NMR study of the kinetics of the proton-exchange reactions in aqueous solutions of copper(II)-amino acid parent complexes

Conformational Analyses of Physiological Binary and Ternary Copper(II) Complexes with l-Asparagine and l-Histidine; Study of Tridentate Binding of Copper(II) in Aqueous Solution

This study explores the structural properties and energy landscapes of the physiologically important bis(l-asparaginato)copper(II) [Cu(l-Asn)2] and (l-histidinato)(l-asparaginato)copper(II) [Cu(l-His)(l-Asn)]. The conformational analyses in the gas phase and implicitly modeled water medium, and magnetic parameters of electron paramagnetic resonance spectra were attained using density functional theory calculations. The apical CuII coordination and hydrogen bonding were analyzed. Predicted lower-energy structures enabled the confirmation and, for apical bonding, also the refinement of structural proposals from literature. Available experimental results were indecisive regarding the amido-group binding in the CuII equatorial plane in solutions, but the examination of the relative stability of Cu(l-Asn)2 conformers in 30 binding modes confirms the glycine-like mode as the most stable one. Previously reported experimental results for Cu(l-His)(l-Asn) were interpreted for l-His to have a tridentate histamine-like mode. However, the aqueous conformers with l-His in the glycinato mode are also predicted to have low energies, which does not contradict the tridentate l-His binding. The predicted magnetic parameters of conformers with an apical oxygen atom (intramolecular or from a water molecule) can reproduce the experimental data. An extent of conformational flexibility and abundance of l-His-containing ternary copper(II) amino acid complexes under physiological conditions may be related.

Zn(II) complexes of glutathione disulfide: structural basis of elevated stabilities

Glutathione disulfide (GSSG), a long disregarded redox partner of glutathione (GSH), is thought to participate in intracellular zinc homeostasis. We performed a concerted potentiometric and NMR spectroscopic study of protonation and Zn(II) binding properties of GSSG ((γECG)(2)) and a series of its nine analogs with C-terminal modifications, tripeptide disulfides: (γECS)(2), (γECE)(2), (γECG-NH(2))(2), (γECG-OEt)(2), and (γEcG)(2); dipeptide disulfides, (γEC)(2) and (γEC-OEt)(2); and mixed disulfides, γECG-γEC and γECG-γEC-OEt. The acid-base and Zn(II) complexation properties in this group of compounds are strictly correlated to average C-terminal electrostatic charges. In particular, it was demonstrated that GSSG assumes a bent (head-to-tail) conformation in solution at neutral pH, which is controlled by electrostatic attraction between the protonated γ-amino groups of the Glu residue and the deprotonated C-terminal Gly carboxylates. This interaction modulates the ability of GSSG to coordinate Zn(II), both indirectly, by affecting the basicities of the amino groups, and directly, through the participation of the Gly carboxylates in the outer coordination sphere of the Zn(II) ion. A specific coiled structure of the major [Zn-GSSG](2-) complex is additionally stabilized by the formation of hydrogen bonds between glycinyl carboxylates and two Zn(II)-coordinated water molecules. The elevated stability of Zn(II)-GSSG complexes was demonstrated by competition with FluoZin-3, a fluorescent sensor with high Zn(II) affinity, commonly used in in vitro and in vivo studies. The potential biological functions and reactivity of GSSG complexes of Zn(II) ions are discussed.

A two-dimensional (magnetic field and concentration) electron paramagnetic resonance method for analysis of multispecies complex equilibrium systems. information content of EPR spectra

A two-dimensional simulation method has been developed for the interpretation of electron paramagnetic resonance (EPR) spectra consisting of a multitude of strongly overlapping signal components. The set of EPR spectra for complex equilibrium systems is analyzed simultaneously as a function of metal and ligand concentrations and pH. The formation constants of the various species are adjusted together with the magnetic parameters of the component EPR spectra. At most 10 EPR-active and 5 EPR-silent species can be involved to simulate a maximum of 36 experimental spectra, while the number of adjusted parameters is at most 100. Statistical parameters are suggested to give the confidence intervals for parameter estimation and to distinguish alternative speciation models. The efficiency of the program is demonstrated for the copper(II)--L-asparagine system, in which 10 species, including 3 pairs of isomers, are characterized with magnetic parameters and formation constants. On the basis of the magnetic parameters, a structural assignment is made for the detected species. The two-dimensional approach can also supply the formation constant of the EPR-silent species, as demonstrated for the copper(II)--glycyl-L-serine system.