Dissociations of copper(ii)-containing complexes of aromatic amino acids: radical cations of tryptophan, tyrosine, and phenylalanine
Physical Chemistry, Chemical Physics - PCCP
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 0 -chloro-2,2 0 :6 0 ,2 00 -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
... ntrast, 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 H 2 O and CH 3 OH (giving [Cu II (4Cl-tpy)(H 2 O)] 2+ and [Cu II (4Cl-tpy)(CH 3 OH)] 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 NH 3 proceeds via a "phenonium-type" intermediate. Dissociative electron transfer in [Cu II (4Cl-tpy)(M-NH 3 )] 2+ results in [M-NH 3 ] + . The [Phe-NH 3 ] + ion dissociates facilely by eliminating CO 2 and giving a metastable phenonium-type ion that rearranges readily into the styrene radical cation.