Some observations on the automation by flow injection analysis of the spectrophotometric determination of amino acids and proteins witho-phthalaldehyde

Automation by flow injection analysis with Spectrophotometric detection of the determination of total amino acids and proteins witho-phthalaldehyde is not straightforward. The use of spectrophotometry, instead of spectrofluorimetry, and of N-acetyl-L-cysteine, instead of the conventional mercaptoethanol is advantageous because of the lower variability of absorptivities with respect to fluorescence yields, and the larger stability of the derivatives. Under adequate working conditions and with leucine as reference, the procedure can be used for the evaluation of total amino acids. A similar procedure is proposed for the analysis of proteins in a sample. Limits of detection are 1 × 10^–5 M for amino acids, and 1 × 10^–6 M for proteins, respectively.     

Electrochemical Synthesis and Characterization of Nickel(II) Complexes with N‐2‐Pyridyl‐sulfonamide Ligands

Heteroleptic nickel(II) complexes [NiL₂L′] of a series of monoanionic and potentially bidentate N‐2‐pyridyl‐sulfonamide ligands [HL] and 2,2′‐bipyridine or 1,10‐Phenanthroline (L′) have been prepared by electrochemical oxidation of a nickel anode in an acetonitrile solution of the ligands. The complexes have been characterized by microanalysis, IR and electronic spectroscopy, magnetic measurements and LSI mass spectrometry. The crystal structure of [Ni(Ms6mepy)₂(bipy)] has been determined by x‐ray diffraction and shows the metal in an octahedral NiN₆ environment. Octahedral structures are also proposed for the other complexes with the N‐2‐pyridyl‐sulfonamide ligands acting as N,N′ or N, O bidentate systems, depending on the position of the methyl substituent on the pyridine ring.     

Two‐Dimensional Holo‐ and Hemidirected Lead(II) Coordination Polymers: Synthetic,Spectroscopic, Thermal,and Structural Studies of [Pb(μ‐SCN)2(μ‐ebp)1.5]n and {[Pb(μ‐OAc)(μ‐ebp)](ClO4)}n (ebp=4,4′‐[(1E)‐Ethane‐1,2‐diyl]bis[pyridine]; OAc=Acetato)

Two new coordination polymers of Pb^II complexes with bridging 4,4′‐[(1E)‐ethane‐1,2‐diyl]bis[pyridine] (ebp), thiocyanato, and acetato ligands, [Pb(μ‐SCN)₂(μ‐ebp)_1.5]_n ( 1 ) and {[Pb(μ‐OAc)(μ‐ebp)](ClO₄)}_n ( 2 ), were synthesized and characterized by elemental analysis, FT‐IR, ¹H‐ and ¹³C‐NMR, thermal analysis, and single‐crystal X‐ray diffraction. In 1 , the Pb²⁺ ions are doubly bridged by both the ebp and the SCN⁻ ligands into a two‐dimensional polymeric network. The seven‐coordinate geometry around the Pb²⁺ ion in 1 is a distorted monocapped trigonal prism, in which the Pb²⁺ ions have a less‐common holodirected geometry. In 2 , the Pb²⁺ ions are bridged by AcO⁻ ligands forming linear chains, which are also further bridged by the neutral ebp ligands into a two‐dimensional polymeric framework. The Pb²⁺ ions have a five‐coordinate geometry with two N‐atoms from two ebp ligands and three O‐atoms of AcO⁻. Although ClO acts as a counter‐ion, it also makes weak interactions with the Pb²⁺ center. The arrangement of the ligands in 2 exhibits hemidirected geometry, and the coordination gap around the Pb²⁺ ion is possibly occupied by a configurationally active lone pair of electrons.     

Synthesis, characterization, and tautomerism of four novel copper(I) complexes from 3-hydroxy-3-(p-R-phenyl)-2-propenedithioic acids

The paper describes the synthesis and structural characterization of four novel copper(I) complexes [CuL(PPh(3))(2)] (L = 3-hydroxy-3-(p-R-phenyl)-2-propenedithioate). In addition, a tautomeric equilibrium in solution was found and Hammett correlations with (13)C NMR parameters were studied. The structure of one complex was fully established by X-ray diffraction analysis.     

New ent‐Abietanoids from Isodon rubescens

Six new ent‐abietane diterpenoids, rubescensins I–M ( 1 – 5 ) and P ( 6 ), along with two related acetonide derivatives ( 7 and 8 ), were isolated from Isodon rubescens. Their structures were elucidated by detailed spectroscopic analysis. Compound 5 is the first N‐containing diterpenoid from the genus Isodon, exhibiting notable cytotoxicity against human tumor K562 cells.     

Utility of N‐[4‐(N‐Substituted Sulfamoyl)Phenyl] Cyanothioformamides in the Synthesis of Heterocyclic Compounds

The novel cyanothioformamides 2a‐d were prepared by treatment of isothiocyanatosulfonamides 1a‐d with potassium cyanide at room temperature. Cyclocondensation of compounds 2b,c with phenyl isocyanate as electrophile furnished the corresponding imidazolidines 3a,b . The reactivity of compound 3a towards some nitrogen nucleophiles was investigated. Thus, the thiosemicarbazone 4 and imidazo[4,5‐b]quinoxaline 6 were synthesized by condensation of compound 3a with thiosemicarbazide and o‐phenylenediamine, respectively. Treatment of 3a with hydrochloric acid afforded compound 7 . Our investigation was extended to include the reactivity of cyanothioformamide 2 towards o‐aminophenol, anthranilic acid, and o‐phenylenediamine and yielded the corresponding heterocycles 9 , 11 and 13 derivatives, respectively. Structures of the synthesized compounds were established by their elemental analysis and spectral data.     

Selenium in electrolytic manganese as a reference material for the quality control of aluminium melts

Electrolytic manganese is an important alloying element for aluminium and steel melts. It is mainly added to melts of aluminium in the holding furnace as tablets or minitablets (compressed compacts of manganese and aluminium powders). Selenium derivates are usually added during the production of electrolytic manganese, so some selenium is present in the alloys produced when electrolytic manganese is added to the aluminium furnace. Since the selenium contents of many alloys are of concern from health and environmental perspectives, their values should be provided. In this work, a laboratory reference material (LRM) based on electrolytic manganese was produced to assure our routine quality control method, where selenium is analysed by hydride generation followed by optical emission spectrometry with inductively coupled plasma (HG–ICP–OES). Therefore, the present paper describes in detail the preparation procedure for and the results from homogeneity and stability studies performed on electrolytic manganese LRM. For this purpose, a commercial electrolytic manganese lot was selected and the main factors involved in the preparation of the material (pretreatment step, homogenization, bottling and storage) were carefully studied and established in order to guarantee the long-term stability of the LRM. The results obtained showed that the LRM developed was a fit-for-purpose material for the quality control of the routine analysis of selenium.     

Molecular dynamics simulations of the mononuclear zinc-beta-lactamase from Bacillus cereus complexed with benzylpenicillin and a quantum chemical study of the reaction mechanism

Herein, we present results from MD simulations of the Michaelis complex formed between the B. cereus zinc-beta-lactamase enzyme and benzylpenicillin. The structural and dynamical effects induced by substrate-binding, the specific role of the conserved residues, and the near attack conformers of the Michaelis complex are discussed. Quantum chemical methods (HF/6-31G* and B3LYP/6-31G*) are also applied to study the hydrolysis reaction of N-methylazetidinone catalyzed by a monozinc system consisting of the side chains of the histidine residues (His86, His88, and His149) complexed with Zn-OH and the side chains of Asp90 and His210. From this model system, we built molecular-mechanics representations of the prereactive complex and transition state configurations docked into the active site. Linear-scaling semiempirical calculations coupled with a continuum solvent model were then performed on these static models. We propose that the experimental rate data for the B. cereus enzyme is compatible with a one-step mechanism for the hydrolysis of beta-lactam substrates in which His210 acts as a proton donor.     

Thermal properties of cyanatocopper(II) complexes with pyridine,bipyridine and phenanthroline

The thermal properties of cyanatocopper complexes with pyridine, bipyridine and phenanthroline are described in this paper. It was found that the thermal stabilities of the complexes were found to increase in the order pybipy2(NCS)₂ (L=pyridine and its methyl derivates) which are composed of the liberation of ligandsL and redox reactions of thiocyanate ligands with the central Cu(II) atom [1, 2]. The decomposition temperature of thiocyanate ligands depends on the properties of the ligandsL present in the coordination sphere. An analogous course of thermal decomposition was also observed for [Cu(bipy)₂(NCX)](NCX) (X=S or Se) [3]. For the phenanthroline complexes [Cu(phen)₂(NCX)₂] (X=S or Se) the thermal stability was found to increase significantly (by about 140C) and the decomposition stoichiometry was also changed [3].The present paper contains the results of thermoanalytical studies on bipyridine and phenanthroline cyanatocopper complexes, and a comparison of the thermal properties of pyridine cyanato and isothiocyanatocopper complexes.     

Water absorption of a diglycidyl ether of bisphenol A/1,3-bisaminomethylcyclohexane (DGEBA/1,3-BAC) epoxy resin system

The diffusive and dynamic mechanical behavior of the DGEBA/1,3-BAC epoxy resin system was studied during water absorption. The diffusion of water was investigated at 100% relative humidity, by immersion of specimens in water at 60, 80 and 100°C. In all absorption experiments, water diffusion followed Fick's law. Diffusion coefficients and saturated water concentrations are given for these temperatures. The activation energy for diffusion was determined from the relationship between the diffusion coefficient and the reciprocal of the absolute temperature. The value obtained was 31.2 kJ mol^–1. Dynamic mechanical analysis of samples immersed in 100°C water and with various water contents showed both a shift of T_g, defined by thetan peak, to lower temperatures and a slight decrease in the dynamic modulus in the presence of water. These effects are probably a result of plasticization.This work was supported by the Xunta de Galicia through grant XUGA-17201A92.