Evaluation of the stability of ethanol in water certified reference material: measurement uncertainty under transport and storage conditions

This study simulated the transport and storage conditions of ethanol in water certified reference material (CRM) produced by the Chemical Metrology Division of Inmetro—DQUIM with the purpose of estimating the measurement uncertainty related to stability. The short-term stability study was performed on five different mass fractions (w) in terms of mg ethanol/g solution of the ethanol in water CRM. The nominal values are w = 0.5, 0.9, 1.1, 3.8 and 4.6 mg/g, at temperatures of 4 and 60 °C. On the other hand, the long-term stability study was developed on four different mass fractions (nominal values): w = 0.5, 0.9, 1.1 and 4.6 mg/g, at a temperature of 20 °C. This paper will show the data from the long-term stability study that took place over 52 weeks. The method used complies with ISO Guide 35, the BCR Guideline for Feasibility Studies and ISO Guide 34. According to the statistical parameters used in both studies, the stability of ethanol in water CRM was confirmed for all of the mass fractions studied.     

Alanine methyl groups as NMR probes of molecular structure and dynamics in high-molecular-weight proteins

The importance and utility of Ala(β) methyl groups as NMR probes of molecular structure and dynamics in high-molecular-weight proteins is explored. Using (2)H and (13)C relaxation measurements in {U-(2)H; Ala(β)-[(13)CHD(2)]}-labeled Malate Synthase G (MSG)--an 82-kDa monomeric enzyme that contains 73 Ala(β) methyl groups--we show that the vast majority of selectively labeled Ala(β) methyls are highly ordered. A number of NMR applications used for solution studies of structure and dynamics of large protein molecules can benefit from proximity of Ala(β) methyls to the protein backbone and their high degree of ordering. In the case of MSG, these applications include the measurement of (1)H-(13)C residual dipolar couplings in Ala(β) methyls, characterization of slow (μs-to-ms) dynamics at the substrates' binding sites, and methyl-TROSY-based NOE spectroscopy performed on {U-(2)H; Ala(β)-[(13)CH(3)]; Ile(δ1)-[(13)CH(3)]; Leu,Val-[(13)CH(3)/(12)CD(3)]}-labeled samples where the number of methyl probes for derivation of distance restraints is maximized compared to the state-of-the-art ILV labeling methodology.     

Clearance of host cell impurities from plasmid-containing lysates by boronate adsorption

The ability of boronate adsorption to clear Escherichia coli impurities directly from plasmid-containing lysates (∼pH 5.2) was evaluated. Results show that 3-aminophenyl boronate (PB) controlled pore glass (CPG) is able to adsorb not only those species that bear cis-diol groups (RNA, lipopolysaccharides-LPS), and are thus able to form covalent bonds with boronate, but also cis-diol-free proteins and genomic DNA (gDNA) fragments, while leaving most plasmid DNA in solution. Control runs performed with phenyl Sepharose and with PB-free CPG beads ruled out hydrophobic interactions with the phenyl ring and non-specific interactions with the glass matrix, respectively, as being responsible for RNA and gDNA adsorption. In batch mode, up to 97.6 ± 3.1% of RNA, 94.6 ± 0.8% of proteins and 96.7 ± 11.7% of gDNA were cleared after 30 min, with a plasmid yield of 64%. In fixed-bed mode, most of the plasmid was recovered in the flowthrough (96.2 ± 4.0%), even though the RNA (65.5 ± 2.8%), protein (84.4 ± 1.3%) and gDNA clearance (44.7 ± 14.1%) were not as effective. In both cases, the LPS content was removed to a residual value of less than 0.005 EU/ml. The method is fast and straightforward, circumvents the need for pre-treatment of the feed and may contribute to shorten plasmid purification processes, as the treated streams can proceed directly to the final polishing steps.     

An improved procedure for the diastereoselective addition of triorganozincates to N-(tert-butanesulfinyl)imines: use of catalytic dialkylzinc

The addition of triorganozincates to (R)-N-(tert-butanesulfinyl)benzaldimine has been performed with very good results by using a catalytic amount of Me₂Zn (0.15 equiv) to generate the organozincate. Yields and/or diastereoselectivities of the formed α-branched sulfinamides improve in comparison with the values obtained in the same reactions carried out with an excess of a previously formed triorganozincate. On the other hand, the transfer of the two alkyl groups of a dialkylzinc reagent to the imine has been achieved by using 0.5 equiv of dialkylzinc and 1.5 equiv of MeMgBr to generate the trialkylzincate. In both methods, good to excellent yields and diastereomeric ratios (up to 98:2) have been obtained.     

A Bistable Molecular Switch Driven by Photoinduced Hydrogen‐Atom Transfer

The occurrence of photoinduced hydrogen atom transfer between two remote spots of a molecule is experimentally demonstrated. This photoprocess involves the intermediacy of an intramolecular “crane”. In an experimental case study, 7‐hydroxy‐4‐methylquinoline‐8‐carbaldehyde monomers isolated in low‐temperature Ar matrices are investigated. On UV (λ>295 nm) irradiation, a hydrogen atom is transferred from the O⁷H group to the N¹ atom of the quinoline ring. Subsequent irradiation with UV (λ>360 nm) light reveals that the phototransformation is partially photoreversible. In the studied hydrogen‐atom‐transfer process, the exocyclic carbaldehyde group plays the role of an intramolecular crane. The possible application of systems analogous to 7‐hydroxy‐4‐methylquinoline‐8‐carbaldehyde as optically driven molecular switches is discussed.     

Solution NMR evidence for symmetry in functionally or crystallographically asymmetric homodimers

A recurrent theme of many structural studies of homo-oligomeric protein systems is concerned with verification that the conformation observed in a crystal represents the functionally relevant structure. An asymmetric conformation adopted by two chemically identical subunits in homo-oligomers can represent an intrinsic property of a protein or be an artifact induced by crystal packing forces. Solution NMR studies can distinguish between these two possibilities. Using methyl-based NMR spectroscopy, we provide evidence for symmetry in the absence of ligands in several homodimeric proteins that are either asymmetric functionally and/or adopt different conformations of the two subunits in available X-ray structures.     

Influence of Excitation Wavelength (UV or Visible Light) on the Photocatalytic Activity of Titania Containing Gold Nanoparticles for the Generation of Hydrogen or Oxygen from Water

Gold nanoparticles supported on P25 titania (Au/TiO(2)) exhibit photocatalytic activity for UV and visible light (532 nm laser or polychromatic light λ > 400 nm) water splitting. The efficiency and operating mechanism are different depending on whether excitation occurs on the titania semiconductor (gold acting as electron buffer and site for gas generation) or on the surface plasmon band of gold (photoinjection of electrons from gold onto the titania conduction band and less oxidizing electron hole potential of about -1.14 V). For the novel visible light photoactivity of Au/TiO(2), it has been determined that gold loading, particle size and calcination temperature play a role in the photocatalytic activity, the most active material (Φ(H2) = 7.5% and Φ(O2) = 5.0% at 560 nm) being the catalyst containing 0.2 wt % gold with 1.87 nm average particle size and calcined at 200 °C.     

Development of a flow method for the determination of phosphate in estuarine and freshwaters—Comparison of flow cells in spectrophotometric sequential injection analysis

A sequential injection system with dual analytical line was developed and applied in the comparison of two different detection systems viz; a conventional spectrophotometer with a commercial flow cell, and a multi-reflective flow cell coupled with a photometric detector under the same experimental conditions. The study was based on the spectrophotometric determination of phosphate using the molybdenum-blue chemistry. The two alternative flow cells were compared in terms of their response to variation of sample salinity, susceptibility to interferences and to refractive index changes. The developed method was applied to the determination of phosphate in natural waters (estuarine, river, well and ground waters). The achieved detection limit (0.007 μM PO₄³⁻) is consistent with the requirement of the target water samples, and a wide quantification range (0.024–9.5 μM) was achieved using both detection systems.     

Chemical evolution from simple inorganic compounds to chiral peptides

Numerous experiments performed in the past 50 years have strongly changed ideas of how life could have emerged on the primitive Earth. This review deals with the synthesis of biomolecule precursors under the conditions prevailing on the primordial Earth, and describes possible scenarios for their combination and elongation to form peptides and proteins. Furthermore it proposes different answers to one of the big secrets of nature: why DNA-coded biohomochiral life emerged using amino acids in their l-form?