Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55 μm

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–1.55 μm. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by 60 nm Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BV_ceo, was about 6 V. Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at 1.3 μm. When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and 5 mA/W at 1.3 and 1.55 μm, respectively.     

Mapping of possible prion protein self-interaction domains using peptide arrays

Background  

The common event in transmissible spongiform encephalopathies (TSEs) or prion diseases is the conversion of host-encoded protease sensitive cellular prion protein (PrP^C) into strain dependent isoforms of scrapie associated protease resistant isoform (PrP^Sc) of prion protein (PrP). These processes are determined by similarities as well as strain dependent variations in the PrP structure. Selective self-interaction between PrP molecules is the most probable basis for initiation of these processes, potentially influenced by chaperone molecules, however the mechanisms behind these processes are far from understood. We previously determined that polymorphisms do not affect initial PrP^C to PrP^Sc binding but rather modulate a subsequent step in the conversion process. Determining possible sites of self-interaction could elucidate which amino acid(s) or amino acid sequences contribute to binding and further conversion into other isoforms. To this end, ovine – and bovine PrP peptide-arrays consisting of 15-mer overlapping peptides were probed with recombinant sheep PrP^C fused to maltose binding protein (MBP-PrP).     

Syntheses of anomerically phosphodiester-linked oligomers of the repeating units of the Haemophilus influenzae types c and f capsular polysaccharides

Spacer-equipped dimers and trimers of the repeating units of the capsular polysaccharide of Haemophilus influenzae type c, -4)-3-O-Ac-beta-D-GlcpNAc-(1-->3)-alpha-D-Galp-(1-OPO(3-)-, and type f, -3)-beta-D-GalpNAc-(1-->4)-3-O-Ac-alpha-D-GalpNAc-(1-OPO(3-)-, have been synthesized for use in immunological studies. H-Phosphonate chemistry was used for the formation of the interglycosidic phosphate diester linkages. Two types of building blocks, a spacer glycoside disaccharide starting monomer (15 and 22) and an anomeric monoester alpha-H-phosphonate disaccharide elongating monomer (12 and 27), were built up for each serotype structure from properly protected monosaccharide precursors using mainly thioglycosides as glycosyl donors. Stereospecificity in the formation of the alpha-linked monoester H-phosphonate was possible in type c through crystallization of the pure alpha-anomer of the precursor hemiacetal from an alpha/beta-mixture, whereas in type f, the hemiacetal was isolated directly as exclusively the alpha-anomer. Subsequent phosphonylation using triimidazolylphosphine was performed without anomerization. Formation of the anomeric phosphate diester linkages was performed using pivaloyl chloride as coupling reagent followed by I(2)/H(2)O oxidation of the formed diester H-phosphonates. Original experiments afforded no diester product at all, but optimization of the oxidation conditions (lowering the temperature and dilution with pyridine prior to I(2) addition) gave the dimers in good yields (71% and 81%) and, subsequently, after removal of a temporary silyl protecting group in the dimers, the trimers in fair yields (36% and 37%), accompanied by hydrolysis of the dimer phosphate linkage. One-step deprotection through catalytic hydrogenolysis efficiently afforded the target dimer (30 and 36) and trimer structures (32 and 39). The synthetic scheme allows for further elongation to give higher oligomers.     

Pimaricin—VIII : Structural and configurational studies by electron impact and field desorption mass spectrometry, 13C (25.2MHz) and 1H (270 MHz)-NMR spectroscopy

Attempts to obtain a molecular ion from the polyene macrolide antibiotic pimaricin by EI and FD mass spectrometry were unsuccessful The loss of carbon dioxide and a varying number of water molecules from the molecular ion made a molecular-weight determination impossible ¹³C NMR spectroscopy of N-acetylpimaricin, its dodecahydroderivative, and of HH-2, a hydrogenation-hydrogenolysis product of N-acetylpimaricin, confirmed that the antibiotic has structure 3 containing a hemi-ketalic ring and lacking an OH group at C-8. The value of the anomeric coupling constant, J_c-1-h-1, indicates that the mycosamine moiety is β-glycosidically bound to the aglycone.The structure and configuration of the antibiotic have been studied by analysis of the 270 MHz ¹H-NMR spectra of N acetylpimaricin and HH-2. By comparison of the two spectra and by extensive decoupling experiments, all signals in the spectrum of N-acetylpimaricin have been assigned to the protons in structure 3. From chemical-shift, coupling constant, and integral values, it was deduced that the mycosamine ring is pyranoid with a chair conformation, the hemi-ketal is 6 membered and occupies a chair conformation with the substituents in equatorial positions, the epoxy protons as well as the olefinic protons are trans to each other, and the antibiotic is diastereomerically pure.     

Use of molecular dynamics in the design and structure determination of a photoinducible beta-hairpin

The study presented here consists of three parts. In the first, the ability of a set of differently substituted diazobenzene-based linkers to act as photoswitchable beta-turn building blocks was assessed. A 12-residue peptide known to form beta-hairpins was taken as the basis for the modeling process. The central (beta-turn) residue pair was successively replaced by six symmetrically ((o,o), (m,m), or (p,p)) substituted (aminomethyl/carboxymethyl or aminoethyl/carboxyethyl) diazobenzene derivatives leading to a set of peptides with a photoswitchable backbone conformation. The folding behavior of each peptide was then investigated by performing molecular dynamics simulations in water (4 ns) and in methanol (10 ns) at room temperature. The simulations suggest that (o,o)- and (m,m)-substituted linkers with a single methylene spacer are significantly better suited to act as photoswitchable beta-turn building blocks than the other linkers examined in this study. The peptide containing the (m,m)-substituted linker was synthesized and characterized by NMR in its cis configuration. In the second part of this study, the structure of this peptide was refined using explicit-solvent simulations and NOE distance restraints, employing a variety of refinement protocols (instantaneous and time-averaged restraining as well as unrestrained simulations). We show that for this type of systems, even short simulations provide a significant improvement in our understanding of their structure if physically meaningful force fields are employed. In the third part, unrestrained explicit-solvent simulations starting from either the NMR model structure (75 ns) or a fully extended structure (25 ns) are shown to converge to a stable beta-hairpin. The resulting ensemble is in good agreement with experimental data, indicating successful structure prediction of the investigated hairpin by classical explicit-solvent molecular dynamics simulations.     

Appearance of pure fluorocarbon micelles surveyed by fluorescence quenching of amphiphilic quinoline derivatives in fluorocarbon and hydrocarbon surfactant mixtures

A halide-sensitive fluorescence probe was utilized to evaluate the miscibility of fluorocarbon and hydrocarbon surfactants in aqueous micellar systems. The fluorescence of 6-methoxy-N-1,1,2,2-tetrahydroheptadecafluorodecylquinolinium chloride, FC10MQ, was quenched by halide ions dissociated from the surfactant. The fluorescence in micellar solutions showed an initially rapid decay. This suggests that halide ions effectively quench FC10MQ fluorescence at the micellar surface. The subsequent slow decay corresponds to the quenching of FC10MQ fluorescence in the aqueous bulk phase by the free counterions. The Stern-Volmer plots for fluorescence quenching gave a distinct break at the critical micelle concentration of the cationic surfactants. The abrupt increase in fluorescence quenching is attributed to the solubilization of the probe in the micelles. The fluorescence quenching behavior provides direct information about the immiscibility of fluorocarbon and hydrocarbon species in micelles, and the results indicate that almost pure fluorocarbon micelles appear in surfactants mixtures.