenylalanine

Abstract

The dissociations of two types of copper(II)-containing complexes of tryptophan (Trp), tyrosine (Tyr), or phenylalanine (Phe) are described. The first type is the bis-amino acid complex, [Cu^II(M)₂]˙²⁺, where M = Trp, Tyr, or Phe; the second [Cu^II(4Cl-tpy)(M)]˙²⁺, where 4Cl-tpy is the tridendate ligand 4′-chloro-2,2′:6′,2″-terpyridine. Dissociations of the Cu(II) bis-amino acid complexes produce abundant radical cation of the amino acid, M˙⁺, and/or its secondary products. By contrast, dissociations of the 4Cl-tpy-bearing ternary complexes give abundant M˙⁺ only for Trp. Density functional theory (DFT) calculations show that for Tyr and Phe, amino-acid displacement reactions by H₂O and CH₃OH (giving [Cu^II(4Cl-tpy)(H₂O)]˙²⁺ and [Cu^II(4Cl-tpy)(CH₃OH)]˙²⁺) are energetically more favorable than dissociative electron transfer (giving M˙⁺ and [Cu^I(4Cl-tpy)]⁺). The fragmentation pathway common to all these [Cu^II(4Cl-tpy)(M)]˙²⁺ ions is the loss of NH₃. DFT calculations show that the loss of NH₃ proceeds via a “phenonium-type” intermediate. Dissociative electron transfer in [Cu^II(4Cl-tpy)(M–NH₃)]˙²⁺ results in [M–NH₃]˙⁺. The [Phe–NH₃]˙⁺ ion dissociates facilely by eliminating CO₂ and giving a metastable phenonium-type ion that rearranges readily into the styrene radical cation.