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Zoltan Kovacs

Publications and source records attributed to Zoltan Kovacs.

9 recordsLinked to original sources
PubMed2006

Equilibrium and formation/dissociation kinetics of some Ln(III)PCTA complexes.

The protonation constants () of 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (PCTA) and stability constants of complexes formed between this pyridine-containing macrocycle and several different metal ions have been determined in 1.0 M KCl at 25 degrees C and compared to previous literature values. The first protonation constant was found to be 0.5-0.6 log units higher than the value reported previously, and a total of five protonation steps were detected (log = 11.36, 7.35, 3.83, 2.12, and 1.29). The stability constants of complexes formed between PCTA and Mg2+, Ca2+, Cu2+, and Zn2+ were also somewhat higher than those previously reported, but this difference could be largely attributed to the higher first protonation constant of the ligand. Stability constants of complexes formed between PCTA and the Ln3+ series of ions and Y3+ were determined by using an "out-of-cell" potentiometric method. These values ranged from log K = 18.15 for Ce(PCTA) to log K = 20.63 for Yb(PCTA), increasing along the Ln series in proportion to decreasing Ln3+ cation size. The rates of complex formation for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA) were followed by conventional UV-vis spectroscopy in the pH range 3.5-4.4. First-order rate constants (saturation kinetics) obtained for different ligand-to-metal ion ratios were consistent with the rapid formation of a diprotonated intermediate, Ln(H(2)PCTA)(2+). The stabilities of the intermediates as determined from the kinetic data were 2.81, 3.12, 2.97, and 2.69 log K units for Ce(H(2)PCTA), Eu(H(2)PCTA), Y(H(2)PCTA), and Yb(H(2)PCTA), respectively. Rearrangement of these intermediates to the fully chelated complexes was the rate-determining step, and the rate constant (k(r)) for this process was found to be inversely proportional to the proton concentration. The formation rates (k(OH)) increased with a decrease in the lanthanide ion size [9.68 x 10(7), 1.74 x 10(8), 1.13 x 10(8), and 1.11 x 10(9) M(-1) s(-1) for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA), respectively]. These data indicate that the Ln(PCTA) complexes exhibit the fastest formation rates among all lanthanide macrocyclic ligand complexes studied to date. The acid-catalyzed dissociation rates (k1) varied with the cation from 9.61 x 10(-4), 5.08 x 10(-4), 1.07 x 10(-3), and 2.80 x 10(-4) M(-1) s(-1) for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA), respectively.

Acetates↗
PubMed2006

Sensitivity enhancement of multidimensional NMR experiments by paramagnetic relaxation effects.

One of the main goals of NMR method development is to increase the sensitivity of multidimensional NMR experiments or reduce the required acquisition time. In these experiments, more than 80% of the NMR instrument time is spent on the recycle delay, where the instrument idles to wait for the recovery of proton magnetization. In this study, we report a method of using paramagnetic relaxation effects to shorten the recycle delays required in multidimensional NMR experiments of biological macromolecules. This approach significantly reduces the NMR instrument time required. Ni(2+) ion, complexed with the chelating molecule DO2A, is used to decrease the proton T(1) relaxation time of biological macromolecules without the significant line-broadening effects that are associated with most paramagnetic ions. The Ni(DO2A) also significantly decreases the T(1) relaxation time of water, thus providing additional sensitivity gain by eliminating the saturation of labile amide resonances.

Amides↗
PubMed2004

Solution dynamics and stability of lanthanide(III) (S)-2-(p-nitrobenzyl)DOTA complexes.

Addition of a benzyl substituent to the macrocyclic ring of DOTA has a substantial impact on the conformational ring flipping motion of the macrocycle in the resulting LnDOTA complexes. The p-NO2-benzyl substituent in the Ln(p-NO2-Bn-DOTA)- complexes lies in an equatorial position and effectively "locks" the conformation of the ring into the deltadeltadeltadelta configuration. The presence of the p-NO2-benzyl group also increases the population of the square antiprismatic (SAP) coordination isomer for all Ln(p-NO2-Bn-DOTA)- complexes relative to that seen for the respective LnDOTA- complexes. Despite this increase in SAP isomer population, the rate of water exchange in these complexes remains comparatively fast. The kinetic and thermodynamic stabilities of the Ln(p-NO2-Bn-DOTA)- complexes are also slightly lower than the corresponding LnDOTA- complexes but appear to be sufficiently high for in vivo use.

Chelating Agents↗
PubMed2004

Solid-phase synthesis of DOTA-peptides.

A general synthetic route to two DOTA-linked N-Fmoc amino acids (DOTA-F and DOTA-K) is described that allows insertion of DOTA at any endo-position within a peptide sequence. Three model pentapeptides were prepared to test the general utility of these derivatives in solid-phase peptide synthesis. Both DOTA derivatives reacted smoothly by means of standard HBTU activation chemistry to the point of insertion of the DOTA amino acid, but extension of the peptide chain beyond the DOTA-amino acid insertion required the use of pre-activated C-pentafluorophenyl ester N-alpha-Fmoc amino acids. Three Gal-80 binding peptides (12-mers) were then prepared by using this methodology with DOTA positioned either at the N terminus or at one of two different internal positions;the binding of the resulting GdDOTA-12-mers to Gal-80 were compared. The methodology described here allows versatile, controlled introduction of DOTA into any location within a peptide sequence. This provides a potential method for the screening of libraries of DOTA-linked peptides for optimal targeting properties.

Amino Acids↗
PubMed2002

In vivo behavior of copper-64-labeled methanephosphonate tetraaza macrocyclic ligands.

Copper-64 ( T(1/2)=12.7 h; beta(+): 0.653 MeV, 17.4%; beta(-): 0.578 MeV, 39%) is produced in a biomedical cyclotron and has applications in both imaging and therapy. Macrocyclic chelators are widely used as bifunctional chelators to bind copper radionuclides to antibodies and peptides owing to their relatively high kinetic stability. In this paper, we evaluated three tetraaza macrocyclic ligands with two, three, and four pendant methanephosphonate functional groups. DO2P [1,4,7,10-tetraazacyclododecane-1,7-di(methanephosphonic acid)], DO3P [1,4,7,10-tetraazacyclododecane-1,4,7-tri(methanephosphonic acid)], and DOTP [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methanephosphonic acid)] were all radiolabeled with (64)Cu in high radiochemical yields. Copper-64-labeled DO2P and DOTP were highly stable in rat serum out to 24 h, while (64)Cu-DO3P remained 73% intact, with the remainder possibly forming a (64)Cu(.)2DO3P dimer by 24 h. The biodistribution experiments were performed in normal Sprague-Dawley rats. Of the three complexes, (64)Cu-DO2P demonstrated the most optimal clearance through the blood and liver. Copper-64-DO3P and (64)Cu-DOTP exhibited higher liver uptake and longer retention of liver activity, possibly because of the large negative charge of the complexes under physiological conditions. All three (64)Cu-labeled complexes showed high accumulation in bone, likely due to the binding of the methanephosphonate groups to hydroxyapatite. These results suggest that this series of methanephosphonate macrocyclic ligands may be useful as potential bone-imaging agents. The thermodynamic stability constants of the Cu(II) complexes with these three ligands were determined, and were found to be significantly higher than those of their acetate analogues. The Cu(II)-DO2P complex exhibited the highest stability constant among divalent transition metal ion DO2P complexes. Metabolism studies of (64)Cu-DO2P in rat liver suggest that the DO2P ligand may be used as a bifunctional chelator for copper radionuclides in radiodiagnostic or radiotherapeutic studies.

Animals↗
PubMed2002

Magnetic resonance imaging detects a specific peptide-protein binding event.

DOTA was conjugated to the N-terminus of a 12-mer peptide by using standard peptide synthesis chemistry. The peptide, first isolated by phage display, maintained a high affinity for its protein-binding target, Gal-80, even with GdDOTA attached. The high affinity constant (KA = 5 x 105 M-1) combined with the high relaxivity of the resulting GdDOTA-peptide.protein complex (r1bound = 44.8 +/- 1.7 mM-1 s-1) allowed detection of Gal-80 at muM levels using a standard magnetic resonance imaging protocol. This novel peptide-based, binding-activated MRI method could potentially be used to screen a wide variety of biomolecules.

Amino Acid Sequence↗
PubMed1998

Synthesis, Potentiometry, and NMR Studies of Two New 1,7-Disubstituted Tetraazacyclododecanes and Their Complexes Formed with Lanthanide, Alkaline Earth Metal, Mn(2+), and Zn(2+) Ions.

Two new 1,7-disubstituted-1,4,7,10-tetraazacyclododecane ligands, DO2P and DO2PME, and their complexes with Mg(2+), Ca(2+), Sr(2+), Mn(2+), Zn(2+) and Ln(3+) were prepared and characterized by pH potentiometry. The pH titration data showed that DO2P and DO2PME both form 1:1 M:L complexes with all divalent and trivalent metal ions. Protonated complexes did not appear to form with the bis(phosphonate ester) ligand, DO2PME, but were evident for all of the metal ion-DO2P complexes. The alkaline earth metal ion-DO2P complexes formed both ML and MHL complexes while the lanthanide ion (Ln(3+)), Zn(2+), and Mn(2+) complexes of DO2P formed ML, MHL, and MH(2)L species. Zn(2+) formed the most stable complex with both ligands. The stability (beta(101)) of the LnDO2PME(+) complexes increased by about 2 orders of magnitude along the lanthanide series (La(3+) to Lu(3+)) while the stability of the LnDO2P(-) complexes over this same series increased by over 3 orders of magnitude. The bis(phosphonate) ligand, DO2P, and some of its complexes formed with Ln(3+) ions were further examined by NMR spectroscopy. (1)H and (31)P spectra of DO2P collected as a function of pH provided evidence that the first two protonations on the ligand take place largely at the tertiary nitrogens. The similarity of the (31)P chemical shifts of EuDO2P(-) and EuDOTP(5)(-) indicate that DO2P forms an "in-cage" complex with Eu(3+) using all four macrocyclic ring nitrogens and the two phosphonate sidearms as ligands. (17)O NMR shifts of the water signal indicated that the DyDO2P(-) complex has two inner-sphere coordinated water molecules. In the presence of excess of DO2P, a 1:2 metal:ligand, LnDO2P(HDO2P)(4)(-), complex forms with the second ligand interacting only weakly with the coordination sites left vacant by the first DO2P. Both water proton relaxivity data for GdDO2P(-) and (31)P NMR spectra of EuDO2P(-) provide evidence for formation of an "out-of-cage" LnH(2)DO2P(+) complex at low pH values (<6.5) in which the two phosphonate groups of DO2P are only involved in bonding with the lanthanide cation.

Journal Article↗
PubMed1997

Alkaline Earth Metal and Lanthanide(III) Complexes of Ligands Based upon 1,4,7,10-Tetraazacyclododecane-1,7-bis(acetic acid).

The macrocyclic ligand DO2A (1,4,7,10-tetraazacyclododecane-1,7-bis(acetic acid)) was prepared and used as a building block for four new macrocyclic ligands having mixed side-chain chelating groups. These ligands and their complexes with Mg(II), Ca(II), and Ln(III) were studied extensively by potentiometry, high-resolution NMR, and water proton relaxivity measurements. The protonation constants of all compounds compared well with those of other cyclen-based macrocyclic ligands. All Ca(II) complexes were found to be more stable than the corresponding Mg(II) complexes. Trends for the stabilities of the Ln(III) complexes are discussed and compared with literature data, incorporating the effects of water coordination numbers, Ln(III) contraction, and the nature of the side chains and the steric hindrance between them. (1)H NMR titrations of DO2A revealed that the first and second protonations take place preferentially at the secondary ring nitrogens, while the third and fourth involved protonation of the acetates. (17)O NMR shifts showed that the DyDO2A(+) complex had two inner-sphere water molecules. Water proton spin-lattice relaxation rates for the GdDO2A(+) complex were also consistent with water exchange between bulk water and two inner-sphere Gd(III) coordination positions. Upon formation of the diamagnetic complexes of DO2A (Ca(II), Mg(II), La(III), and Lu(III)), all of the macrocyclic ring protons became nonequivalent due to slow conformational rearrangements, while the signals for the acetate CH(2) protons remained a singlet.

Journal Article↗