Alpha-lactalbumin tertiary structure changes on hydrophobic interaction chromatography surfaces

Hydrogen exchange (HX) detected by mass spectrometry (MS) was used to analyze the structure of calcium-free alpha-lactalbumin, a model protein with marginal stability. Two chromatographic peaks were observed from samples of pure protein eluted from SOURCE phenyl hydrophobic interaction chromatography (HIC) media. Whole-protein HX measurements showed that the less-retained peak had solvent exposure similar to native protein in the absence of the HIC surface while the retained protein was nearly, although not fully, solvent exposed. The formation of these two peaks was kinetically limited. The protein also refolded successfully following elution. In addition, proteolytic fragmentation was used to analyze HX at the peptide level. This approach revealed that helix C was the most stable region of alpha-lactalbumin under native conditions and in the flow-through peak. Helix C also formed the core of residual native structure in the partially unfolded protein in the retained peak. The results suggest that residues that are most solvent accessible under native conditions may be those most likely to unfold upon adsorption.     

Die Reaktion von TiCl4 und ZrCl4 mit (SeCN)2

Reaction of TiCl₄ and ZrCl₄ with (SeCN)₂ By reaction of TiCl₄ and ZrCl₄ with (SeCN)₂ in CS₂ the compounds TiCl₃NCSe and ZrCl₃NCSe were obtained. According to their vibrational spectra, the selenocyanate groups form bridges between the metal atoms via their N and Se atoms.     

Determination of radium isotopes by BaSO4 coprecipitation for the preparation of alpha-spectrometric sources

A coprecipitation procedure for the preparation of -spectrometric sources for radium, using BaSO₄ as carrier, has been applied to the determination of alpha radium isotopes in water samples. The use of¹³³Ba as a suitable tracer for radium determination and possible losses of radon isotopes from the sources are studied and discussed.     

The N(4S) + N2O(X̃1∑) reaction

The title reaction, which is spin‐forbidden for N₂(X¹∑) + NO(X²Π) production, has been studied from 960 to 1130 K in a high‐temperature photochemistry reactor. No reaction could be observed, indicating k < 1 × 10^?15 cm³ molecule^?1 s^?1. It is concluded that there is no significant contribution from the spin‐allowed exothermic path leading to N₂(X¹∑) + NO(a⁴Π). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 387–389, 2001     

Discrimination against 13C during degradation of simple and complex substrates by two white rot fungi

Changes in isotopic 13C signatures of CO2-C evolved during decomposition of a sugar (glucose), a fatty acid (palmitic acid), a protein (albumin), a structural biopolymer (lignin) and bulk plant tissue (aerial shoots from Lolium perenne) were monitored over a period of 76 days. All materials were sterilized and inoculated with either of two different species of white rot fungi, Phanerochaete chrysosporium or Coriolus versicolor, and incubated in sealed bottles at 28 degrees C. The CO2 concentration in the jars was periodically determined using an infrared gas analyzer and its isotopic (13C) signature was assessed using a trace gas (ANCA TGII) module coupled to an isotope ratio mass spectrometer (IRMS, Europa 20-20). L. perenne material inoculated with C. versicolor showed the highest C mineralization activity with approximately 70% of total C evolved as CO2 after 76 days of incubation, followed by glucose. Substrates inoculated with C. versicolor generally decomposed faster than when degraded by P. chrysosporium, except for lignin, where no significant differences between the two fungi types were found and CO2-C released was less than 2% of the initial C. Considerable 13C isotopic fractionation during the degradation of plant tissue and of pure biochemical compounds was revealed as well as progressive shifts in cumulative CO2-13C isotopic signatures over time. During the first stages of decomposition, the CO2-C released was usually depleted in 13C as compared with the initial solid substrate, but with ongoing decomposition the CO2-C evolved became progressively more enriched in 13C. P. chrysosporium usually showed a slightly higher 13C fractionation than C. versicolor during the first decomposition phase. At posterior decomposition stages isotopic discrimination was often stronger by C. versicolor. These findings on isotopic 13C discrimination during microbial degradation both of simple biochemical compounds and of complex vegetal tissue confirmed not only the existence of significant 13C isotopic fractionation during plant residue decomposition, but also the existence of non-random isotopic distribution within substrates. They also demonstrated the ability of microorganisms to selectively discriminate against 13C even when degrading an isolated simple substrate.     

In situ FTIR spectroscopy study of the break-in phenomenon observed for PPy/PVS films in acetonitrile

The in situ Fourier transform infrared (in situ FTIR) technique was used for the first time to investigate the break-in phenomenon observed for polypyrrole/poly(vinyl sulfonate) (PPy/PVS) films in acetonitrile containing 0.1 M LiClO(4). Consecutive potential scans provided a continuous increase of the infrared band intensities, simultaneous to an increase observed in the charge involved in the voltammetric peaks, suggesting a rise in the number of the polymeric chains participating in the infrared signal at the same time as the electroactive participants increase in the redox process. Moreover, in situ FTIR spectra evidence that the new infrared-activated chains in each voltammetric cycle adopt the same polymeric structure achieved by the chains activated in the initial cycles. However, if we achieve a cathodic potential limit of -2.1 V (vs Ag/AgCl), a restructuring of the polymeric morphology is observed. In situ FTIR spectra obtained for PPy/ClO(4) films under the same conditions pointed to a steady-state behavior from the very early voltammetric scans. Moreover, the intensities of FTIR bands obtained for PPy/ClO(4) films in the early voltammetric cycles are much higher than those obtained for PPy/PVS films after several potential scans. Only when high cathodic and high anodic potential limits were used for the consecutive cycles did the FTIR band intensities from PPy/PVS become similar to those obtained from PPy/ClO(4), indicating that in both films a similar number of polymeric chains were infrared active. Polarization at a high anodic potential (+1.3 V vs Ag/AgCl) produced overoxidation of the polymer appearing characteristic 1725 cm(-1) band assigned to the formation of carbonyl groups. Furthermore, the approximately 1540 cm(-1) band shifted to higher wavenumbers, indicating that overoxidation reduced the length of conjugated chains in the polypyrrole.     

Directed graphs of structurally stable potential energy surfaces representing a-priori reaction pathways

An intrinsic property of potential energy surfaces (PES) that holds within the adiabatic approximation is established: its structural stability.We derive the condition that ensures this property: There cannot be any integral curve of the gradient field of the PES that connects two classical transition state configurations without passing through another critical configuration in between.Under this situation, we can establish a one-to-one correspondence: a whole class of adiabatic PES defining one reaction mechanism is associated to a directed graph. Thus, the problem of finding a-priori pathways involving a given number m of chemical species narrows down to a classifying certain directed graphs with m sinks. The combinatorial method is derived in this paper.Detailed examples on a-priori pathways for degenerate thermal rearrangements and on 1-2 hydrocarbon shifts are worked out and found in agreement with experimental evidence.     

Absence of Mn-centered oxidation in the S(2) --> S(3) transition: implications for the mechanism of photosynthetic water oxidation

A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S(3) --> [S(4)] --> S(0) transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kbeta X-ray emission spectroscopy (Kbeta XES) to this problem for the first time. The Kbeta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S(2) --> S(3) transition, in contrast to the S(0) --> S(1) and S(1) --> S(2) transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S(3)-state, manganese mu-oxo bridge radical formation is proposed for the S(2) --> S(3) transition, and three possible mechanisms for the O-O bond formation are presented.     

ZrO2-SiO2 materials prepared by sol-gel

Gels with composition xZrO₂−(100−x)SiO₂, X = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors.

Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment.

Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.