Fast-response system for signal acquisition and processing in electrothermal atomic absorption spectrometry

An electrothermal atomic absorption (a.a.) spectrometer is modified to allow accurate correlation of absorbance and pyrometrically measured temperature with time. To by-pass the slow electronics of the spectrometer, the signal is fed from the preamplifier immediately after the photomultipier tube to a 12-bit A/D converter mounted in a 320K IBM-PC microcomputer. The wall temperature of the graphite furnace, measured with an automatic optical pyrometer, is recorded simultaneously with absorbance by feeding the pyrometer output to a second channel of the A/D board. Fast deuterium-arac background correction is also done. Background-corrected absorbance, background absorbance and temperature are recorded at 60 Hz. A simple algorithm allowed experimental activation energies to be calculated automatically. All programming was in BASIC or PASCAL.     

Fast folding of a four-helical bundle protein

The FK506-FKBP12 binding-domain of the kinase FRAP (FRB) forms a classic up-down four-helical bundle. The folding pathway of this protein has been investigated using a combination of equilibrium and kinetic studies. The native state of the protein is stable with respect to the unfolded state by some 7 kcal mol(-1) at pH 6.0, 10 degrees C. A kinetic analysis of unfolding and refolding rate constants as a function of chemical denaturant concentration suggests that an intermediate state may be populated during folding at low concentrations of denaturant. The presence of this intermediate state is confirmed by refolding experiments performed in the presence of the hydrophobic dye 8-anilinonaphthalene-1 sulfonate (ANS). ANS binds to the partially folded intermediate state populated during the folding of FRB and undergoes a large change in fluorescence that can be detected using stopped-flow techniques. Analysis of the kinetic data suggests that the intermediate state is compact and it may even be a misfolded species that has to partially unfold before it can reach the transition state. Folding and unfolding rate constants in water are approximately 150-200 s(-1) and 0.005-0.06 s(-1), respectively, at neutral pH and 10 degrees C. The folding of FRB is somewhat slower than for other all-helical proteins, probably as a consequence of the formation of a metastable intermediate state. The folding rate constant in the absence of any populated intermediate can be estimated to be 8800 s(-1). Despite the presence of an intermediate state, which effectively slows folding, the protein still folds rapidly with a half-life of 5 ms at 10 degrees C. The dependence of the rate constants on denaturant concentration indicates that the transition state for folding is compact with some 80% of the surface area exposed in the unfolded state buried in the transition state. Data presented for FRB is compared with kinetic data obtained for other all-helical proteins.     

Molecular and electronic structure of four- and five-coordinate cobalt complexes containing two o-phenylenediamine- or two o-aminophenol-type ligands at various oxidation levels: an experimental, density functional, and correlated ab initio study

The bidentate ligands N-phenyl-o-phenylenediamine, H(2)((2)L(N)IP), or its analogue 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, ((4)L(N)IP), react with [Co(II)(CH(3)CO(2))(2)]4H(2)O and triethylamine in acetonitrile in the presence of air yielding the square-planar, four-coordinate species [Co((2)L(N))(2)] (1) and [Co((4)L(O))(2)] (4) with an S=1/2 ground state. The corresponding nickel complexes [Ni((4)L(O))(2)] (8) and its cobaltocene reduced form [Co(III)(Cp)(2)][Ni((4)L(O))(2)] (9) have also been synthesized. The five-coordinate species [Co((2)L(N))(2)(tBu-py)] (2) (S=1/2) and its one-electron oxidized forms [Co((2)L(N))(2)(tBu-py)](O(2)CCH(3)) (2 a) or [Co((2)L(N))(2)I] (3) with diamagnetic ground states (S=0) have been prepared, as has the species [Co((4)L(O))(2)(CH(2)CN)] (7). The one-electron reduced form of 4, namely [Co(Cp)(2)][Co((4)L(O))(2)] (5) has been generated through the reduction of 4 with [Co(Cp)(2)]. Complexes 1, 2, 2 a, 3, 4, 5, 7, 8, and 9 have been characterized by X-ray crystallography (100 K). The ligands are non-innocent and may exist as catecholate-like dianions ((2)L(N)IP)(2-), ((4)L(N)IP)(2-) or pi-radical semiquinonate monoanions ((2)L(N)ISQ)(*) (-), ((4)L(N)ISQ)(*) (-) or as neutral benzoquinones ((2) L(N)IBQ)(0), ((4) L(N)IBQ)(0); the spectroscopic oxidation states of the central metal ions vary accordingly. Electronic absorption, magnetic circular dichroism, and EPR spectroscopy, as well as variable temperature magnetic susceptibility measurements have been used to experimentally determine the electronic structures of these complexes. Density functional theoretical (DFT) and correlated ab initio calculation have been performed on the neutral and monoanionic species [Co((1)L(N))(2)](0,-) in order to understand the structural and spectroscopic properties of complexes. It is shown that the corresponding nickel complexes 8 and 9 contain a low-spin nickel(II) ion regardless of the oxidation level of the ligand, whereas for the corresponding cobalt complexes the situation is more complicated. Spectroscopic oxidation states describing a d(6) (Co(III)) or d(7) (Co(II)) electron configuration cannot be unambiguously assigned.     

Determination of Cadmium in Plant Tissues by Electrothermal Atomisation Atomic Absorption Spectrometry with Matrix/Analyte Modification and Smith-Hieftje Background Correction

Abstract

Pressure decomposition in a microwave oven provides a rapid means of sample preparation for plant tissue analyses. The use of delayed atomisation cuvettes, Smith-Hieftje background correction and matrix/analyte modification enables accurate determinations of cadmium concentrations in plant materials to be made. However, care should be taken to restrict the concentrations of modifier used, as too high a concentration may lead to problems with both tube life and over-correction by the Smith-Hieftje background correction system.     

Material Design in Poly(Lactic Acid) Systems: Block Copolymers,Star Homo- and Copolymers,and Stereocomplexes

Abstract

This paper gives a general overview of several approaches we have investigated for designing new PLA-based polymers with a broad range of properties and improved processability. These approaches include: copolymerization (block and stereoblock copolymers), microstructure and architecture control, and stereocomplexation. Multiblock copolymers with alternating “soft” and “hard” segments, synthesized over a broad range of chemical compositions, show properties ranging from hard plastics to elastomers. Stereoblock copolymers with alternating amorphous and semicrystalline PLA blocks combine the advantages of PLA homopolymers (crystallinity) and random copolymers (processability). Independent control of polymer architecture and microstructure allows for the synthesis of star polymers with various arm morphologies. A new method for stereocomplex formation between L-PLA and D-PLA, which combines in-situ polymerization with stereocomplexation, is also described. For the synthesis of these new materials we took advantage of: 1) chirality of lactide monomer, 2) retention of configuration during polymerization, 3) living nature of the ring-opening polymerization (ROP) of lactide in the presence of active hydrogen groups such as OH and NH₂, and 4) control of the level of transesterification reactions.     

Improved Flexural Strength of N-Vinyl-Pyrrolidone Modified Acrylic Acid Copolymers for Glass-Ionomers

Abstract

The flexural strengths of N-vinylpyrrolidone modified glass-ionomer cements were investigated. The optimal molar ratio of the monomers in copolymers, composed of the three components acrylic acid, itaconic acid and N-vinylpyrrolidone, was determined using a SAS statistical program. The copolymers were prepared using a free-radical polymerization process. The viscosities of aqueous solutions of these polymers were determined. Cements were formed by the reaction of these solutions with glass particles. Flexural strength (FS) was used as the basic screening property to find the optimum molar ratio. Statistical models were applied to predict the optimum molar ratios. All strength values were recorded on the specimens conditioned in distilled water at 37°C for 7 days. The optimal molar ratio for these copolymers was 7:1:3 for poly(acrylic acid-co-itaconic acid-co-N-vinylpyrrolidone), based on flexural strength and viscosity. The effect of molecular weight (MW) on FS was also evaluated. Copolymer with a MW of 10, 800 (M_n) showed 85% higher FS than the Ketac-Molar (KM) system, along with a reasonable working viscosity.     

Intramolecular bonding in a statistical associating fluid theory of ring aggregates

The first-order thermodynamic perturbation theory of Wertheim (TPT1) is extended to treat ring aggregates, formed by inter- and intramolecular association. The expression for the residual association contribution to the Helmholtz free energy for ring aggregates, incorporating the appropriate terms in Wertheim's fundamental graph sum of the TPT1 density expansion, is derived to calculate the distribution of the molecular bonding states. This requires the introduction of two new parameters to characterise each possible ring type: the ring size τ, which is equal to one in the case of intramolecular association, and a parameter W that captures the likelihood of two ring-forming sites bonding. The resulting framework can be incorporated in equations of state that account for the residual association contribution to the free energy, such as the statistical associating fluid theory (SAFT) family, or the cubic plus association (CPA) equation of state. This extends the applicability of these equations of state to mixtures with an arbitrary number of association sites capable of hydrogen bonding to form intramolecular and intermolecular rings. The formalism is implemented within SAFT-VR Mie to calculate the fluid-phase equilibria of model chain-like molecules containing two associating sites A and B, allowing for the formation of open-chain aggregates and intramolecular bonds. The effect of adding a second component that competes for the association sites that mediate intramolecular association in the chain is also examined. Accounting for intramolecular bonding is shown to have a significant impact on the phase equilibria of such systems.