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

摘要

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)(CH3QH)]~(·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.