Speeding-up of the determination of amino acids by means of high pressure liquid chromatography with reaction photometric detection using ninhydrin reagent

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Speeding-up of the determination of amino acids by means of high pressure liquid chromatography with reaction photometric detection using ninhydrin reagent

     

Interfacial effects in electron transmission through Ag films on Cu(111)

The transmission coefficient for very low energy electrons ($\text{? 10}\text{eV}$) normally incident on thin epitaxial films of Ag on Cu(111) is modulated by reflection of the electronic wavefunction at the interfaces. The observation of such quantum size oscillations in this epitaxial system demonstrates that this effect is truly interfacial in origin.     

Thin film quantum size effects: I. The effect of defect structure at the vacuum/film interface

The quantum size oscillations observed in the transmitted current for very low energy electrons normally incident on thin epitaxial (111) films of Cu and Ag on W(110) are sensitive to the structure of both the vacuum/film and the film/substrate interface. We report here the contribution of the vacuum/film interface as a function of oxygen adsorption, ion bombardment damage and annealing temperature. We find that the quantum size oscillations are always reduced in amplitude with increasing disorder on an atomic scale at the film surface. Random oxygen adsorption generally suppresses the quantum size effect (QSE) structure. The QSE amplitude is also significantly reduced by defect structure in the form of a random, irregular array of monatomic steps at the film surface. Using the LEED (00) beam width as an index of surface roughness, we find that the amplitude of the quantum size oscillations decreases linearly with surface step atom density. The QSE amplitude is reduced by a factor of two for step atom densities as low as 6.5%, and nearly extinguished for a step atom density of 12%. The QSE structure provides a relative indication of surface roughness at least as sensitive as LEED beam broadening and work function change measurements. We conclude that a relatively well-ordered and uniform film surface is a necessary (but not sufficient) condition for the observation of quantum size structure.     

Experimental study of neutral-current and charged-current neutrino cross sections

Samples of 9200 muon-neutrino and 3800 muon-antineutrino interactions on nuclei were obtained with the fine-grain calorimeter of the CHARM Collaboration at the CERN 200 GeV narrow-band neutrino beam. The interactions were classified as either neutral-current or charged-current processes on an event-by-event basis. Neutral-current and charged-current cross sections in neutrino and antineutrino interactions are presented. From these results we deduce a statistically significant contribution of right-handed coupling to the neutral hadronic current, and a value of the electroweak mixing angle corresponding to sin²θ = 0.220 ± 0.014.     

Experimental study of differential cross sections dσdy in neutral current neutrino and antineutrino interactions

We present differential cross sections dσ/dy corrected for resolution and acceptance, for events induced by both the neutral- and charged-current interactions of neutrinos and antineutrinos. They are based on 8553 neutrino and 3578 antineutrino events obtained using the CHARM fine-grain calorimeter in the CERN 200 GeV narrow-band beam. From these differential cross sections we demonstrate that the coupling strength of the weak neutral current to the strange quark is compatible with being equal to that of the down quark. Assuming this equality we then describe the weak neutral current in terms of one parameter, sin²θ, which we find to be 0.222 ± 0.016. The charged-current differential cross sections yield values of the fractional momentum-weighted content of the nucleon for non-strange (0.12 ± 0.04) and strange (0.06 ± 0.04) sea quarks. Furthermore, from the strength of the allowed y² term in the neutral-current differential cross sections we put a limit of 3% on the presence of scalar or pseudoscalar contributions to the weak neutral current. This can alternatively be expressed in terms of the Callan-Gross violation parameter R, where we find R = 0 .10 ± 0.10.     

NMR Spectroscopic Characterization of the C-Mannose Conformation in a Thrombospondin Repeat Using a Selective Labeling Approach

Despite the great interest in glycoproteins, structural information reporting on conformation and dynamics of the sugar moieties are limited. We present a new biochemical method to express proteins with glycans that are selectively labeled with NMR-active nuclei. We report on the incorporation of ¹³C-labeled mannose in the C-mannosylated UNC-5 thrombospondin repeat. The conformational landscape of the C-mannose sugar puckers attached to tryptophan residues of UNC-5 is characterized by interconversion between the canonical ¹C₄ state and the B₀₃ / ¹S₃ state. This flexibility may be essential for protein folding and stabilization. We foresee that this versatile tool to produce proteins with selectively labeled C-mannose can be applied and adjusted to other systems and modifications and potentially paves a way to advance glycoprotein research by unravelling the dynamical and conformational properties of glycan structures and their interactions.     

Engineering encodable lanthanide-binding tags into loop regions of proteins

Lanthanide-binding tags (LBTs) are valuable tools for investigation of protein structure, function, and dynamics by NMR spectroscopy, X-ray crystallography, and luminescence studies. We have inserted LBTs into three different loop positions (denoted L, R, and S) of the model protein interleukin-1β (IL1β) and varied the length of the spacer between the LBT and the protein (denoted 1?3). Luminescence studies demonstrate that all nine constructs bind Tb3+ tightly in the low nanomolar range. No significant change in the fusion protein occurs from insertion of the LBT, as shown by two X-ray crystallographic structures of the IL1β-S1 and IL1β-L3 constructs and for the remaining constructs by comparing the 1H?15N heteronuclear single-quantum coherence NMR spectra with that of the wild-type IL1β. Additionally, binding of LBT-loop IL1β proteins to their native binding partner in vitro remains unaltered. X-ray crystallographic phasing was successful using only the signal from the bound lanthanide. Large residual dipolar couplings (RDCs) could be determined by NMR spectroscopy for all LBT-loop constructs and revealed that the LBT-2 series were rigidly incorporated into the interleukin-1β structure. The paramagnetic NMR spectra of loop-LBT mutant IL1β-R2 were assigned and the Δχ tensor components were calculated on the basis of RDCs and pseudocontact shifts. A structural model of the IL1β-R2 construct was calculated using the paramagnetic restraints. The current data provide support that encodable LBTs serve as versatile biophysical tags when inserted into loop regions of proteins of known structure or predicted via homology modeling.     

Influence of the Absolute Configuration of NPE‐Caged Cytosine on DNA Single Base Pair Stability

Photolabile protecting groups are a versatile tool to trigger reactions by light irradiation. In this study, we have investigated the influence of the absolute configuration of the 1‐(2‐nitrophenyl)ethyl (NPE) cage group on a 15‐base‐pair duplex DNA. Using UV melting, we determined the global stability of the unmodified and the selectively (S)‐ and (R)‐NPE‐modified DNA sequences, respectively. We observe a differently destabilizing effect for the two NPE stereoisomers on the global stability. Analysis of the temperature dependence of imino proton exchange rates measured by NMR spectroscopy reveals that this effect can be attributed to decreased base pair stabilities of the caged and the 3′‐neighbouring base pair, respectively. Furthermore, our NMR based structural models of the modified duplexes provide a structural basis for the distinct effect of the (S)‐ and the (R)‐NPE group.