Dynamics of xenon binding inside the hydrophobic cavity of pseudo-wild-type bacteriophage T4 lysozyme explored through xenon-based NMR spectroscopy

Wild-type bacteriophage T4 lysozyme contains a hydrophobic cavity with binding properties that have been extensively studied by X-ray crystallography and NMR. In the present study, the monitoring of 1H chemical shift variations under xenon pressure enables the determination of the noble gas binding constant (K = 60.2 M(-1)). Although the interaction site is highly localized, dipolar cross-relaxation effects between laser-polarized xenon and nearby protons (SPINOE) are rather poor. This is explained by the high value of the xenon-proton dipolar correlation time (0.8 ns), much longer than the previously reported values for xenon in medium-size proteins. This indicates that xenon is highly localized within the protein cavity, as confirmed by the large chemical shift difference between free and bound xenon. The exploitation of the xenon line width variation vs xenon pressure and protein concentration allows the extraction of the exchange correlation time between free and bound xenon. Comparison to the exchange experienced by protein protons indicates that the exchange between the open and closed conformations of T4 lysozyme is not required for xenon binding.     

The effect of the crystal field strength on the optical spectra of Cr3+ in gallium garnet laser crystals

We have investigated the basic spectroscopic properties of Cr³⁺-doped gallium garnets. Weak crystal fields (Dq/B=2.39–2.55) lead to broad⁴ T ₂⁴ A ₂ fluorescence in the 700–950 nm range depending on the chemical composition of the crystals. At room temperature the⁴ T ₂⁴ A ₂ transition is homogeneously broadened, whereas the 4 K spectra of the² E⁴ A ₂ transition show inequivalent Cr³⁺ sites and inhomogeneous linewidths of 8 cm^–1 in these laser crystals. From 300 to 4 K, the oscillator strength of the⁴ T ₂⁴ A ₂ transition is reduced by a factor of about 0.6 due to the temperature dependent electron phonon coupling. The transition probability of the² E⁴ A ₂ transition correlates with the energetic position of the⁴ T ₂ level, which is admixed into the² E level via spin-orbit coupling. The comparison of the² E⁴ A ₂ phonon sideband structure in garnets of different chemical compositions allows the assignment of some peaks to GaO₄-tetrahedron modes of the garnet structure.     

Validating automated analytical instruments to meet regulatory and quality standard requirements

Analytical laboratories are more and more faced to meet official regulatory requirements as described in FDA and EPA good laboratory practice, good automated laboratory practice and good manufacturing practice regulations or to officially establish quality systems, such as specified in the ISO 9000 Series quality standards, in the ISO Guide 25 or in the EN 45001 guidelines. The impact on analytical instrumentation will be the requirement for stringent validation of analytical equipment and methods which increase the overall analysis costs. An overview is presented on the validation requirements using e.g. gas chromatography, high performance liquid chromatography, capillary electrophoresis and UV-visible spectroscopy and on the strategy to meet such needs at minimal extra costs with the help of an instrument vendor. It is recommended to use instrument hardware that has already built-in tools for self-verification and which is to be validated at the vendor's site. Performance testing in the user's laboratory is done using standard operating procedures as supplied with the instrument. If resources in the user's laboratory are limited, the performance verification is done by the vendor. Software and the entire computer system is validated prior to shipment at the vendor's site. Acceptance testing is done in the user's environment following the vendor recommendations. Analytical methods are validated automatically at the end of method development using a dedicated software. The software can be customized such that it can also be used for daily automated system suitability testing. Security and integrity of analytical data are ensured by saving the raw data together with instrument conditions and instrument log-books in check-sum protected binary register files for long-term archiving.     

Amperometric detection of amines and amino acids in flow injection systems with a nickel oxide electrode

The use and characteristics of a nickel oxide electrode as a detector for amines in a flow injection system are described. The anodic electrode reaction mechanism involves a higher oxidation state of nickel maintained by the applied potential (+0.49 V vs. SCE). The electroanalytical parameters are investigated and the currents for a series of amines and amino acids are compared. Two electrode configurations are compared. The flow injection technique is shown to be suitable for buffered 25-μl samples of pH as low as 3. The linear range for glycine is 10^-6–10^-3 M with detection limits of a few nanograms.     

Optical absorption of the β‐phase of poly(9,9‐dioctylfluorene)

We outline a theory for the optical absorption of the β‐phase of poly(9,9‐dioctylfluorene) (PFO) that is based on the Frenkel exciton model. The absorption peak at 435 nm is attributed to polymer segments having torsion angles equal to π that are weakly perturbed by the presence of random monomer junctions with torsion angles equal to 0. The broad band below 435 nm is associated with disordered segments having a broad distribution of random torsion angles. The effects of small random deviations from π in the torsion angles are discussed. The calculations support the interpretation that the β‐phase is characterized by alternating segments of highly ordered and strongly disordered regions. PFO is a widely studied fluorene‐based polymer with interesting and potentially useful photophysical properties. In this work, Frenkel exciton states in the β‐phase of PFO are studied, and a two‐region model—weak torsional disorder and strong torsional disorder—is presented. The peak in the optical absorption at 435 nm is associated with the weakly disordered regions. The broad background in the absorption is attributed to the strongly disordered regions. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1109–1111     

Access to Enantiomerically Pure P-Stereogenic Primary Aminophosphine Sulfides under Reductive Conditions

Stereochemically pure phosphines with phosphorus-heteroatom bonds and P-centered chirality are a promising class of functional building blocks for the design of chiral ligands and organocatalysts. A route to enantiomerically pure primary aminophosphine sulfides was opened through stereospecific reductive C−N bond cleavage of phosphorus(V) precursors by lithium in liquid ammonia. The chemoselectivity of the reaction as a function of reaction time, substrate pattern, and chiral auxiliary was investigated. In the presence of exclusively aliphatic groups bound to the phosphorus atom, all competing reductive side reactions are totally prevented. The absolute configurations of all P-stereogenic compounds were determined by single-crystal X-ray diffraction analysis. Their use as synthetic building blocks was demonstrated. The lithium salt of (R)-BINOL-dithiophosphoric acid proved to be a useful stereochemical probe to determine the enantiomeric purity. Insights into the coordination mode of the lithium-based chiral complex formed in solution was provided by NMR spectroscopy and DFT calculations.