L-histidine System Research Articles

Thermodynamic Parameters of the Mixed-Ligand Complexation of Cobalt(II) with Triglycine and L-Histidine in an Aqueous Solution

The Co2+–triglycine–L-histidine system is studied in an aqueous solution at Т = 298.15 K (KNO3) via potentiometry and calorimetry. The formation of mixed-ligand CoL2Y− complex is established, and the standard thermodynamic characteristics (ΔrH°, ΔrG°, ΔrS°) of the complexation reaction are determined. A structure is proposed for this complex.

The combined action of l-cysteine and l-histidine as a significant eco-friendly protective system to enhance the corrosion protection performance of AA2024-T3 alloy in 0.1 M NaCl solution: Electrochemical and surface studies

In the present study, the combined action of l-cysteine and l-histidine on the corrosion behavior of AA2024-T3 was studied. The inhibitive behavior of the organic l-cysteine + L-histidine system (CH-System) was evaluated by electrochemical tests and surface analyses. The polarization results indicated a 400 mV increase and 380 mV decrease in the pitting and corrosion potentials after 4 h of exposure, respectively. Also, the results of electrochemical impedance spectroscopy (EIS) showed a considerable increase in the impedance of AA2024-T3 samples in the presence of CH-System related to l-cysteine and l-histidine alone. In addition, the surface studies were carried out to investigate the chemical composition of surface products. The obtained results revealed that a thin organic film precipitated on the surficial aluminum oxide was responsible for the significant inhibition behavior of the CH-System in the saline electrolyte. The complex formation due to the interaction of positive (amine) and negative (carboxylic) groups presented in the l-cysteine and l-histidine molecules is the main co-inhibition mechanism in the CH-system.

Thermodynamics of Mixed-Ligand Complexation of Copper(II) with Triglycine and L-Histidine

The Cu2+–triglycine–L-histidine system was studied by potentiometry and calorimetry in an aqueous solution at T = 298.15 K (KNO3). Mixed-ligand CuLY and CuLHY+ complexes formed in the system. The standard thermodynamic characteristics (ΔrH°, ΔrG°, ΔrS°) of their formation reactions were determined. The possible structures of the mixed-ligand complexes were given.

Standard thermodynamic functions of complexation between copper(II) and glycine and L-histidine in aqueous solutions

The Cu2+–glycine–L-histidine system is studied calorimetrically at 298.15 K and an ionic strength of 0.2, 0.5, and 1.0 in aqueous solutions containing potassium nitrate. The standard thermodynamic parameters (Δr H°, Δr G°, Δr S°) of complexation processes are determined.

New program for computation of the thermodynamic, spectral, and NMR relaxation parameters of coordination compounds in complex systems

A new approach providing a direct calculations of equilibrium constants, parameters of the chemical exchange reactions, and spectral characteristics of complexes on the basis of data of several methods including pH-potentiometry, multi-wavelength electronic spectroscopy, and NMR relaxation within a single computer program was proposed and realized in the program STALABS. Application of the STALABS program has been demonstrated on the example of investigation of the complex nickel(II) - L-histidine system by joint usage of the above three methods.

L-proline and l-histidine co-catalyzed Baylis–Hillman reaction

Baylis–Hillman reaction between an aldehyde and an activated alkene, such as alkyl vinyl ketones, acrylates, acrylonitrile, and vinylsulfones, giving the β-hydroxy-α-methylene carbonyl compounds, is a versatile and atom-economical carbon–carbon bond-forming reaction. This reaction usually is catalyzed by strong Lewis bases such as tertiary amines and suffers from slow reaction rate. In this paper the Baylis–Hillman reaction between arylaldehydes and methyl vinyl ketone was successfully realized by a catalytic amount of l-proline and l-histidine system to give the corresponding normal Baylis–Hillman adducts in moderate to high yields for the first time. The effects of solvent and reagent electro property on the yields of this reaction were also investigated. Proline and histidine are naturally widely present amino acids in food system; furthermore, aldehydes and activated alkenes can also be easily produced in food processing, so proline and histidine co-catalyzed Baylis–Hillman reaction may find application in explaining some phenomena in food processing.

An ESR study of the copper(II)-glycyl- l-histidine system in aqueous solution by the simultaneous analysis of multi-component spectra. Formation constants and coordination modes

A series of multi-component ESR spectra of fluid aqueous solutions containing glycyl- l-histidine and copper(II) at 1:1 and 20:1 concentration ratios, taken in a circulating system at various pH, have been analysed simultaneously. The isotropic ESR parameters (g-factors, hyperfine coupling constants for the 63Cu and 65Cu isotopes and maximum four non-equivalent 14N nuclei, and relaxation parameters) and the formation constants of the various complexes were optimized. In the acidic region, the new species [CuLH 2] 3+, has been shown and the formation of the pH-metrically identified species [CuLH] 2+, [CuL] + and [CuLH −1] has been supported. The coordination in the first three complexes is reminiscent of that in simple dipeptides: it is likely to take place at first by the carboxylate O, then by the amino N and peptide O, and then by the amino N, deprotonated peptide N and carboxylate O atoms in equatorial positions, respectively, while the imidazole ring remains protonated. In the species [CuLH −1], the non-protonated imidazole nitrogen displaces the carboxylate oxygen from the equatorial coordination. In the alkaline region of the equimolar solutions the spectra could be described consistently in terms of the formation of four species: three of them, the complex [Cu 2L 2H −3] − and the two coordination isomers of the complex [CuLH −2] − are ESR-active, while the tetramer [Cu 4L 4H −8] 4− is ESR-inactive. For the complex [CuLH −2] −, one of the isomers is the species [CuLH −1(OH)] −, while the other one has the imidazole ring deprotonated. At ligand excess the region pH 7–10 is dominated by the pH-metrically identified complexes [CuL 2] and [CuL 2H −1] −. In these species the first ligand is bound equatorially by the amino, deprotonated peptide and imidazole nitrogen atoms. The second dipeptide is co-ordinated either by the amino nitrogen in equatorial and the peptide oxygen in axial position (4N isomer), or vice versa (3N isomer) in both bis complexes.

An electron spin resonance study of coordination modes in the copper(II)–histamine and copper(II)– l-histidine systems in fluid aqueous solution

Copper(II)–histamine and copper(II)– l-histidine equilibrium systems were studied in fluid aqueous solution by ESR spectroscopy. Eighty-seven spectra taken in a circulating system at various ligand-to-metal concentration ratios and pH were analysed. The experimental curves were decomposed to one to four component spectra which were built up from the hyperfine lines of 63Cu and 65Cu, and a maximum of four non-equivalent 14N nuclei. The isotropic ESR parameters ( g-factors, hyperfine coupling constants and relaxation parameters) and the relative concentrations of the different species were optimized. New, pH-potentiometrically non-identified species were also considered in the equilibrium models. In the copper(II)–histamine system the complex [CuLH −2] was added to the species [CuLH] 3+, [CuL] 2+, [CuLH −1] +, [Cu 2L 2H −2], [CuL 2H] 3+ and [CuL 2] 2+. In the copper(II)– l-histidine system, in addition to the complexes [CuLH] 2+, [CuL] +, [CuLH −1], [Cu 2L 2H −2], [CuL 2H 2] 2+, [CuL 2H] + and [CuL 2], the new species [CuLH 2] 3+ and [CuLH −2] − were found. The relative concentrations obtained from the ESR spectra are in good accordance with the concentrations calculated from the literature pH-potentiometric formation constants. The two ligands in their ‘LH’ states are bound differently: the histamine by the imidazole, and the l-histidine through the amino and the carboxylate groups in equatorial positions (complexes [CuLH], [CuL 2H] and [CuL 2H 2]). For the [CuL], [CuL 2H] and [CuL 2] complexes of both histamine and l-histidine, the first ‘L’ ligand is coordinated equatorially by the amino and imidazole nitrogens. The second ‘L’ ligand in the [CuL 2] complexes is either bound in the former way, or its imidazole group occupies an axial site. The carboxylate group of l-histidine is coordinated to the metal ion in each complex, in either an equatorial or an axial position. The deprotonation of the [CuL] complex takes place from the imidazole ring, which is followed by the proton loss of the equatorial water molecule in the highly alkaline region.

Vanadium(III) complexes with L-histidine in aqueous solution

Complex compounds of vanadium(III) with L-histidine were studied using absorption, CD and potentiometric measurements in aqueous solutions. In solutions at pH 2–4.5 consecutive mononuclear species MLH, ML 2H 2, MLH 2, ML 2H 4 and ML were observed and their stability constants were evaluated. More complex equilibria in the vanadium(III)-L-histidine system were observed at higher pH values, where mixed hydroxy- and/or oxo-species appeared in solution. At pH 6–8.5 range V 2OL 4 species predominantly exists in solution. CD spectra has confirmed the low symmetry of this species, because of the distinct splitting of 3 T 1 g → 3 T 2 g d-d transition of V III.

Polarographic study of mixed-ligand complexes of In(III) with L-histidine and glutamine/L-proline

In the In(III)–L-histidine system, the existence of 1:1 and 1:2 complex species has been established at pH 4.1 while 1:2 complex species predominates at pH ⩾ 5.9. The logarithmic values of overall stability constants for In(L-histidinate) ++, In(L-histidinate) + 2, In(L-histidinate)(L-glutaminate) + and In(L-histidinate)(L-prolinate) + are 10.05, 17.86, 16.37 and 18.14, respectively. The higher-than-statistical value of mixing constant for the mixed-ligand complex species In(L-histidinate)(L-prolinate) + reveals that it is more stable than the parent bis complexes.

Structure of the Species in the Copper (II)–L-Histidine System

The presence of simultaneously existing multiple species were taken into account in interpreting the structures of the complex species in the Cu(II)–L-histidine system. A refined proton displacement technique was utilized for structure elucidation of the species in solution. The absorbance and the molar extinction of the individual species were computed from the total absorbance and the species distribution over the pH range studied. The i.r. spectra of L-histidine and Cu(II)–L-histidine in D2O were obtained as a function of pD. They were interpreted in view of the species detected in a given pH range. The results of the species distribution as a function of pH, proton displacement, visible spectra of the individual species, and infrared data, were all combined to make structural interpretations. The following modes of metal coordination were proposed for the major species in the Cu(II)–L-histidine system: MHA, O(carboxyl) and N(imidazole); MH2A2, O(carboxyl) and N(amino) in both L-histidines; MA, O(carboxyl), N(imidazole), and N(amino); MHA2O(carboxyl), N(imidazole), and N(amino) in one L-histidine and O(carboxyl) and N(amino) in the other; MA2, O(carboxyl). N(imidazole), and N(amino) in both L-histidines, or O(carboxyl), N(imidazole), and N(amino) in one L-histidine and N(amino) and N(imidazole) in the other or an equilibrium mixture of both structures.

Equilibria of the Simultaneously Existing Multiple Species in the Copper(II)–L-Histidine System

The complete species distribution and the stability constants of the complex species in the Cu(II)–L-histidine system have been worked out for the pH range 2–11. The method of "analytical potentiometry" has been applied successfully to this system having excess ligand with respect to Cu(II) ion. The equilibria of three systems of Cu(II) (M) and L-histidine (H3A) in molar ratios of 1:2, 1:4, and 1:8 were investigated by this technique in 0.15 M NaCl at 25°. The following species were detected in this system: MHA, MA, MH2A2, MHA2, MA2, MH−1A2, MH−1A, and M2H−2A2. Statistical analysis on the numerical results has been performed and is given as standard deviation together with the individual values for the stability constants.