Biomedical subjects

Antal Rockenbauer

Publications and source records attributed to Antal Rockenbauer.

24 records · Page 2Linked to original sources

PubMed2003

An electron paramagnetic resonance study of copper(II)-beta-substituted beta-amino acid systems by the two-dimensional simulation method: first evidence of primarily steric effects of substituents on equilibria of metal complexes.

We have studied the complex equilibria of copper(II) with a series of beta-substituted beta-amino acids (R: H, Me, Et, iBu, iPr, cHex, 1-EtPr, and tBu) in aqueous solution by pH potentiometry and electron paramagnetic resonace (EPR) spectroscopy in the range pH = 2-8 at various metal and ligand concentrations. The basicities of the corresponding donor groups differed only slightly in the series of ligands. A purely mathematical method, the matrix rank analysis carried out on the EPR spectrum package recorded in the presence of copper(II), indicated the formation of 6 independent paramagnetic species. Accordingly, Cu(2+) (aqua complex) and the complexes [CuLH](2+), [CuL](+), [CuL(2)H(2)](2+), [CuL(2)H](+), and [CuL(2)] were considered in the subsequent analysis of series of spectra, and also two isomers of [CuL(2)] were identified. The formation constants and the EPR parameters, e.g. the isotropic g-factors and the copper and nitrogen hyperfine couplings for the above species, were determined in the same optimization procedure by the simultaneous evaluation of spectra. The ligands "LH" are suggested to bind in equatorial positions through their carboxylate groups, while the amino acids in the L protonation state are likely to occupy two equatorial sites via the amino and carboxylate groups. For the isomers of [CuL(2)], the donors of the same kind are in the cis or trans position. As far as we know, this is the first reported case in which a strong correlation has been found between the steric effects of substituents characterized by Meyer's steric parameter V(a) and the protonation constants of metal complexes. The observed trend for the preference for nonprotonated complexes [CuL](+) and [CuL(2)] to increase with the steric demand of the substituent was explained by the increasing shielding effect of the substituent hindering protonation of the nonprotonated complex.

Amino Acids↗

PubMed2003

Great structural variety of complexes in copper(II)-oligoglycine systems: microspeciation and coordination modes as studied by the two-dimensional simulation of electron paramagnetic resonance spectra.

A series of isotropic EPR spectra recorded at various concentrations and pH (in the range 2-12) on equilibrium systems containing copper(II) and diglycine, triglycine, or tetraglycine were analyzed. A purely mathematical method, matrix rank analysis gave the number of independent EPR-active species. Two-dimensional evaluation then resulted in the formation constants and magnetic parameters of 14 metal complexes (including microspecies) in each system. The independent paramagnetic species formed with each ligand are as follows: Cu(2+) (aqua complex), [CuLH](2+), [CuL](+), [CuLH(-1)], [CuLH(-2)](-), [CuL(2)H(2)](2+), [CuL(2)H](+), [CuL(2)], [CuL(2)H(-1)](-), and [CuL(2)H(-2)](2-). Moreover, for diglycine, the diamagnetic complex [Cu(2)L(2)H(-3)](-), and for triglycine and tetraglycine, the EPR-active species [CuLH(-3)](2-) were identified. Further, equilibria of two microspecies were demonstrated for [CuL(2)], [CuL(2)H(-1)](-), and [CuL(2)H(-2)](2-). The magnetic parameters allowed a detailed description of the coordination modes. The most important findings: (1) For the mono complexes, the in-plane sigma-bonds between copper(II) and the equatorial N donors are particularly strong when the same ligand forms several adjacent chelate rings with the participation of amino N, deprotonated peptide N(s), and the carboxylate group. (2) Structures with coupled chelate rings are likewise favored in the bis complexes. Different protonation states of the two ligands are observed in the major isomer of [CuL(2)] ((LH(-1) + LH) coordination), and in the isomers of [CuL(2)H(-2)](2-) ((LH(-2) + L) coordination) for triglycine and tetraglycine.

Computer Simulation↗

PubMed2002

Spin trapping of superoxide in the presence of beta-cyclodextrins.

The trapping of superoxide anion with DMPO and DEPMPO has been carried out in the presence of a methylated beta-cyclodextrin, Me-beta-CD; inclusion of the spin adducts in the cavity of Me-beta-CD resulted in a seven-fold increase of their half-life (t1/2 = 96 min for DEPMPO-superoxide spin adduct) and in their protection towards glutathione peroxidase (Gpx) and also ascrobate anion in the case of DEPMPO.

Cyclodextrins↗

PubMed2002

Synthesis, structure and catecholase activity of dinuclear copper and zinc complexes with an N(3)-ligand.

The preparation and characterization of dinuclear [M(II)(dbcat)(idpa)](2) (M[double bond]Zn (1), Cu (3); dbcat[double bond]3,5-di-tert-butylcatecholate; idpa[double bond]3,3'-iminobis(N,N-dimethylpropylamine)) complexes are described. Crystallographic characterization of the complex [Cu(II)(dbcat)(idpa)](2) has shown that the co-ordination geometry around copper(II) ions is distorted square pyramidal (triclinic, P-1, a=10.576(1) A, b=11.927(1) A, c=12.621(1) A, alpha=77.89(1) degrees, beta=88.65(1) degrees, gamma=70.21(1) degrees, V=1462.7(2) A(3), Z=2, R=0.0387). Both 1 and 3 were suitable catalysts for the catalytic oxidation of dbcatH(2) to dtbq (dtbq=3,5-di-tert-butyl-1,2-benzoquinone) with dioxygen at ambient conditions in good yields. However, on the basis of kinetic studies the copper- and zinc-catalyzed reactions showed different mechanisms. In the first case valence tautomerism [Cu(II)(dbcat)(idpa)]<==>[Cu(I)(dbsq)(idpa)] precedes the reaction with O(2), while with the zinc complex metal-bound catecholate reacts directly with O(2) with the formation of free superoxide anion.

Catechols↗

PubMed2002

Microspeciation in the copper(II)-L-histidylglycine system. An ESR study by the two-dimensional computer simulation method.

Twelve ESR-active (and one inactive) copper(II) complexes of L-histidylglycine (HL) were characterized via their formation (micro)constants and ESR parameters obtained by two-dimensional ESR spectroscopic evaluation in aqueous solution. In strongly acidic media, the ligand is coordinated through its N-terminal donor groups: the complex [CuLH(2)](3+) involves monodentate imidazole binding, whereas [CuLH](2+) involves bidentate ligation through the amino and imidazole N atoms. This histamine-like bonding mode also predominates in the isomers of [CuL(2)], formed at ligand excess near pH 7: in the major 4N isomer, both ligands occupy two equatorial sites, while in the 3N isomer, the second dipeptide is coordinated equatorially by the amino and axially by the imidazole groups. At above pH 3-4, deprotonation of the peptide group also starts: in approximately 60% of the molecules of [CuL](+), the peptide group is deprotonated, while in the minor isomer histamine-like coordination occurs. At higher pH, the active dimer [Cu(2)L(2)H(-2)], the mixed hydroxo complexes (the inactive [Cu(2)L(2)H(-3)](-) and the active [CuLH(-2)](-)), and the bis complexes [CuL(2)H](+) and [CuL(2)H(-1)](-) all involve tridentate equatorial ligation of the backbone by the amino and deprotonated peptide N and the carboxylate O atoms. In the active dimer, the neutral imidazole groups form bridges between CuLH(-1) units. In [CuL(2)H](+), the second ligand is bound equatorially via its imidazole group; in [CuL(2)H(-1)](-), the L ligand occupies the fourth equatorial site and an axial site through its amino and imidazole N atoms, respectively.

Amino Acids↗

PubMed2002

Observation of protein-derived (BSA) oxygen-centered radicals by EPR spin-trapping techniques.

Electron paramagnetic resonance (EPR) spin-trapping experiments, employing the novel spin-trap DEPMPO, provide evidence for the formation of protein-peroxyl radicals from the reaction of bovine serum albumin (BSA) or lysozyme with HO. in the presence of O2. Spin-trapping leads to the detection of anisotropic spectra of partially immobilized protein-peroxyl spin-adducts; positive identification is based on a novel spectrum simulation approach (through which broadened anisotropic spectra are simulated and compared with experiment) and by comparison of results with those obtained when MeO2. is trapped and the adduct frozen in a solid matrix.

Animals↗