Systematic theoretical investigations on the tautomers of thymine in gas phase and solution

The tautomers of thymine are systematically calculated using various methods. The order of the relative stability, dipole moment as well as solvent effect of the 13 isomers are investigated. The one-step transition process by proton transfer in gas phase and in solution (H₂O or CH₃OH), which acts as both proton donor and acceptor, are exhaustively studied, meanwhile, the internal rotation between two related isomers in the gas phase is also investigated. Furthermore, the energy barrier of each transition is calculated and the results show that both H₂O and CH₃OH could reduce the energy barrier. Comparisons between the activation energies of all the producing processes of each tautomer as well as temperature effect on the producing processes are made. The calculation results also indicated that the diketo is the most stable isomer both in gas phase and in solution, additionally, it is thermodynamically as well as dynamically favored.     

Sequential injection with lab-at-valve (LAV) approach for potentiometric determination of chloride

Sequential injection with “Lab-at-Valve (LAV)” approach is demonstrated for potentiometric determination of chloride. The LAV flow-through electrode system consists of two Ag/AgCl electrodes: one as a reference electrode, silver chloride activated surface-silver wire soaked in a constant-concentration chloride ion solution in a small tube covered with a polymer-membrane, another as a working electrode (a similar silver chloride activated surface-silver wire) placed in a flow channel. The electrode system is attached at one port of a 10 port multiposition valve. A modified autoburette was used as a propelling device. Using SI operation via a program written in-house, based on LabVIEW^®, a standard/sample is inserted, via the selection valve, in potassium nitrate as an electrolyte and water is used as a carrier. The zones are transported from the holding coil to the flow cell to monitor the difference in potential due to concentration cell behavior. The potential difference is then recorded as a peak. Peak height is proportional to logarithm of chloride concentration. The SI-LAV for chloride determination is very simple, fast, precise, accurate, automatic and economical. Applications to mineral drinking water and surface water have been made. The results agree with those of IC and titrimetric methods.     

The mechanism of u.v. stabilization of polypropylene films by 2-hydroxy-4-octyloxybenzophenone

The rates of oxygen absorption by unstabilized and u.v.-stabilized polypropylene (PP) films irradiated with a high-pressure mercury lamp have been determined as a function of the light intensity (I). The induction time is nearly proportional to I^?0.5 for unstabilized PP and for PP stabilized with Tinuvin P, and proportional to about I^?0.9 for PP stabilized with 2-hydroxy-4-octyloxybenzophenone (HOBP). The change in concentration of HOBP in PP during the photo-oxidation is determined with radioactive HOBP. During the induction time, the observed decrease obeys zero-order kinetics even when the starting concentration of HOBP is as low as 0.008%. The decrease is caused by reaction of HOBP with the polymer and by fragmentation into gaseous products. These results are interpreted on the basis of the same reaction scheme as that worked out for the uninhibited and inhibited autoxidation of PP. It is concluded that HOBP acts not only as a u.v. absorber or as an excited-state quencher, but also as a scavenger of free radicals.     

Molecular structure and microbiological activity of alkali metal 3,4-dihydroxyphenylacetates

The changes in physical, chemical and biological properties of chemical compounds decide about their biological activity. In this paper the molecular structure of alkali metal 3,4-dihydroxyphenylacetates is studied in comparison to 3,4-dihydroxyphenylacetic acid (3,4-DHPAA) using FT-IR, FT-Raman and UV–Vis spectroscopy as well as density functional theory (DFT) calculations. The B3LYP/6-311++G⁷ method is used to calculate optimized geometrical structures of studied compounds, atomic charges (Mulliken, APT, NBO), dipole moments, energies as well as the wavenumbers and intensities of the bands in vibrational spectra. Theoretical parameters are compared to the experimental data. The relationship between spectroscopic parameters of studied compounds and their biological activity is analyzed. Antioxidant activity is studied using FRAP and DPPH methods. IC₅₀ parameter is also calculated. Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli and Klebsiella oxytoca are used in microbiological analysis of 3,4-DHPAA as well as its sodium and potassium salts.     

Micellar selectivity triangle for classification of chemical selectivity in electrokinetic chromatography

A novel micellar selectivity triangle (MST) is developed and used to characterize and classify the chemical selectivities of pseudo-phases in electrokinetic chromatography (EKC). The MST scheme is, in concept, similar to the widely known solvent selectivity triangle (SST) originally developed by Snyder. However, the MST is based on linear solvation energy relationships. Thus it incorporates the solvation characteristics of both the pseudo-phase and the bulk solvent; while the SST is basically for classification of pure solvents. The similarities and differences of these pseudo-phases are determined by the relative scales of hydrogen bond donating ability (X_b), hydrogen bond accepting ability (X_a) and dipolarity (X_s). The MST scheme is used for characterization and classification of a wide range of pseudo-phases such as micelles, polymers, vesicles, liposomes, as well as mixed systems such as mixed micelles, mixed polymer–surfactants, organically modified pseudo-phases, etc. Over seventy pseudo-phases were examined and four clusters of pseudo-phases with different selectivity patterns are recognized that include pseudo-phases with strong hydrogen bond acidities (e.g. fluorinated micelles or micelles modified with fluorinated alcohols), strong hydrogen bond acceptor pseudo-phases (such as bile salts, liposomes, microemulsions, as well as biphasic octanol–water system), strong dipolar phase of a class of polymeric pseudo-phase, and pseudo-phases with intermediate hydrogen bonding and dipolarity [like sodium dodecyl sulfate (SDS) and its analogs as well as organically modified SDS]. The MST scheme is also useful in identifying pseudo-phases that closely resemble the selectivities of octanol–water for determination of octanol–water partition coefficients by EKC.     

Environmentally Benign,Rapid, and Selective Extraction of Gold from Ores and Waste Electronic Materials

The extraction of gold from ores and electronic waste is an important topic worldwide, as this precious metal has immense value in a variety of fields. However, serious environmental pollution and high energy consumption due to the use of toxic oxidation reagents and harsh reaction conditions is a well‐known problem in the gold industry. Herein, we report a new chemical method based on the combined use of N ‐bromosuccinimide (NBS) and pyridine (Py), which has a greatly decreased environmental impact and reagent cost, as well as mild reaction requirements. This method can directly leach Au⁰ from gold ore and electronic waste to form Au^III in water. The process is achieved in a yield of approximately 90 % at room temperature and a nearly neutral pH. The minimum dose of NBS/Py is as low as 10 mm , which exhibits low toxicity towards mammalian cells and animals as well as aquatic creatures. The high leaching selectivity of Au over other metals during gold leaching is demonstrated, showing that this method has great potential for practical industrial application towards the sustainable refining of gold from ores and electronic waste.     

Environmentally Benign,Rapid, and Selective Extraction of Gold from Ores and Waste Electronic Materials

The extraction of gold from ores and electronic waste is an important topic worldwide, as this precious metal has immense value in a variety of fields. However, serious environmental pollution and high energy consumption due to the use of toxic oxidation reagents and harsh reaction conditions is a well‐known problem in the gold industry. Herein, we report a new chemical method based on the combined use of N ‐bromosuccinimide (NBS) and pyridine (Py), which has a greatly decreased environmental impact and reagent cost, as well as mild reaction requirements. This method can directly leach Au⁰ from gold ore and electronic waste to form Au^III in water. The process is achieved in a yield of approximately 90 % at room temperature and a nearly neutral pH. The minimum dose of NBS/Py is as low as 10 mm , which exhibits low toxicity towards mammalian cells and animals as well as aquatic creatures. The high leaching selectivity of Au over other metals during gold leaching is demonstrated, showing that this method has great potential for practical industrial application towards the sustainable refining of gold from ores and electronic waste.     

Chlorine-isotopic exchange between lithium chloride and substituted 2-chloropyridine in homogeneous solution

The kinetics of chlorine-isotopic exchange between lithium chloride-36 and cyano- and nitro- substituted 2-chloropyridines were measured in sulpholane, acetone or methanol solution. Activating effects of ortho-nitro and ortho-azu substitution are compared: a nitro-group is 50 × as activating as the aza-group in the p-nitrochlorobenzene system, where as it is the aza-function which is 3 times as activating as the nitro group in the o-nitrochlorobenzene system. The effect of Me substituents placed ortho to an activating nitro-group was studied by comparing 2-chloro-3-cyano-5-nitropyridine and its 6-methyl- and 4,6-dimethyl-derivatives.     

Dissolution equation of polymer

The dissolution of the polar polymer, polyacrylamide (PAM), in water is studied kinetically. The rate of dissolution, ?, of PAM is determined by means of measuring the conductivity of water as dissolution of the polymer proceeds. The thickness of the swollen surface layer, δ, and the rate of swelling, ?, are determined with the aid of penetration and followed surface track by conductometric electrode. A kinetic equation reflecting PAM dissolution in water is obtained directly as 2? + ? = D?_s/δ, where D?_s is the average diffusion coefficient of solvent in polymer and is shown to be true in cases of nonpolar as well as polar polymers. © 1996 John Wiley & Sons, Inc.     

Analytical applications of a differential technique in laser-induced fluorimetry: accurate and precise determination of uranium in concentrates and for designing microchemielectronic devices for on-line determination in processing industries

A novel instrumental technique for the direct, fast, accurate, and precise determination of uranium in concentrates and other U-rich materials (as well as to mineralized rocks) is presented. The proposed technique is an absolute methodology, based on the comparison of the fluorescence of the accurately known standard with a sample of similar but unknown concentration in the low operational range of the instrument (on same sample-dilution basis), by the use of H₃PO₄-NH₄H₂PO₄ as a fluorescence-enhancing reagent. The relative standard deviation of the proposed technique was 0.5-0.9% (n=9) at 18.1, 36.2, 61.2, and 99.6% U₃O₈. The proposed technique is suitable for the determination of uranium in samples arising from exploration projects, ores from mining operations, mill process samples, uranium ore concentrates leading to fuel fabrication as well as samples from environmental monitoring containing up to 100% uranium. The results are in good agreement with those obtained by titrimetric, gravimetric, and TBP extraction-H₂O₂ spectrophotometric methods. The precision of the technique is within the acceptable ‘pure geochemistry’ type of analysis (R.S.D. ∼ 1.0%) and is comparable even those obtained with titrimetric and gravimetric assay. The proposed differential technique coupled with flow injection may open up new advancement in instrumentation leading to design and development of microchemielectronic devices for direct on-line determination, more compatible with the tools of computer age as also to help in handling of radioactive solutions in chemical laboratories in uranium processing industries.     

Decomposition of oxalic acid with nitric acid

Oxalic acid or oxalate is widely used as a precipitant and a detergent in the field of nuclear energy. The present work aimed at developing a method of decomposing oxalic acid with HNO₃ in the presence of Mn²⁺ ion. The use of Mn²⁺ ion as low as 10^?3 mol/l facilitated the complete decomposition of oxalic acid, and the acidity of the resulting solution became as low as 0.1 eq/1 under the optimum conditions. The decomposition of oxalic acid is a first order reaction and proceeds at temperatures above 80°C; the activation energy of the reaction is 18.6 kcal/mol. This decomposition method is applicable to the dissolution of an oxalate precipitate.     

Can an Amine Be a Stronger Acid than a Carboxylic Acid? The Surprisingly High Acidity of Amine–Borane Complexes

The gas‐phase acidity of a series of amine–borane complexes has been investigated through the use of electrospray mass spectrometry (ESI‐MS), with the application of the extended Cooks kinetic method, and high‐level G4 ab initio calculations. The most significant finding is that typical nitrogen bases, such as aniline, react with BH₃ to give amine–borane complexes, which, in the gas phase, have acidities as high as those of either phosphoric, oxalic, or salicylic acid; their acidity is higher than many carboxylic acids, such as formic, acetic, and propanoic acid. Indeed the complexation of different amines with BH₃ leads to a substantial increase (from 167 to 195 kJ mol^?1) in the intrinsic acidity of the system; in terms of ionization constants, this increase implies an increase as large as fifteen orders of magnitude. Interestingly, this increase in acidity is almost twice as large as that observed for the corresponding phosphine–borane analogues. The agreement between the experimental and the G4‐based calculated values is excellent. The analysis of the electron‐density rearrangements of the amine and the borane moieties indicates that the dative bond is significantly stronger in the N‐deprotonated anion than in the corresponding neutral amine–borane complex, because the deprotonated amine is a much better electron donor than the neutral amine. On the top of that, the newly created lone pair on the nitrogen atom in the deprotonated species, conjugates with the BN bonding pair. The dispersion of the extra electron density into the BH₃ group also contributes to the increased stability of the deprotonated species.     

Modification of sialic acid-MBTH assay for quantitative determination of nonenzymatically glucosylated proteins

A colorimetric assay for the quantitative determination of nonenzymatically glucosylated proteins is presented. The method employed is a modification of the method of Massamiri et al. (1978, Anal. Biochem., 91, 618–625) for quantitating sialic acid residues. The procedure involves periodate oxidation of the glucosylated protein releasing formaldehyde. The latter is then detected by complexing with the reagent, 3-methyl-2-benzothiazolinone hydrazone (MBTH). Fructose is used as a standard as it is structurally analogous to the ketoamine form on the protein. The MBTH assay can accurately detect fructose in concentrations as low as 1 μM. The assay was utilized to quantitate in vitro nonenzymatically glucosylated calmodulin and bovine serum albumin.     

Synthesis of mesoporous nano-TiO<Subscript>2</Subscript> doped with Sn by auto-assembly method and photo-catalytic property

Frank and Bard firstly reported that suspending TiO2 powder could photo-catalyze the solution con-taining CN?. Research on photo-degradation of delete-rious pollutants with nano-semiconductor material has become one of the most popular topics. Owing to it…     

Optimal population analysis

An easy method of approximating the electron density of a molecule by expressing it as a sum of squares of atomic orbitals is described. The coefficients are determined by minimizing the error in the electric field. The optimal population analysis is an integration of this optimized density over the orbitals belonging to one atom. The Mulliken population analysis can be interpreted as the integral of a Mulliken density that is a similar expression with fixed coefficients. The two densities are compared in détail using calculations on the water molecule and, more briefly, HF, LiH, BH, and C₂H₄. The error in the densities is identified and examined. The optimal population analysis is strongly recommended as a practical improvement to the Mulliken populations.     

The method of partial crystallization in determining the homogeneous distribution coefficient in coprecipitating systems

The method of determining the homogeneous distribution coefficient by the partial crystallization procedure has been examined. It has been found that in chemically very similar systems, such as FeSO₄·7H₂O as host and⁶⁰Co as guest, the partial crystallization technique gives very reliable data. However, in the case of CoSO₄·7H₂O as host and copper tracer as guest, the distribution coefficient found by the partial crystallization procedure is much lower than that obtained by the traditional method of complete equilibration. In this system the distribution coefficient increases as the period of partial crystallization is lengthened. The probable explanation of this particular behaviour is discussed. This paper was presented at the Convention of Chemists 1970 held in Madras, India in 1970.     

An integrative in silico approach to the structure of Omp33-36 in Acinetobacter baumannii

Omp33-36 in A. baumannii, a bacterium causing serious nosocomial infections, is a virulence factor associated with the pathogen metabolic fitness as well as its adherence and invasion to human epithelial cells. This protein is also involved in interaction of the bacteria with host cells by binding to fibronectin. Moreover, Omp33-36 renders cytotoxicity to A. baumannii in addition to inducing apoptosis and modulation of autophagy. In the present study, an integrated strategy is launched to pierce into the 3D structure of Omp33-36 protein. The signal peptide within the sequence was determined, then, topology as well as secondary and tertiary structures of the protein were predicted. The mature protein assigned as a 14-stranded barrel in which residues 1–19 is removed as signal peptide. The obtained 3D models were evaluated in terms of quality; and then, served as queries to find similar protein structures. The hits were analyzed regarding topology among which 14-stranded were considered. The most qualified model was refined and then its sequence aligned to its counterpart similar structure protein (CymA from Klebsiella oxytoca). The determined structure of Omp33-36 could justify its porin function and carbapenem-resistance associated with the loss of this protein.     

Simultaneous separation of mercury and lignosulfonate from aqueous solution using supported liquid membrane

This paper presents an experimental investigation on facilitated and simultaneous transport of mercury and lignosulfonate (LS) through a flat sheet supported liquid membrane (SLM) having Nylon 6,6 as support, trioctylamine (TOA) as carrier and dichloroethane as solvent. The experiments were performed at various operating conditions such as strip phase concentration, feed pH, carrier concentration and feed concentration to find the best set of parameters that would yield the maximum separation of pure mercury as well as its mixture with LS. The experiments were performed in co-transport mode using NaOH as the strip phase. It was observed that extraction of mercury as well as its mixture increases with increase in concentration of NaOH up to a certain limit 0.1 M NaOH in case of pure solution and 0.2 M NaOH in case of mixture). Feed phase pH significantly affects the mercury separation process. However, initial feed concentration does not affect the extraction process appreciably. Separation of mixture of mercury and LS behaves in a similar way as their pure solution; however the extraction is low in comparison to pure solution. The extraction of mercury from its pure solution is about 81% in 1 h. The extraction of mercury and LS from their mixture is about 52.6% and 50.2%, respectively at optimum condition and in a period of 2 h.     

Fluorination rates of polyolefins as a function of structure and gas atmosphere

Polyolefins and fluoropolymers were reacted with elemental fluorine under carefully controlled conditions in a thermobalance adapted to be compatible with fluorine gas. The fluorination reactions were monitored by measuring the mass increase as a result of hydrogen substitution by fluorine. The mass increase was directly proportional to the square root of the fluorination time, which indicates that fluorine gas diffusion to the unreacted surface is the rate determining step. The fluorination rate was increased by increasing the fluorine concentration and the fluorination temperature. The fluorination rate is higher when nitrogen rather than helium is used as diluting gas. The fluorination rate for the reaction in which CO₂ is used as diluting gas is the same as during fluorination with nitrogen as diluting gas, while the presence of oxygen dramatically decreased the fluorination rate. Oxygen is incorporated during fluorination with oxygen as diluting gas, while no functionalization was observed when CO₂ was employed as diluting gas. The effect of polymer structure on fluorination was studied. Poly(vinylfluoride) gained mass during fluorination, while no reaction was observed for poly(vinylidenefluoride). The reaction rate for polypropylene was higher than that of polyethylene. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Determination of Chlorophenols in Sediments Using Ultrasonic Solvent Extraction Followed by Stir Bar Sorptive Extraction Coupled to TD-GC-MS

A rapid method for analysis of chlorophenol content in sediments was developed using ultrasonic solvent extraction followed by stir bar sorptive extraction. The sediment sample is extracted using methanol and dichloromethane (9:1) in an ultrasonic bath. The extract is then redissolved in water and phenolic compounds are derivatised using acetic acid anhydride. The derivates are preconcentrated on a stir bar coated with polydimethylsiloxane that is commercially available as Gerstel Twister which is then thermally desorbed and analysed by GC-MS. Method performance is illustrated and evaluated for the enrichment of up to 100 ng g⁻¹ of selected chlorophenols in freshwater sediment samples. The method showed good linearity, recoveries and blank levels, as well as advantages such as sensitivity, simplicity, low cost and high feasibility.