| This research work examines the dissociation chemistries of ternary complexes containing Cu(II), a peptide or amino acid (M), and typically an auxiliary ligand (L). Many of these complexes dissociate to give radical cations of the peptides or amino acids, which have fascinating chemistries that differ substantially from their protonated counterparts. In particular, we studied the amino acids histidine (H), lysine (K), arginine (R), tryptophan (W) and tyrosine (Y), and peptides containing a residue of W or Y, with various auxiliary ligands.;Dissociations of complexes [CuII(L)(M)2] ·2+ and [CuII(M)4]·2+ involving tyrosine and tryptophan ester and amide lead to stable dimeric radical cations ([(M)2]·+). The yields of dimer radical cations are dependent on both the auxiliary ligand as well as the tryptophan or tyrosine derivatives used. Stabilizing interactions, most likely involving hydrogen bonding, between the two amino acid derivatives are proposed to account for observation of the dimer radical cations. Dissociations of these ions yield protonated or radical cationic amino acid derivatives, which are determined by proton competition between monomeric units.;Two isomeric amino-acid radical cations are generated by the CID of complexes for H, K and R via control of the auxiliary ligands. Type 1 radical cations, which are stable, are proposed to result from neutral (canonical) amino-acid coordination, whereas Type 2 radical cations, which are metastable, are from zwitterionic amino-acid coordination to copper in the complex. The stable Type 1 radical cations of 14 and K are alpha radical cations which lose water to form [b1 - H]·+ and further lose CO, while that of R behaves differently by losing dehydroalanine. The Type 2 radical cations easily fragment via the loss of carbon dioxide, effectively preventing their direct observation. The ratio of Type 1/Type 2 ions is controlled by the nature of the auxiliary ligand, with Type 2 being formed exclusively in the presence of monodentate acetone.;Complexes [CuII(Ma)(Mb)] ·2+ where Ma and Mb are dipeptides or tripeptides each containing either a tryptophan (W) or tyrosine (Y) residue and lack an auxiliary ligand have been studied. CIDs of complexes containing identical peptides with a tryptophan residue generate abundant radical cations of the peptides; by contrast, for complexes containing peptides with a tyrosine residue, the main fragmentation channel is dissociative proton transfer giving [Ma + H]+ and [CuII(Mb - H)]·+. When two different peptides, each containing a tryptophan residue, are present in the complex, radical cations are again the major products, with their relative abundance being dependent on the locations of the tryptophan residue in the peptides. In the CIDs of mixed complexes in which one peptide contains a tryptophan residue while the other a tyrosine residue, the major fragmentation channel is formation of the radical cation of the tryptophan-containing peptide.;Tryptophan or tyrosine, present in the form of an amino acid or as a residue in an oligopeptide, typically gives abundant molecular radical cations from fragmentation of the [CuII(L)(M)]· 2+ complex ion. The dissociation reactions of radical cations of tryptophan- and tyrosine-containing di- and tripeptides have been compared experimentally using collision-induced dissociation (CID). Many of the fragment ions are products of N--Calpha bond cleavages at the heteroresidues, forming either [zk -- H]·+ or [cn-k + 2H]+ ions. Density functional theory calculations at the B3LYP/6-311++G(d,p) level, using [GW]·+ and [GY] ·+ as prototypical examples, establish that the fragmentation involves a proton transfer from the beta-carbon to the carbonyl oxygen of the amide group, thereby forming an intermediate in which the charge and unpaired electron are separated and both delocalized in the forms of a protonated amide moiety and a benzylic radical. The energy barrier of this process for [GW] ·+ is higher than that for [GY]·+. The fragmentation products [zk -- H]·+ and [cn-k + 2H]+ are determined by the relative proton affinities of the two competing components in the intermediate. Other dissociation pathways are also evident: fragmentation of the Calpha--C bond and hydrogen transfer to the N-terminal fragment lead to the formation of [an + H]·+ ions and elimination of carbon dioxide. This pathway is competitive with the N---COE bond cleavage; another competitive pathway is the formation of [b n - H]·+. |
