Voltammetric,spectroscopic and thermal investigations of the interaction of levofloxacin with cysteine at physiological pH

In this study, levofloxacin (LEVOF) hemihydrate interaction with L-cysteine (RSH) was investigated by using square-wave voltammetry (SWV) in Britton–Robinson (B–R) buffer pH 7.4. Addition of the LEVOF to RSH solution resulted in dropping of the main reduction peak current of RSH (the current of mercurous cysteine thiolate Hg₂(RS)₂). The vary in the peak current of Hg₂(RS)₂, after the adding of the LEVOF is indicated an interaction between RSH and LEVOF molecules. Moreover, the interaction between two molecules also confirmed with UV-Vis, FT-IR spectroscopic measurements and thermal analysis data. Binding constant of LEVOF with RSH was calculated by both voltammetric and UV-Vis spectroscopic data.     

In vitro study of the interaction of cysteine with a thiazide diuretic (hydrochlorothiazide) at different ph by voltammetric and spectroscopic techniques

The interaction of cysteine (RSH) with a thiazide diuretic, hydrochlorothiazide (HCTZ) was characterized by UV-Vis absorption spectroscopy and square-wave voltammetry in Britton-Robinson (B-R) buffer solution (with pH 5, 7 and 9). On the square-wave voltamogram of cysteine, the reduction peak current of mercurous cysteine thiolate (Hg₂(RS)₂) decreased and its peak potential shifted to more positive values with the addition of HCTZ. This results showed that the RSH interacted with HCTZ. The stoichiometry of HCTZ-RSH molecular complex was determined by voltammetric data with the result of 1: 1. By using linear regression analysis of the voltammetric data at pH 5, 7 and 9, the apparent formation constants of HCTZ-RSH complex were calculated to be 9.54 × 10³, 2.80 × 10⁴ and 2.55 × 10⁴ M^?1, respectively. At the same time, this interaction was also supported by UV-Vis spectroscopic measurements. According to the voltammetric and spectroscopic results, it was suggested that the interaction mode between RSH and HCTZ molecules might be a combination of hydrophobic interactions and hydrogen bonds.     

Voltammetric study on interaction of cysteine with type I-collagen in the aqueous medium

Interaction of cysteine with type I-collagen from bovine achilles tendon in the aqueous solutions has been examined using square wave voltammetry (SWV) and cyclic voltammetry (CV) techniques. In the absence of cysteine, type I-collagen gives a reversible peak at ?0.114 V in Britton-Robinson (B-R) buffer (pH 4.0). The electrochemical parameters (I _p/f, E _p/f, E _p/pH, I _p/pH, I _p/v, I _p/v^1/2) of type I-collagen have been also studied. In addition, it has been determined that there is a linear relationship between current and concentration of type I-collagen. On the other hand, cysteine exhibits a reversible peak at ?0.383 V due to the reduction of mercurous cysteine thiolate. By using a hanging mercury drop electrode in aqueous solutions, SWV and CV voltammograms obtained for type I-collagen in the presence of cysteine indicate that there is an interaction between type I-collagen and cysteine. In the presence of cysteine, peak current of type I-collagen decreases and a new peak is observed at ?0.121 V for cysteine which is bonded to type I-collagen.     

Electron spin resonance studies on some copper(II) complexes with a few nitrogen donors derived from pyridine

Electron spin resonance spectral studies have been made on copper(II) chloride, bromide, thiocyanate and sulphate complexes with some pyridine derivatives, viz. nicotinic acid (NA), nicotinamide (NICA) and isonicotinamide (INA) in solid and DMF-solution states to see the effect of different anions on the Spin-Hamiltonian parameters at the paramagnetic site for a particular ligand. The spectra of the complexes for a particular anion are almost comparable suggesting the same local symmetry for them. Analysis of the ESR data reveals axial symmetry for all the complexes, except Cu(NA)₂SO₄ for which a rhombic symmetry is suggested. The study shows the interaction of solvent (DMF) molecules with copper(II) ion in the axial plane as evident from the differences in 295 and 77 K g_| values. Moreover, the spectra are consistent with the complete absence or negligbly small copper(II)—copper(II) interactions (in few cases) in these complexes. The various Spin-Hamiltonian parameters calculated from ESR data indicate the presence of an unpaired electron in the d_x²−y² orbital of the copper(II) ion with the additional possibility of a d_xy ground state for Cu(NA)₂SO₄.     

Kinetics of oxidation of cysteine and captopril via Cs3[Mo(CN)8] and Cs3[W(CN)8]

The kinetics of the oxidation of cysteine and captopril via octacyanomolybdate(V) and octacyanotungstate(V) in a buffered acidic media (pH range 2.20–4.80) have been studied spectrophotometrically. The rate law for the oxidation is: Rate = k [RSH] [Ox] [H⁺]⁻¹, where RSH is cysteine or captopril and Ox is Cs₃[Mo(CN)₈] or Cs₃[W(CN)₈]. The activation parameters (E_a, ΔH^#, ΔG^#, ΔS^#) for the oxidation of cysteine and captopril via Cs₃[Mo(CN)₈] or Cs₃[W(CN)₈] have been determined. The results indicate that Cs₃[Mo(CN)₈] is more reactive than Cs₃[W(CN)₈] as an oxidizing agent. Effects of pH, ionic strength, temperature, dielectric constant of the reaction medium and copper(II) ions on the oxidation rate have been studied. Mechanisms for the oxidation of cysteine to cystine and captopril to the corresponding disulfide have been proposed.     

Preliminary Study on Reverse Pulse Amperometric Detection in Capillary Electrophoresis for Simultaneous Determination of Cysteine and Cystine

Simultaneous determination of cysteine (RSH) and cystine (RSSR), two important sulfur-containing amino acids, in capillary electrophoresis (CE) has been an analytical task. Dual-microelectrode amperometric detection seems to be a good scheme, but significant difficulty in electrode construction and poor detection limit for RSSR determination remain major short-comings. In reverse pulse amperometric (RPA) detection, the applied potentials are repeatedly pulsed back and forth between the reducing initial potential (e.g., E_i = ?1.4 V) and the oxidizing final potential (e.g., E_f = 0.0 V) at a single, gold-mercury amalgam (Au/Hg) microelectrode. At E_i, RSSR is reduced to RSH which causes catalytical oxidation of the Au/Hg amalgam microelectrode when the potential is pulsed to E_f. The resulting anodic current is then recorded. Therefore, by using RPA detection after CE separation, RSH and RSSR can be simultaneously determined.     

Amperometric argentimetric and mercurimetric titration of sulfhydryl: Mercurimetric titration

The ampero-mercurimetric titration of cysteine (RSH), glutathione (GSH), thioglycolic acid (TSH) and 2-mercaptoethanol (ESH) at the RMPE at pH 9 yields a sharply defined end-point corresponding to the composition (XS)₂Hg. Under the same conditions with the KPtWK two lirciiks an; (ibst'rvc-d in the titration lines with RSH and GSH, corresponding to compositions (RS)₂Hg and (RS)₂Hg₂ respectively. On the other hand, with TSHl and ESH only one clear break is found at the RPtWE corresponding to ('l'S)₂Hg or (ES)₂Hg Conditions liave been describcd for the accurate mercurimetric titration of the thiol compnunds at pH 2 in the pesence of 0.01 M sulfur dioxide at the RMPE, use of this can be made in the determination of disufides and in the determination of the equilibrium concentrations of the reaction betwcen a disulfide and a sulfite at varying pH.     

Lysine modification of human serum albumin and its effect on protein conformation and nalidixic acid binding

In order to investigate the involvement of lysine residues of human serum albumin (HSA) in nalidixic acid (NA) binding, various modified preparations of HSA such as 44% carbamylated (C₄₄), 83% carbamylated (C₈₃) and 85% acetylated (A₈₅) were made by treating the HSA solution with a different molar excess of potassium cyanate and acetic anhydride. The extent of modification, charge homogeneity and conformational changes of these derivatives were checked by TNBSA reaction method, polyacrylamide gel electrophoresis (PAGE) and gel filtration using Sephacryl S-200 HR column, respectively. Binding of NA to HSA and its derivatives was examined using fluorescence quenching titration method to determine the binding constant. The emergence of a single band in PAGE and single symmetrical peak in gel filtration results confirmed the charge and size homogeneity of these derivatives. Hydrodynamic properties such as Stokes radius and frictional ratio, as obtained from the analytical gel filtration results suggested molecular expansion in C₈₃ and A₈₅ HSAs while C₄₄ HSA retained the native conformation. Addition of NA to both native and modified HSA derivatives quenched the fluorescence intensity of the protein at 344 ​nm to a different extent. Whereas the values of the Stern-Volmer constant (K_SV) and bimolecular quenching rate constant (k_q) suggested, NA-HSA complex formation, binding constant (K_a) value suggested an intermediate binding affinity between NA and HSA. Furthermore, the decrease in the K_a value with the extent of modification was indicative of the involvement of lysine residues in NA-HSA interaction.     

Studies in sesquiterpenes—LVIII : Deodardione,a sesquiterpene diosphenol and,limonenecarboxylic acid,a possible norsesquiterpene—compounds from the wood of cedrus deodara loud.

Isolation and structure elucidation of a C₁₁ mono-carboxylic acid, apparently a nor-sesquiterpene, and, a sesquiterpene diosphenol from the essential oil of Cedrus deodara Loud. are described.     

Polarographic studies on o-aminobenzenethiol

Polarographic behaviour of o-aminobenzenethiol (RSH) is studied at dme in 0.1M-HClO₄, 0.1M-NaOH,Britton-Robinson buffers of different pH values and aqueous acetic acid. A well defined single wave is obtained in 0.1M-HClO₄ and buffers of pH 2.0 at all concentrations of the depolarizer whereas in all the other buffers, a well defined single wave is obtained only if the concentration is less than 2×10^–4 M. In other cases, two waves are observed. With the increase in pH, theE _1/2 becomes more negative and finally attains a constant value. Thus the dissociation constant of sulphydryl group under the experimental conditions is 5.14. The diffusion coefficient in buffer of pH 2.0 is calculated to be 5.76×10^–6 usingIlkovi equation.The reaction is reversible and the oxidation product is a mercurous compound (RSHg) which converts quickly to (RS)₂Hg. Even in acetic acid media, well defined reversible anodic wave is obtained in all proportions of acetic acid and water.With 4 Figures     

Quantum-chemical DFT study of acid properties of thiols RSH and ligand power of their anions RS<Superscript>−</Superscript>

The DFT calculations of electronic and geometry structure, dissociation energies of S-H and S-C bonds, and acid dissociation constants (pK _a) for ten RSH thiols were fulfilled. Calibration corrections to the calculated pK _a^DFT values, which allowed obtaining a good correspondence with experimental data were introduced. Donor power of thiolate anions RS- was estimated for the prediction of stability of their compounds with ions of d-elements.     

Electrocatalytic Oxidation of 2‐Thiouracil and 2‐Thiobarbituric Acid at a Carbon‐Paste Electrode Modified with Cobalt Phthalocyanine

Voltammetric behavior of two mercaptopyrimidine derivatives (2‐thiouracil and 2‐thiobarbituric acid) has been studied by cyclic voltammetry at a cobalt phthalocyanine (CoPc)‐modified carbon‐paste electrode. The results of voltammetric determinations showed that the CoPc in the matrix of modified electrode acts as catalyst for electrooxidation of these thiols (RSH), lowering the overpotential of the reaction and significantly increasing the sensitivity for detection of thiols in neutral conditions. The results of voltammetric and polarization measurements in solutions with various pHs were used for prediction of the mechanism of electrocatalytic oxidation at the surface of modified electrode. These results showed that at the modified electrode, electrochemical oxidation of thiolate anion (RS^?) is the rate‐determining step. It was found that the modified electrode exhibits good selectivity for catalytic oxidation of mercaptopyrimidines over other biologically important mercaptans such as cysteine, glutathione and thioglycolic acid. The results demonstrate that the peak current for thiol oxidation has a linear variation with the concentration in the range of 1×10^?2–1×10^?5 M. This system can be used for sensitive and selective voltammetric detection of mercaptopyrimidine derivatives.     

Pulse radiolysis of pyridinecarboxylic acids in aqueous solution

The reactivity of OH, e^-_aq and H radicals towards aqueous carboxypyridines: picolinic acid (2-pyridinecarboxylic acid), PA; isonicotinic acid (4-pyridinecarboxylic acid), i-NA; 2,6-pyridinedicarboxylic acid, 2,6-PDCA; and 3,5-pyridinedicarboxylic acid, 3,5-PDCA was investigated in the pH-range 1–13.8. The absorption spectra of the OH-adducts, H-adducts and pyridinyl radicals are given as well as the formation and decay kinetics. In acid (but not in alkaline) solution, the reaction of H-atoms leads to the formation of two distinct products, namely H-adduct and pyridinyl radicals. The yields of pyridinyl radical are: 20% for PA, 75% for i-NA, 60% for 2,6-PDCA and 25% for 3,5-PDCA (a yield of 50% has been found earlier for nicotinic acid, NA).     

系列Mn(II)配位超分子的合成、晶体结构和表面光电压研究

采用水热合成方法得到了三种Mn(II)配位超分子: Mn(2,5-dcp)₂(H₂O)₂ (1), Mn(INA)₂(H₂O)₄ (2)和Mn(phen)₂Cl₂ (3) (2,5-dcp＝pyridine-2,5-dicarboxylic acid, INA＝iso-nicotinic acid, phen＝1,10-phenanthroline). 通过X射线单晶衍射、红外光谱(IR)、紫外光谱(UV-Vis)及表面光电压光谱(SPS)等方法对化合物进行了表征. 三种化合物中均存在大量的氢键, 使化合物晶体构成了无限延伸的三维网络结构. 表面光电压谱显示了化合物1～3在300～600 nm范围内呈现出正的光伏响应带, 具有P-型半导体的特征. 讨论了氢键在超分子构建中的作用以及不同配位环境对于配合物表面光电压的影响.     

The electrochemistry of proteins and related substances: I. Cystine and cysteine at the mercury electrode

Studies of the electroreduction of cystine (RSSR) and oxidation of cysteine (RSH) at several pH's at a hanging Hg drop electrode by cyclic voltammetry and at a Hg pool by coulometry are described. The proposed mechanism for RSSR involves reduction of an adsorbed monolayer (maximum coverage of 41 μC cm^?2) to form solution phase RSH at the adsorption prewave. A diffusion controlled reduction of RSSR to RSH occurs at more negative potentials. Oxidation of RSH involves formation of an adsorbed organomercury species, e.g., Hg(RS)₂ (maximum coverage of 80 μC cm^?2), which is reducible back to RSH. At higher RSH concentrations, anodic and cathodic current spikes appear on the cyclic voltammograms which are ascribed to formation of a tight or compact film when monolayer coverage of Hg(RS)₂ is attained, corresponding to strong interactions between adsorbed species.     

Complexes of nalidixic acid with some vital metal ions: Synthesis,chemical structure elucidation,and antimicrobial evaluation

Complexes of nalidixic acid with Ca(II), Mg(II), Zn(II), Fe(III), and VO(II) ions, have been synthesized and characterized by means of microanalysis, conductance, IR, NMR and UV-Vis spectroscopy and thermogravimetric (TGA/DTG and DTA) measurements. The ligand coordinates in a bidentate mode via one carboxylate oxygen atom and the oxygen atom of the pyridine carbonyl group. The kinetic thermodynamic parameters such as: activation energy E*, ΔH*, ΔS*, and ΔG* have been estimated using Coats and Redfern as well as Horowitz-Metzger equations. Antimicrobial activity of the obtained complexes against some kinds of bacteria and fungi has been evaluated.     

Excess volumes for binary liquid mixtures of trichloroethylene with anisole,pyridine, quinoline and cyclohexane at 298.15 and 308.15 K

Dilatometric measurements of excess volumes V^E have been made for binary liquid mixtures of trichloroethylene (CHClCCl₂) with anisole, pyridine, quinoline and cyclohexane at 298.15 and 308.15 K. At both temperatures, and values of V^E have been found to be slightly negative for CHClCCl₂ + anisole, CHClCCl₂ + pyridine, highly negative for CHClCCl₂ + quinoline, and highly positive for CHClCCl₂ + cyclohexane. The negative values of V^E for the systems CHClCCl₂ + anisole, CHClCCl₂ + pyridine, and CHClCCl₂ + quinoline, have been explained as due to the existence of specific interaction of CHClCCl₂ with anisole, pyridine and quinoline in the liquid state.     

Chemical interactions between silver nanoparticles and thiols: a comparison of mercaptohexanol against cysteine

The interaction between citrate capped silver nanoparticles and two different thiols, mercaptohexanol(MH) and cysteine, was investigated. The thiols interacted with silver nanoparticles in a significantly contrasting manner. With MH, a sparingly soluble silver(I) thiolate complex AgSRm(Rm = –(CH2)6OH) was formed on the silver nanoparticle surface. Cyclic voltammograms and UV-vis spectra were used to infer that the AgSRm complex on the nanoparticle surface undergoes a phase transition to give a mixture of AgSRm and Ag2S-like complexes. In contrast, when silver nanoparticles were exposed to cysteine, the citrate capping agent on the silver nanoparticles was replaced by cysteine to give cysteine capped nanoparticles. As cysteine capped nanoparticles form, the electrochemical data displayed a decrease in oxidative peak charge but the UV-vis spectra showed a constant signal. Therefore, cysteine capped nanoparticles were suggested to have either inactivated the silver surface or else promoted detachment from the electrode surface.     

Molecular structure, vibrational spectra, quantum chemical calculations and photochemistry of picolinamide and isonicotinamide isolated in cryogenic inert matrixes and in the neat low-temperature solid phases

Picolinamide (PA) and isonicotinamide (INA), two structural isomers of pyridinecarboxamide, have been investigated by matrix isolation and low-temperature solid-state infrared spectroscopy, combined with UV (lambda > 235 nm) photoexcitation and density functional theory and ab initio (MP2) theoretical studies. In consonance with the theoretical data, both PA and INA were found to exist in a single conformation in cryogenic rare gas matrixes. Comparison between the experimental spectra of the matrix-isolated compounds with those theoretically predicted allowed for full assignment of the experimental spectra. In situ UV (lambda > 235 nm) irradiation of the matrixes showed that only PA reacts, with production of isocyanic acid and pyridine, the first photoproduct further reacting to yield CO + NH and cyanic acid. The different photochemical behavior of the two compounds was explained taking into consideration their different structures. The infrared spectra of (i) the low-temperature glassy state resulting from fast deposition of vapors of the compounds onto a substrate cooled to 10 K, (ii) the crystal resulting from the annealed amorphous film of the compound, and (iii) the room-temperature crystals (alpha-phase) of the studied compounds were also obtained, fully assigned and correlated with intermolecular interactions present in the condensed phases, in particular H-bond interactions, showing that these latter are stronger in INA than in PA.