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

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 NH ₃. 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. © the Owner Societies.