Investigation of the stereochemistry of N-substituted 2,5-dimethyl-4-piperidinones by NMR spectroscopy

The conformations of the cis and trans isomers of N-substituted 2,5-dimethyl-4-piperidinones were studied by means of the¹H and¹³C NMR parameters. It was established that in the case of bulky and electron-acceptor substituents attached to the N atom the cis isomers are virtually completely represented by the chair (2a,5e) conformation, while the trans isomers are characterized by the chair (2e,5e) twistboat (2a,5e) chair (2a,5a) conformational equilibrium. It is demonstrated that 1-tert-butyltrans-2,5-dimethyl-4-piperidinone hydrochloride has the twist (2a,5e) conformation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1760–1768, August, 1990.     

Electronic structure and conformational flexibility of 1,2-dihydropyridine, 1,2- and 1,6-dihydropyrimidines,and their ylide derivatives

Molecular and electronic structures of 1,2-dihydropyridine, 1,2- and 1,6-dihydropyrimidine, and their oxo, imino, and methylene derivatives were studied by the semiempirical quantum-chemical AM1 method. In all compounds, the heterocycle exhibits a high conformational flexibility. The transition from a planar equilibrium conformation to a distorted sofa conformation with the =C-NH-C-C(N)= torsion angle of ±20° causes an increase in the energy by less than 1.7 kcal mol^–1. All molecules have similar -electronic structures, which, apparently, determines the similarity in their conformational behavior. The bending strain and the nonaromatic character of the cyclic -system are the factors that stabilize the nonplanar conformation of the ring in unsubstituted dihydroazines and ylide derivatives, respectively.Translated fromIzvestiva Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1934–1937, August, 1996.     

Synthesis and conformational analysis of 18-membered Aib-containing cyclohexapeptides

The synthesis and conformational analysis of two Aib-containing cyclic hexapeptides, cyclo(Gly-Aib-Leu-Aib-Phe-Aib) 1 and cyclo(Leu-Aib-Phe-Gly-Aib-Aib) 2, is described. The linear precursors of 1 and 2 were prepared using solution phase techniques, and the cyclization efficiency of three different coupling reagents (HATU, PyAOP, DEPC) was examined. The success of the cyclization was found to be reagent dependent. Solid-state conformational analysis of 1 and 2 was performed by X-ray crystallography and has revealed some unusual features as all three Aib residues of 1 assume nonhelical conformations. Furthermore, the residue Aib⁴ adopts an extended conformation (?=−175.9(3)°, ψ=+178.6(2)°), which is, to the best of our knowledge, the first observation of an Aib residue adopting an extended conformation in a cyclopeptide. The structure of 1 is also a rare example in which an Aib residue occupies the (i+1) position of a type II′ β-turn, stabilized by a bifurcated hydrogen bond. The cyclic peptide 2 adopts a more regular conformation in the solid state, consisting of two fused β-turns of type I/I′, stabilized by a pair of intramolecular hydrogen bonds. In addition, the conformational study of the cyclic peptide 1 in DMSO-d₆ by NMR spectroscopy and molecular dynamics simulations revealed a structure, which is very similar to its structure in the crystalline state.     

Stereochemistry of nitrogen heterocycles. 62. Conformational analysis of isomers of 2-methyl-cis-decahydro-5-quinolinol

Isomers of 2-methyl- and 1,2-dimethyl-cis-decahydro-5-quinolinol with a syn orientation of the hydroxy and amino groups and different orientations of the methyl group relative to the methylene group at C₍₈₎H₂ were subjected to conformational analysis. In the case of a cis orientation of the methyl and methylene groups the equilibrium is shifted completely to favor the conformation with an intramolecular hydrogen bond, whereas in the case of their trans orientation the mole fraction of this conformation amounts to 21–24% for the secondary amino alcohol and 18–21% for the tertiary amino alcohol. The energies of the hydrogen bonds were determined from the intensities of the absorption bands of the free and associated hydroxy groups in the IR spectra: for the secondary hydroxy amine, according to the band of the free hydroxy group, G⁰ _OH/N is –0.8 kcal/mole, whereas, according to the band of the associated hydroxy group, it is –0.9 kcal/mole; the values for the tertiary hydroxy amine are, respectively, –0.7 and –0.8 kcal/mole.See [1] for Communication 61.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1514–1519, November, 1987.     

Structure of dilignol,β-O-4-[1-(4-hydroxyphenyl)-2-(2-methoxyphenoxy)-1-ethanol] in crystal and in solution

The structure of dilignol, 1-(4-hydroxyphenyl)-2-(2-methoxyphenoxy)-1-ethanol was studied by x-ray diffraction structural analysis and¹H PMR spectroscopy. The dilignol molecules crystallize in an extended conformation with trans-Ar-C-C-O and trans-C-C-O-Ar structures: the angle between the plane of the aromatic systems was 74.0°. This conformation was stabilized by intermolecular hydrogen bonds, which linked the molecules into chains. Weak Y-shaped hydrogen bonds were found between molecules of adjacent chains. These results were compared with the literature data on the crystal structure of -O-4 dilignols. A similar conformation of dilignol stabilized by an intramolecular hydrogen bond, whose existence was indicated by IR spectroscopy, was found to predominate in acetone and chloroform solutions. A conclusion was drawn concerning the conformational mobility of -O-4 dilignols and the factors determining the conformational composition of dilignols in solution were discussed.S. M. Kirov Leningrad Forest Technology Institute, Institute of Wood Chemistry, Academy of Sciences of the Latvian SSR. Institute of Organic Synthesis, Academy of Sciences of the Latvian SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 5, pp. 135–141, September–October, 1989.     

Conformational flexibility of six-membered 1,2-dihydrocycles based on the results of AM1 calculations

The conformational flexibility of 2H-pyran, 1,2-dihydropyridine, 2H-thiopyran, their oxo, imino, and methylene derivatives, and the 5-oxo, imino, and methylene derivatives of 1,3-cyclohexadiene was studied by the semiempirical quantum-chemical AM1 method. All compounds have planar equilibrium conformations and similar electronic structures. The transition to a distorted sofa conformation with the =C-X-C(=Y)-C= torsion angle (X = CH₂, NH, O, or S; Y = H₂, O, NH, or CH₂) of ±20° increases the energy of the molecule by less than 1.5 kcal mol^–1. The reasons for the high conformational flexibility of these dihydrocycles were analyzed. The nonaromatic character of the cyclic -system is the factor that determines the conformational flexibility of the dihydrocycle in ylide derivatives.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2650–2652, November, 1996.     

Determination of pyrimethanil and kresoxim-methyl in soils by headspace solid-phase microextraction and gas chromatography-mass spectrometry

A method using headspace solid-phase microextraction (HS-SPME) then gas chromatography–mass spectrometry with selected ion monitoring (GC–MS, SIM) has been developed for determination of trace amounts of the fungicides pyrimethanil and kresoxim-methyl in soil and humic materials. Both fungicides were extracted on to a fused-silica fibre coated with 85 m polyacrylate (PA). Response-surface methodology was used to optimise the experimental conditions. For soil samples the linear dynamic range of application was 0.004–1.000 g g^–1 for pyrimethanil and 0.013–1.000 g g^–1 for kresoxim-methyl. The detection limits were 0.001 g g^–1 and 0.004 g g^–1 for pyrimethanil and kresoxim-methyl, respectively. HP-SPME–GC–MS analysis was highly reproducible—relative standard deviations (RSD) were between 6.7 and 12.2%. The method was validated by analysis of spiked matrix samples and used to investigate the presence of pyrimethanil and kresoxim-methyl above the detection limits in soil and humic materials.     

Application of l-cysteine-capped nano-ZnS as a fluorescence probe for the determination of proteins

Nanometer-sized l-cysteine-capped ZnS particles have been synthesized and used as a fluorescence probe to investigate the effect of proteins on fluorescent intensity. With =190 nm, maximum and constant synchronous fluorescence enhancement was produced at 267 nm and pH 5.12 in the presence of proteins. A highly sensitive synchronous fluorescence method for the rapid determination of proteins has been developed. Under optimum conditions, calibration graphs are linear over the range 0.03–8.0 g mL^–1 for bovine serum albumin (BSA), 0.01–6.0 g mL^–1 for human serum albumin (HSA), 0.05–8.0 g mL^–1 for -globulin (-G), and 0.04–4.0 g mL^–1 for ovalbumin, respectively. The relative standard deviations of seven replicate measurements were 1.75% for 1.0 g mL^–1 BSA, 1.90% for 1.0 g mL^–1 HSA, 1.65% for 1.0 g mL^–1 -G, and 2.32% for 1.0 g mL^–1 ovalbumin.     

Thermal denaturation of ribonuclease T1 a DSC study

The thermal denaturation of microbial Ribonuclease T1 (RNAase T1) as a function ofpH, was studied by means of DSC microcalorimetry. The midpoint denaturation temperatures, enthalpy changes and heat capacity changes of Ribonuclease T1 were compared with those obtained for pancreatic Ribonuclease A (RNAase A). It was found that the microbial T1 protein undergoes a more complex conformational transition than the simple two-state transition shown by Ribonuclease A. The hypothesis of the presence of a molten globule form is discussed. The conformational stability of RNAase T1 is lower than that of RNAase A at highpH values. Indeed, the maximum stability of RNAase T1 occurs atpH 5, whereas that of RNAase A occurs atpH 8. AtpH=3.7 an irreversible aggregation phenomenon was indicated by the existence of a reproducible exothermic peak. The conformational transition of RNAase T1 is reversible in the range ofpH 4.5–7.0, whereas it becomes irreversible atpH8.0 as for RNAase A.This work was financed by the National Research Council (C.N.R.-Rome) and by Ministry of University and Scientific and Technological Research.     

IR spectroscopic study of the conformational lability of 4′-ethyl-4-cyanobiphenyl

The conformational lability of 4-ethyl-4-cyanobiphenyl molecules in solid crystal (SC) and isotropic liquid (IL) states was investigated by IR spectroscopic techniques (experiment and theory). IR absorption spectra were measured at 28°C–95°C in the frequency range 400 cm^–1–4000 cm^–1. Spectrum simulation was performed using the fragment method with allowance for the conformational fluctuations of molecules. The experimental and calculated spectra were compared and analyzed, and it was shown that in the IL, the samples are mixtures of conformers. The temperature changes in the spectra in the stated range are caused by the conformational lability of molecules.Original Russian Text Copyright © 2004 by L. M. Babkov, I. I. Gnatyuk, G. A. Puchkovskaya, and S. V. TrukhachevTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 398–405, May–June, 2004.     

Silica Gel Loaded with <Emphasis Type="Italic">o</Emphasis>-Dihydroxybenzene: Design,Metal Sorption Equilibrium Studies and Application to Metal Enrichment Prior to Determination by Flame Atomic Absorption Spectrometry

The aminopropyl silica gel (APSG) prepared by reaction of activated silica gel with 3-aminopropyltriethoxysilane on reaction with 3,4-dihydroxybenzaldehyde has resulted in a new chelating matrix, o-dihydroxybenzene (DHB) anchored silica gel, which is characterized by IR, TGA and elemental analyses. APSG is characterized with ¹³C CPMAS NMR spectroscopy. DHB anchored silica gel sorbs quantitatively (97.4–99.2% recovery) Cu(II), Pb(II), Fe(III), Zn(II), Co(II), Ni(II) and Cd(II) at pH 6.0–7.5, 5.0–7.0, 5.5–7.0, 6.0–8.0, 6.5–8.0, 5.5–7.0 and 6.5–7.5, respectively. The sorption capacity varies from 32 to 348µmolg^–1 and is highest for copper. Desorption was found to be quantitative with 1.0–3.0molL^–1 HCl/HNO₃ (for Pb). The optimum flow rate of the solution for quantitative sorption of metal ions on a column (10cm×10mm) packed with 1g of the modified silica gel is 1.0–4.0mLmin^–1, whereas for desorption it is 2.0 –4.0mLmin^–1. The tolerance limits for NaCl, NaBr, NaI, NaNO₃, Na₂SO₄, Na₃PO₄, humic acid, EDTA, ascorbic acid, citric acid, sodium tartrate, Ca(II) and Mg(II) in the sorption of all the seven metal ions are reported. The preconcentration factors are between 100 and 300 and t_1/2 values 17min. The present matrix coupled with FAAS has been used to enrich and determine the seven metal ions in river and tap water samples (RSD 1.4–7.0%) and in synthetic certified water samples SLRS-4 (NRC, Canada) with an RSD of 2.73–2.83%. The cobalt present in pharmaceutical vitamin tablets and Zn in milk powder was preconcentrated on DHB anchored silica gel and determined by FAAS (RSD of 2.00 to 2.72%).     

Electrocatalytic Properties and Voltammetric Determination of Melatonin at a Nanostructured Film Electrode

A multi-wall carbon nanotube (MWNT) film coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behavior of melatonin on the MWNT film coated GCE was investigated. The oxidation peak current of melatonin increases significantly and the oxidation peak position shifts positively at the MWNT film modified GCE compared to that at a bare GCE. This indicates that MWNTs feature highly effective catalysis to the electrochemical oxidation of melatonin. A simple and sensitive electroanalytical method was developed for the determination of melatonin. The oxidation peak current is proportional to the concentration of melatonin from 8×10^–8 to 1×10^–5molL^–1. The detection limit is about 2×10^–8molL^–1 for 3min accumulation. The proposed method was demonstrated to work satisfactorily with commercial capsules.     

Analysis of High-Purity Volatile Compounds

The current status of the analysis of high-purity volatile substances is considered. Two types of impurities in high-purity volatile substances were distinguished: molecularly dissolved substances and suspended particles. The main factors that restrict the limiting capabilities of analytical techniques were revealed. The attained detection limits were 10^–8–10^–10% for metal impurities, 10^–4–10^–8% for organic substances, 10^–5–10^–9% for water, and 10^–5–10^–7 for permanent gases. Suspended particles of 0.04–0.003 m in size were determined by light scattering.     

Determination of subnanomolar concentrations of cobalt by adsorptive stripping voltammetry at a bismuth film electrode

Procedures for trace cobalt determinations by adsorptive stripping voltammetry at in situ and ex situ plated bismuth film electrodes are presented. These exploit the enhancement of the cobalt peak obtained by using the Co(II)–dimethylglyoxime–cetyltrimethylammonium bromide–piperazine-N,N-bis(2-ethanesulfonic acid) system. The calibration graph for an accumulation time of 120 s was linear from 2 × 10^–10 to 2 × 10^–8 mol L^–1. The relative standard deviation from five determinations of cobalt at a concentration of 5 × 10^–9 mol L^–1 was 5.2%. The detection limit for an accumulation time of 300 s was 1.8 × 10^–11 mol L^–1. The proposed procedure was applied to cobalt determination in certified reference materials and in tap and river water samples.     

Chemical and Biological Sensors Based on Electrochemical Detection Using Conducting Electroactive Polymers

The electrochemical behavior of composites of conducting electroactive polyaniline (PAn) and polypyrrole (PPy) formulated within cross-linked hydrogel networks was investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Composite PAn gels displayed similar anodic charge density compared to the pristine conducting polymer (80mC/cm² and 84mC/cm², respectively), suggesting a similar degree of electroactivity between the two systems. Composite gels of PAn displayed fast cation transport with K⁺ diffusivity (D_appt= 5.31×10^–7cm²s^–1) that were three orders of magnitude larger than that of pristine PAn (D_appt=3.12×10^–10cm²s^–1), while PPy composite gels showed similar ferrocene anion diffusivity (D_appt=7.05×10^–5cm²s^–1) compared to electropolymerized PPy (D_appt=6.54×10^–5cm²s^–1). The electrochemical interactions between CYP2D6, a cytochrome P450 isoenzyme, and fluoxetine mediated by electroactive polyaniline films on glassy carbon electrodes (GCEs) were investigated. Cyclic voltammograms indicate that PAn is an effective mediator of CYP2D6 activity under anaerobic conditions. An analytical interrogation methodology based on small-amplitude, pulsed DC was developed and incorporated into the Electroconductive Polymer Sensor Interrogation System (EPSIS). Polypyrrole membranes were rendered biospecific by either copolymerization of pyrrole (Py) with 4-(1-pyrrolyl) butyric acid (4PyBA), followed by direct conjugation with 5-(biotinamido)pentyl amine (5BPA), or by reacting 4PyBA with 5BPA to form pyrrolyl-biotin conjugates. The biotinylated PPy was made responsive to glucose or urea by exploiting strong biotin-streptavidin binding to either streptavidin-glucose oxidase or biotin-urease conjugates. These bioactive conducting polymer membranes were demonstrated as conductimetric glucose and urea biosensing layers using the EPSIS. The rate of conductivity of the bioactive PPy membranes was observed to double upon increasing glucose concentration from 100µM (4×10^–6Scm^–1s^–1) to 600µM (9×10^–6Scm^–1s^–1).     

Potentiometric Flow Injection Analysis of Anionic Surfactants in Industrial Products and Wastes

A new sensor for anionic surfactants with a membrane consisting of 33wt% poly(vinyl chloride) (PVC), 66wt% dioctylphthalate (DOP) plasticizer, and 1wt% tridodecylmethylammonium chloride (TDMAC) is developed and used for flow injection analysis. The sensor displays a working response range of 5×10^–7–5×10^–3M dodecylbenzene sulfonate (DBS^–) with a Nernstian slope of 58.5±0.2mV decade^–1, a response time of 30s and a detection limit of 1.5×10^–7M DBS^–. Selectivity measurements with different anionic species indicate good membrane selectivity towards DBS^–. The sensor is used to measure anionic surfactants (DBS^–) in different wastewater samples, commercial detergent products, and for monitoring the rate of surfactant biodegradation in sewage treatment plants. The results obtained agree fairly well with data obtained by the standard extraction-spectrophotometric method. The proposed potentiometric method offers the advantages of simplicity, accuracy, automation feasibility, and applicability to turbid and colored sample solutions.     

Kinetics and Mechanism of the oxygenation of triphenylphosphine by bis(ethyl-L-cysteinato)dioxomolybdenum(VI)

Summary The kinetics of oxygen-transfer from [MoO₂(Et-L-cys)₂] to PPh₃ and the reaction between [Mo₂O₃(Et-L-cys)₄] and O₂ in benzene solution have been investigated using spectrophotometric techniques between 25 and 40°. The rate laws-d[Mo⁶⁺]/dt = k₁[Mo⁶⁺][PPh₃] with k₁ (at 35°) = 2.95×10^–4dm³mol^–1s^–1 and -d[Mo⁵⁺]/dt = 2k₃[Mo⁵⁺][O₂] with k₃ (at 35°) = 6.3×10^–2 dm³mol^–1s^–1 account for the kinetic data obtained with activation parameters (at 35°) of H = 46 kJ mol^–1, S = –153 JK^–1mol^–1, and H = 50.8 kJ mol^–1, S = –95 JK^–1 mol^–1 respectively.     

Determination of ratios of sulfate to bisulfate ions in aqueous solutions by Raman spectroscopy

Raman spectra are presented for aqueous ammonium bisulfate and zinc sulfate-sulfuric acid solutions over a wide range of concentration and from 20 to 85°C. The heights of the 980 cm ^–1 (primarily SO ₄ ^–2 ) and 1053 cm ^–1 (primarily HSO ₄ ^– ) bands are correlated with the pH of the solutions. The ratio of the species SO ₄ ^–2 and HSO ₄ ^– can be obtained from the ratio of scattering intensities at 980 and 1053 cm ^–1 . The H obtained from plots of the log of the concentration products versus 1/T is –37±5 kJ-mol ^–1 for ammonium bisulfate solutions and –48±4kJ-mol ^–1 for the ZnSO ₄ -H ₂ SO ₄ solutions. This compares to –22 kJ-mol ^–1 for the H of bisulfate ion dissociation as obtained from thermodynamic equilibrium constants.     

Mathematical Modeling of Kinetics of Adenosine 5"-Triphosphate Hydrolysis Catalyzed by the Zn2+ Ion in the pH Range 7.4–8.3

Kinetic data on adenosine triphosphate (ATP) hydrolysis catalyzed by the Zn²⁺ ion in the pH range 7.4–8.3 are analyzed by the method of numerical simulation. The rates of forward and reverse reactions of isomeric conversion of the open conformation of ZnATP^2– (Op), which is inactive in hydrolysis to ADP, to the active cyclic conformation ZnATP^2– (Cy) in the specified range of pH are proportional to the concentration of H₃O⁺ and characterized by the same rate constants as in the range of pH above 8.5. The mechanism of the isomeric conversion Op Cy involves the formation of a pentacovalent state at -P, pseudorotation, and the abstraction of OH^– from -P of the pentacovalent intermediate with the participation of H₃O⁺ in a slow step. The sequence of steps for the formation and transformation of intermediates, which was established earlier for the ZnATP^2– associates in the pH range 7.1–7.4, is applicable to this range of pH as well. In the analyzed range of pH, the contributions from the pH-independent channel of hydrolysis of the ZnATP^2– associates and the pH-dependent channel of CyOH^– and Op(OH^–)₂ species, which determine the formation of ADP and AMP at pH > 8.5, are comparable. Changes in the concentrations of intermediate products (monomeric and associates) in the course of hydrolysis are described. General base catalysis by a nitrogen base in the steps of formation of active centers for hydrolysis, the general acid catalysis of a coordinated water molecule, the exchange of medium OH^– with OH of -phosphate, the catalysis of conversion of the inactive conformation ZnATP^2– to the active one by a proton, and a change of the rate-limiting stage of hydrolysis with a change in pH indicate the enzyme-like mechanism of the reaction.     

Adsorption of Cl? ions on electrodeposited rhodium black layer (rhodized electrode) at low concentrations

The adsorption of Cl^– ions on rhodium black layer (rhodized electrodes) was studied by radiotracer technique at low Cl^– ion concentrations (c10^–5 mol dm^–3) in 1 mol dm^–3 H₂SO₄ supporting electrolyte. The specific adsorption of Cl^– ions was treated in terms of partition between solution phase and electrodeposited Rh black layer. The potential dependence of the partition coefficient is determined.