The Determination of Protonation Constants of Some Amino Acids and Their Esters by Potentiometry in Different Media

In this study, stoichiometric protonation constants of L-tyrosine, L-cysteine, L-tryptophane, L-lysine, and L-histidine, and their methyl and ethyl esters in water and ethanol–water mixtures of 30, 50, and 70% ethanol (v/v), were determined potentiometrically using a combined pH electrode system calibrated as the concentration of hydrogen ion. Titrations were performed at 25^∘C and the ionic strength of the medium was maintained at 0.10 mol⋅L⁻¹ using sodium chloride. Protonation constants were calculated by using the BEST computer program. The effect of solvent composition on the protonation constants is discussed. The log₁₀ K₂ values of esters generally decreased with increasing ethanol content. However, the log₁₀ K₁ values of the esters of L-tyrosine, L-cysteine, and L-tryptophane were found to increase with increasing ethanol content in contrast those of L-lysine and L-histidine esters.     

Energetic and topological analysis of the reaction of Mo and Mo2 with NH3, C2H2, and C2H4 molecules

The Density functional theory has been applied to characterize the structural features of Mo(1,2)-NH(3),-C(2)H(4), and -C(2)H(2) compounds. Coordination modes, geometrical structures, and binding energies have been calculated for several spin multiplets. It has been shown that in contrast to the conserved spin cases (Mo(1,2)-NH(3)), the interaction between Mo (or Mo(2)) and C(2)H(4) (or C(2)H(2)) are the low-spin (Mo-C(2)H(4) and -C(2)H(2)) and high-spin (Mo(2)-C(2)H(4) and -C(2)H(2)) complexes. In the ground state of Mo(1,2)-C(2)H(4) and -C(2)H(2), the metal-center always reacts with the C-C center. The spontaneous formation of the global minima is found to be possible due to the crossing between the potential energy surfaces (ground and excited states with respect to the metallic center). The bonding characterization has been performed using the topological analysis of the Electron Localization Function. It has been shown that the most stable electronic structure for a pi-acceptor ligand correlates with a maximum charge transfer from the metal center to the C-C bond of the unsaturated hydrocarbons, resulting in the formation of two new basins located on the carbon atoms (away from hydrogen atoms) and the reduction of the number of attractors of the C-C basin. The interaction between Mo(1,2) and C(2)H(4) (or C(2)H(2)) should be considered as a chemical reaction, which causes the multiplicity change. Contrarily, there is no charge transfer between Mo(1,2) and NH(3), and the partners are bound by an electrostatic interaction.     

Second derivative spectrophotometric method for simultaneous determination of cobalt, nickel and iron using 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol

A simple, sensitive and rapid derivative spectrophotometric method using 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) has been developed for simultaneous determination of Co(II), Ni(II) and Fe(II) which have very similar chemical behavior and appear together in many real samples. The complexes of all these metal ions with 5-Br-PADAP were formed immediately at pH 7.0 ammonium acetate buffered solution and were stable for at least 24 h. Second derivative spectra were selected for evaluation, because working wavelength determination was more precise and spectral overlap was less than in the ordinary and first derivative spectra. Three wavelengths at which the complexes exhibit extremum ²D values for Co(II), Ni(II) and Fe(II) were selected as analytical wavelengths, i.e., 640, 600 and 740 nm, respectively. Calibration curves drawn with zero-to-peak values at mentioned wavelengths were linear between 80 and 2000 ng ml⁻¹ for each metal ion. Concentrations of Co(II) and Ni(II) were calculated from the total ²D values and the sum of the linear equations for these three cations at 640 and 600 nm, after Fe(II) assay by making use of the ²D value at 740 nm. Limits of detection (LOD) for Co(II), Ni(II) and Fe(II) were 2.7, 13.9 and 3.0 ng ml⁻¹, respectively. The method has been applied to tool steel and heater resistance wire samples successfully.     

Diazoaldehyde Chemistry. Part 1. Transdiazotization of Acylacetaldehydes in Neutral-to-Acidic Medium. A Direct Approach to the Synthesis of α-Diazo-β-oxoaldehydes

First ever non-deformylating transdiazotization of acylacetaldehydes was achieved: the reactions of 2-azido-l-ethylpyridinium tetrafluoroborate ( 4 ) with acylacetaldehydes 3 proceeded partially without deformylation to yield 16 new α-diazo-β-oxoaldehydes 1 along with diazomethyl ketones 2 , especially in the presence of NaOAc (Scheme 1, Tables 1 and 2). The product distribution was substituent-dependent and could be correlated quantitatively. This new diazotization reaction appears as an alternative, direct, and more general method for the synthesis of these diazooxoaldehydes. α-Oxocycloalkanecarbaldehydes 5 gave only traces (if any) of α-diazocycloalkanones 7 , and rearrangement products 6 were isolated (Scheme 2). Mechanisms of the reactions are discussed (Schemes 4 and 5).     

Energetic and topological analyses of the oxidation reaction between Mo(n) (n = 1, 2) and N2O

The interaction between molybdenum, atom, and dimer, with nitrous oxide has been investigated using density functional theory. The analysis of the potential energy surfaces for both reactions has revealed that a single molybdenum atom can activate the N--O bond of N2O requiring a small activation energy. However, the presence of several intersystem crossings between three different spin states, namely, septet, quintet and triplet states, seems to be the major constraint to the Mo + N2O reaction. Contrarily, the low-lying excited states (triplet and quintet) do not participate in the reaction between the molybdenum dimer and N2O. The latter reaction fully evolves on the singlet spin surface. Three different regions have been distinguished along the pathway: formation of an adduct complex, formation of an inserted compound, and the N2 detachment. The connection between the two first regions has been characterized by the formation of a special complex in which the N--O bond is so weakened that it could be considered as a first step in the insertion process. It has been shown that the topological changes along the pathways provide a clear explanation for the geometrical changes that occur along the reaction pathway. In summary, the detachment of the N2 molecule is found to be kinetically an effective process for both reactions, owing to the high exothermicity and consequently to the high internal energy of the insertion intermediates. However, in the case of Mo atom, the reaction should be a slow process due to the presence of spin-forbidden transitions. These results fully agree with previous experimental works.     

Binary and Ternary Copper(II) Complexes with NIPAAm/IA Copolymers and Aminoacids: Interpretation of UV‐Visible Spectra and Cyclic Voltammograms

The complex formations of homopolymers and copolymers of poly(N‐isopropyl acrylamide) (PNIPAAm), polyacrylamide (PAAm) and poly(itaconic acid) (PIA) with Cu(II) ions in aqueous solutions were studied by using UV‐visible spectra in the region of 200–1100 nm and cyclic voltammograms between ?0.800 V and 0.600 V. According to the optical and electrochemical spectra of the polymer‐ and copolymer‐Cu(II) complexes and their ternary complexes with alanine, i.e., absorptions and the shifts in the wavelength of the maximum absorptions, currents and potentials of the peaks in the pH range of 3–12, the intensities of anodic and cathodic peak currents of polymers containing IA groups decrease with increasing pH and they show maximum absorptions at lower wavelengths than do the homopolymers used in this work. The peak point of the visible band shifts from 800 to 650 nm, with increasing pH, while the intensity of the third anodic peak observed after pH=4 increases in the case of both Ala‐Cu(II) and its ternary solution with P(NIPAAm‐co‐IA, 9.8 mol%). Both the pH‐dependent shifts of maximum absorptions and the appearence of the third anodic peaks as the pH raised were interpreted as a presence of tetracoordinated Cu(II) complexes in the solution and on the electrode surface, involving combined carboxyl, amide and amine interaction.     

Voltammetric methods of reboxetine analysis and the mechanism of its electrode reactions

Reboxetine (RBX) electrochemical redox behavior at hanging mercury drop (HMDE) and glassy carbon electrodes (GCE) was studied in various pH Britton-Robinson universal buffers using cyclic voltammetry and square-wave voltammetry. RBX was reduced at the HMDE and oxidized at the GCE with reversible adsorption controlled and irreversible diffusion controlled processes respectively. The anodic peak is due to the amine and the cathodic peak may correspond to oxygen protonation. An oxidation reaction mechanism is proposed. The linear relation between peak currents and RBX concentration allowed simple, sensitive, precise and inexpensive voltammetric procedures to be developed. The limit of detection was 0.04 µM RBX. The procedures were successfully applied to human urine and RBX tablet assay. Therapeutic RBX concentrations in human serum were not detected due to strong drug-protein binding. Using bovine serum albumin, the methods were used to investigate the effect of serum protein binding on RBX determination.      

Synthesis and structural analysis of nonstoichiometric ternary fulleride K 1.5 Ba 0.25 CsC 60

The existence of cation-vacancy sites in fullerides might lead to long-range ordering and generate a new vacancy-ordered superstructure. The purpose of this work is to search whether or not long-range ordering of vacant tetrahedral sites, namely superstructure emerges in nonstoichiometric K _1.5 Ba _0.25 CsC ₆₀ fulleride. Therefore, K _1.5 Ba _0.25 CsC ₆₀ with cation-vacancy sites is synthesized using a precursor method to avoid inadequate stoichiometry control and formation of impurity phases within the target composition. For this purpose, first, phase-pure K ₆ C ₆₀ , Ba ₆ C ₆₀ and Cs ₆ C ₆₀ precursors are synthesized. Stoichiometric quantities of these precursors are used for further reaction with C ₆₀ to afford K _1.5 Ba _0.25 CsC ₆₀ . Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data of the precursors and K _1.5 Ba _0.25 CsC ₆₀ confirms that K ₆ C ₆₀ , Ba ₆ C ₆₀ and Cs ₆ C ₆₀ are single-phase and they crystallize in a body-centered-cubic structure ( Im 3) as reported in the literature. The analysis also shows that K _1.5 Ba _0.25 CsC ₆₀ phase can be perfectly modeled using a face-centered cubic structure. No new peaks appear which could have implied the appearance of a superstructure. This suggests that there is no long-range ordered arrangement of vacant tetrahedral sites in K _1.5 Ba _0.25 CsC ₆₀ .     

Stereoselective synthesis of optically active cyclopenta[c]pyrans and cyclopenta[c]furans by the intramolecular Pauson–Khand reaction

An intramolecular Pauson–Khand reaction of enynes derived from homoallyl, homopropargyl, and allyl alcohols is described. 2-Heteroaryl-substituted homoallyl, homopropargyl, and allyl alcohols are easily and efficiently resolved through enzymatic resolution in high ee (91–99%) and with a known stereochemistry. Each enantiomerically enriched enyne derived from homoallyl and homopropargyl alcohols affords the conformationally most stable diastereomeric cyclopenta[c]pyran ring system as the sole product, whereas enantiomerically enriched enynes derived from allyl alcohols give a diastereomeric cis:trans mixture of the cyclopenta[c]furan ring system.