Francesco Dallavalle

Pentacopper(II) 12-metallacrown-4 complexes with α- and β-aminohydroxamic acids in aqueous solution: a reinvestigation

A reinvestigation of the equilibria of (S)-α-alaninehydroxamic acid (α-Alaha) and (R)-aspartic-β-hydroxamic acid (Asp-β-ha) with copper(II) was performed in aqueous solution in order to clarify some contradictory literature reports regarding the stoichiometry of the polynuclear complexes formed. β-Alaninehydroxamic acid (β-Alaha, HL), for which the formation of a planar 12-metallacrown-4, [Cu5L4H−4]2+, was already reported, was also re-examined for comparison. Among the different techniques used (potentiometry, absorption spectrophotometry, spectropolarimetry and electrospray ionization mass spectrometry), ES data allowed to define unambiguously that all these three ligands form the same pentanuclear species. Therefore it can be concluded that in aqueous solution the hydroxamates of both α- and β-amino acids form 12-metallacrown-4 complexes, and that the formers are less stable.

Dansylated Polyamines as Fluorescent Sensors for Metal Ions: Photophysical Properties and Stability of Copper(II) Complexes in Solution

Protonation and the CuII complexation constants of the dansylated polyamines N-dansylethylenediamine (1), N-dansyldiethylenetriamine (2), N-dansyltriethylenetetramine (3), N′-[2-(dansylamino)ethyl]diethylenetriamine (4), and tris(2-dansylaminoethyl)amine (5) were determined by glass-electrode potentiometry in MeOH/H2O 9 : 1 (v/v) solution. For ligands 3 and 4, the determinations were also performed in aqueous solution. The complexes formed by these ligands in neutral form correspond to those observed for the analogous unsubstituted monoprotonated amines, whereas, when the ligands are deprotonated at the sulfonamide moiety, the species parallel those of the corresponding amines. The molecular structures of the complexes were deduced from the VIS absorption spectra. The crystal structure of the [CuL2H−2] complex 6 of ligand 1 (L) was determined by X-ray diffraction. The study of the photophysical properties of the ligands 3 – 5 showed that they are good fluorescent sensors for copper(II), which induced fluorescence quenching. Time-resolved fluorescence measurements allowed us to clarify the sensing mechanism. The pH dependence of the quenching effect demonstrated that it occurs for all Cu2+ complexes, even for species in which the sulfonamide moiety is not deprotonated. Sensing of Cu2+ was compared with that of other metal ions (Co2+, Ni2+, Zn2+, Cd2+, Hg2+), and selectivity was studied as a function of pH. Ligands 3 and 4 were found to be selective chemosensors for Cu2+ in weakly acidic solution (pH ca. 4 – 5).

Speciation and structure of copper(II) complexes with (S)-phenylalanine- and (S)-tryptophanhydroxamic acids in methanol/water solution: a combined potentiometric, spectrophotometric, CD and ESI-MS study

Abstract Complexation equilibria of ( S )-phenylalaninehydroxamic and ( S )-tryptophanhydroxamic acids (HL) with copper(II) were studied in methanol/water solution in order to avoid the precipitation of a polynuclear species observed in aqueous solution. Among the different techniques used (potentiometry, absorption spectrophotometry, spectropolarimetry and ESI-MS), mass spectrometry data allowed to detect a pentanuclear species [Cu 5 L 4 H −4 ] 2+ instead of the dinuclear one, [Cu 2 L 2 H −1 ] + , usually reported for the α-aminohydroxamates in aqueous solution. A cup structure of the pentanuclear cation is proposed, which is similar to that determined by X-ray analysis for the corresponding species of β-alaninehydroxamic acid.

Synthesis, molecular structure, solution equilibrium, and antiproliferative activity of thioxotriazoline and thioxotriazole complexes of copper(II) and palladium(II)

Preparations of copper(II) and palladium(II) complexes of 4-amino-5-methylthio-3-(2-pyridyl)-1,2,4-triazole (L(1)) and the copper(II) complex of 1,4-dihydro-4-amino-3-(2-pyridyl)-5-thioxo-1,2,4-triazole (HL) are described. These complexes have been characterized by means of spectroscopy and microanalysis. Molecular structures of HL (1), [CuCl(2)(H(2)L)]Cl.2H(2)O (2a), cis-[CuCl(2)(L(1))] (3), and cis-[PdCl(2)(L(1))] (4) have been determined by single-crystal X-ray diffraction. The HL ligand acts as a N,S bidentate through the thioxo moiety and the exo-amino group whilst the ligand L(1) forms N,N coordination complexes through the pyridine and triazole nitrogen atoms. Speciation in solution of the systems Cu/HL and Cu/L(1) have been determined by means of potentiometry and spectrophotometry as well as for the Cu/L(1)/A (HA=glycine) system in order to determine species present at physiological pH. Antiproliferative activity of these complexes and their ligands was evaluated, using the AlamarBlue Assay, on normal human fibroblasts (HF) and human fibrosarcoma tumor (HT1080) cells. The copper compounds cis-[CuCl(2)(H(2)L)]Cl and cis-[CuCl(2)(L(1))] exerted significant antiproliferative activity of both normal and neoplastic cells; although dose-response experiments revealed that the HT1080 cell line was more sensitive to the tested drugs than normal fibroblasts.

Thermodynamics of Self-Assembly of Copper(II) 15-Metallacrown-5 of Eu(III) or Gd(III) with (S)-α-Alaninehydroxamic Acid in Aqueous Solution

The equilibria of self-assembly of 15-metallacrown-5 (15-MC-5) complexes of Cu(2+) and (S)-alpha-alaninehydroxamic acid (alpha-Alaha, HL) with the lanthanide (Ln) ions Eu(3+) or Gd(3+) in aqueous solution are described. The binary Ln(3+)/alpha-Alaha systems were first studied by potentiometric and calorimetric in-cell titrations; the latter technique allowed us to define the most suitable speciation model. On the contrary, because the kinetics of formation of the Ln(3+) 15-MC-5 complexes is slow, their stability constants were determined by out-of-cell (batch) potentiometric titrations. Two 15-MC-5 complexes are formed with both Eu(3+) and Gd(3+), namely, {Ln[Cu(5)L(5)H(-5)]}(3+) and {Ln[Cu(5)L(5)H(-5)](OH)}(2+), with the latter being the hydroxo species of the former. The acidity of the former to give the hydroxo species is remarkably high (log K = 4.40-4.69). Moreover, our potentiometric and spectrophotometric investigations clearly indicate that the hydroxide ion is coordinated to the central Ln ion, as was reported for several 15-MC-5 in the solid state. The formation of {Ln[Cu(5)L(5)H(-5)]}(3+) starts at ca. pH 3.5, which converts at ca. pH 4.5 into the {Ln[Cu(5)L(5)H(-5)](OH)}(2+) species, which predominates up to pH 7, where a purple precipitate occurs. The coexistence of both 15-MC-5 species and the copper(II) 12-MC-4 species of alpha-Alaha ([Cu(5)L(4)H(-4)](2+)) was observed under appropriate experimental conditions (pH and ligand and metal concentrations). A complete ESI-MS investigation of the Ln(3+)/Cu(2+)/alpha-Alaha system at different pHs confirmed the formation of the two 15-MC-5 species. The 15-MC-5 stability constants were employed to quantitatively evaluate the solution behavior of Ln(III) MCs regarding their integrity, ligand substitution, and transmetalation processes. In particular, EDTA or DOTA, added in equimolar amounts, should not appreciably interfere with the MC integrity, as found in previous experimental investigations, although it is expected that at higher amounts of EDTA, the MC should be disrupted. Our results also demonstrate that an excess of alpha-aminohydroxamate does not interfere with the integrity of the MC, and the disappearance of the CD spectra upon addition of the R enantiomer to 15-MC-5 containing the S enantiomer is due to a very rapid ligand exchange with formation of all possible isomers with no selectivity. The stability of the 15-MC-5 complexes in the presence of transferrin, serum albumin, or an excess of Zn(2+) is also discussed. With regards to the latter metal ion, we found that the MCs are stable toward Gd(3+)/Zn(2+) transmetalation. although the presence of a phosphate buffer promotes the disruption of the MC scaffold by formation of stable Gd(3+)/phosphate species.

Copper(II) 12-metallacrown-4 complexes of alpha-, beta- and gamma-aminohydroxamic acids: a comparative thermodynamic study in aqueous solution.

A complete thermodynamic study of the protonation and Cu(II) complex formation equilibria of a series of alpha- and beta-aminohydroxamic acids in aqueous solution was performed. The thermodynamic parameters obtained for the protonation of glycine-, (S)-alpha-alanine-, (R,S)-valine-, (S)-leucine-, beta-alanine- and (R)-aspartic-beta-hydroxamic acids were compared with those previously reported for gamma-amino- and (S)-glutamic-gamma-hydroxamic acids. The enthalpy/entropy parameters calculated for the protonation microequilibria of these three types of ligands are in very good agreement with the literature values for simple amines and hydroxamic acids. The pentanuclear complexes [Cu5L4H(-4)]2+ contain the ligands acting as (NH2,N-)-(O,O-) bridging bis-chelating and correspond to 12-metallacrown-4 (12-MC-4) which are formed by self-assembly between pH 4 and 6 with alpha-aminohydroxamates (HL), while those with beta- and gamma-derivatives exist in a wider pH range (4-11). The stability order of these metallomacrocycles is beta- >> alpha- > gamma-aminohydroxamates. The formation of 12-MC-4 with alpha-aminohydroxamates is entropy-driven, and that with beta-derivatives is enthalpy-driven, while with gamma-GABAhydroxamate both effects occur. These results are interpreted on the basis of specific enthalpies or entropy contributions related to chelate ring dimensions, charge neutralization and solvation-desolvation effects. The enthalpy/entropy parameters of 12-MC-4 with alpha-aminohydroxamic acids considered are also dependent on the optical purity of the ligands. Actually, that with (R,S)-valinehydroxamic acid presents an higher entropy and a lower enthalpy value than those of enantiopure ligands, although the corresponding stabilities are almost equivalent. Moreover, DFT calculations are in agreement with a more exothermic enthalpy found for metallacrowns with enantiomerically pure ligands.

Chiral discrimination of Dns- and unmodified d,l-amino acids by copper(II) complexes of terdentate ligands in high-performance liquid chromatography

Abstract The enantiomeric separation of unmodified d , l -amino acids and Dns-amino acids in RP-HPLC by copper(II) complexes of terdentate diaminoamido ligands added to the eluent has been studied. The work is aimed at investigating whether a copper(II) complex with only one free equatorial position can still perform chiral discrimination of bidentate analytes such as unmodified and Dns-amino acids. The problem is approached by determining the nature and the stability of the initial copper(II) binary complexes and of the ternary complexes with amino acids in solution by potentiometry. Different chromatographic parameters were examined (pH, selector concentration, ionic strength, eluent polarity). All experimental data are consistent with a mechanism of chiral discrimination involving formation of diastereomeric complexes in which the amino acid coordinates at the free equatorial position and at the apical position of the initial complex which maintains its original chelate rings. It may still be considered a ligand exchange, implying that coordinated water molecules have been substituted by the analyte. The poor performance of the tricoordinated copper(II) complexes in the chiral discrimination of unmodified amino acids can be ascribed to the low stability of the mixed complexes and to their loose sterical requirements. With Dns-amino acids the most important feature seems to be the higher/lower affinity (match/mismatch) of the enantiomers for the selector adsorbed on the stationary phase.

Copper(II) complexes of N2-alkyl-(S)-amino acid amides as chiral selectors for dynamically coated chiral stationary phases in RP-HPLC.

Copper(II) complexes of N2-octyl-(S)-phenylalaninamide (Noc-Phe-NH2), N2-dodecyl-(S)-phenylalaninamide (Ndo-Phe-NH2), and N2-octyl-(S)-norleucinamide (Noc-NLeu-NH2), dynamically adsorbed on a reversed-phase C18 column, were able to perform the direct enantiomeric separation of unmodified amino acids, amino acid amides and esters, hydroxy acids, and dipeptides by elution with aqueous or mixed aqueous-organic solutions containing copper(II) sulphate or acetate. The role played by several parameters in the separation procedure was examined with the copper(II) complex of Noc-Phe-NH2 [concentration of the copper(II) ion in the eluent, pH and eluent polarity, amount of adsorbed selector]. The separation was shown to occur entirely on the stationary phase. The mechanism of chiral discrimination is discussed in terms of the chromatographic parameters and of the structure of the copper(II) complexes in solution and in the solid state. The chiral stationary phase maintained its separation ability for about 3 months. However, the column could be easily restored by recovering the selector with methanol and repeating the loading procedure.

Analysis of variance applied to determinations of equilibrium constants.

The statistical analysis of variance has been applied to the values of the equilibrium constants of the glycinate-proton and glycinate-nickel systems, determined in different laboratories by pH-titration in aqueous solution. The analysis shows how the main part of the error derives from the variability from one titration to another even in the same laboratory. Therefore the data for a single titration (k) must be processed separately, thus yielding a mean value for the equilibrium constant logbeta (pqr)(k) of the species M(p)H(q)L(r); from these mean values for different titrations in each laboratory l, a within-laboratory grand average, logbeta (pqr)(l), can be calculated; the variance of this grand average measures the experimental error. A further analysis of the data from the different participating laboratories shows that there were no significant differences between laboratories for the constants reported. From these results it can be inferred that all the values of the mean constants logbeta (pqr)(k) for one species, as determined separately for each titration in four laboratories, belong to the same population. A chi(2) analysis of these populations demonstrates that the stability constants of the species HL, H(2)L(+), NiL(+), NiL(2) (with L(-) = glycinate) are normally distributed, but not that for NiL(-)(3). Therefore, general mean values of the first four constants can be calculated and proposed as reliable standard values at 25 degrees and I = 1.0M Na(Cl): protonation of glycinate, log beta(011) = 9.651(12), log beta(021) = 12.071(26); nickel-glycinate complexes, log beta(101) = 5.615(35), log beta(102) = 10.363(62). These values indicate that the standard deviations are rather higher than those often reported in the literature.

Formation equilibria of ternary complexes of copper(II) with (S)-tryptophanhydroxamic acid and both D- and L-amino acids in aqueous solution

Abstract Binary copper(II)/( S )-tryptophanhydroxamic acid and ternary copper(II)/( S )-tryptophanhydroxamic acid/D- and L-amino acid systems (amino acid, HA=Phe, Pro, Trp) have been studied both by potentiometry and Vis-spectrophotometry. ( S )-tryptophanhydroxamic acid (( S )-2-Amino-3-(indol-3-yl)- N -hydroxypropanamide, HL) forms four binary species ([CuL] + , [Cu 2 L 2 H −1 ] + , [CuL 2 ], [CuL 2 H −1 ] − ) in the pH range 3–11, but precipitation of the dinuclear complex occurs between pH 4 and 6. In order to overcome these difficulties, EDTA was used as a competing ligand in order to reduce the concentration of the insoluble species. In the case of ternary systems the concentration of [Cu 2 L 2 H −1 ] + was lowered by using high amino acid/metal ratios (up to 30:1), therefore a reinvestigation of the amino acid equilibria was performed to obtain precise values for the protonation and Cu II complexation constants. The thermodynamic stereoselectivity in the formation of [CuLA] and [CuLH −1 A] − ternary complexes was significant in the case of both species with proline and for [CuLH −1 A] − of phenylalanine. This stereoselectivity was tentatively explained in terms of steric hindrance between tryptophanhydroxamate and amino acid residues.