title: Reinvestigation of the copper(II)-carcinine equilibrium system: "two-dimensional" EPR simulation and NMR relaxation studies for determining the formation constants and coordination modes.
abstract: The equilibria and solution structure of complexes formed between copper(II) and carcinine (beta-alanyl-histamine) at 2< or = pH< or =11.2 have been studied by EPR and NMR relaxation methods. Beside the species that have already been described in the literature from pH-potentiometric measurements, several new complexes have been identified and/or structurally characterized. The singlet on the EPR spectrum detected in equimolar solutions at pH 7, indicates the formation of an oligomerized (CuL)n(2n+) complex, with [NH2,Nim] coordination. The oligomerization is probably associated with the low stability of the ten-membered macrochelate ring, which would form in the mononuclear complex CuL2+. In presence of moderate excess of ligand the formation of four new bis-complexes (CuL2Hn(2+n), n=2,1 and 0/-1) was detected with [Nim][Nim], [NH2,Nim][Nim] and [NH2,N-,Nim][Nim] type co-ordination modes, respectively. At higher excess of ligand ([L]/[Cu2+]>10) and at pH approximately 7, the predominant species is CuL4H2(4+). The 1H and 13C relaxation measurements of carcinine solutions (0.6 M) in presence of 0 mM< or = [Cu2+](tot)< or = 5 mM at pH=6.8, allowed us to extract the carbon-to-metal distances, the electronic relaxation and tumbling correlation times, as well as the ligand exchange rate for the species CuL4H2(4+). According to these results, the metal ion is [4Nim] co-ordinated in the equatorial plane, while the neutral amino groups are unbounded. Since naturally occurring carcinine shows in vivo antioxidant property, the SOD-like activity of the copper(II)-carcinine system has also been investigated and the complex CuLH(-1) was found to be highly active.
authors.affiliation: Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary.
authors.forename: Zsuzsanna
authors.initials: Z
authors.lastname: Arkosi
authors.forename: Zoltán
authors.initials: Z
authors.lastname: Paksi
authors.forename: László
authors.initials: L
authors.lastname: Korecz
authors.forename: Tamás
authors.initials: T
authors.lastname: Gajda
authors.forename: Bernard
authors.initials: B
authors.lastname: Henry
authors.forename: Antal
authors.initials: A
authors.lastname: Rockenbauer
mesh_terms: D002336:Carnosine / Q000031:analogs & derivatives* / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D012996:Solutions / Q000737:chemistry; D013482:Superoxide Dismutase / Q000032:analysis
chemical_list: D008024:Ligands; D012996:Solutions; D003300:Copper; D002336:Carnosine; D013482:Superoxide Dismutase; C015055:carcinine