Molecular beam epitaxy growth and properties of GaAs/(AlGa)As p-type heterostructures on (100), (011), (111)B, (211)B, (311)B, and (311)A oriented GaAs

We report on a series of Be-doped GaAs/AlGaAs two-dimensional hole gas (2DHG) structures grown on (110), (111)B, (211)B and (311)B oriented substrates and compare their properties with high-mobility samples grown on (311)A using Si doping. The samples were prepared and grown under the same conditions so as to render them comparable. They are found to have mobilities which are strongly anisotropic within the plane. The highest mobility is found on the (110) surface with 100,000 cm² V⁻¹ s⁻¹, while the (211) surface gave the lowest values 10,000 cm² V⁻¹ s⁻¹. However, the later samples are found to have quantum Hall effect critical currents of >70 μA: an exceptionally high value for a hole gas which makes them suitable for metrology. All the samples show strong low-field positive magnetoresistance with resistance increases of up to 30% at magnetic fields of only 0.1 T. The presence of this feature on all the different planes shows that it does not depend upon the details of the band structure. It is identified with the lifting of the degeneracy of the spin sub-bands by the asymmetrical potential giving rise to a classical two-band magnetoreresistance.     

High pressure synthesis of late rare earth RFeAs(O,F) superconductors; R = Tb and Dy

New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures T(c) = 46 and 45 K and very high critical fields, >/=100 T, have been prepared at 1100-1150 degrees C and 10-12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La-Nd, Sm, Gd) high temperature superconductors.     

Free-solvent model shows osmotic pressure is the dominant factor in limiting flux during protein ultrafiltration

In protein ultrafiltration (UF), the limiting flux phenomenon has been generally considered a consequence of the presence of membrane fouling or the perceived formation of a cake/gel layer that develops at high operating pressures. Subsequently, numerous theoretical models on gel/cake physics have been made to address how these factors can result in limiting flux. In a paradigm shift, the present article reestablishes the significance of osmotic pressure by examining its contribution to limiting flux in the framework of the recently developed free solvent osmotic pressure model. The resulting free-solvent-based flux model (FSB) uses the Kedem–Katchalsky model, film theory and the free solvent representation for osmotic pressure in its development. Single protein tangential-flow diafiltration experiments (30 kDa MWCO CRC membranes) were also conducted using ovalbumin (OVA, 45 kDa), bovine serum albumin (BSA, 69 kDa), and immuno-gamma globulin (IgG, 155 kDa) in moderate NaCl buffered solutions at pH 4.5, 5.4, 7 and 7.4. The membrane was preconditioned to minimize membrane fouling development during the experimental procedure. The pressure was randomly selected and flux and sieving were determined. The experimental results clearly demonstrated that the limiting flux phenomenon is not dominated by membrane fouling and the FSB model theoretically illustrates that osmotic pressure is the primary factor in limiting flux during UF. The FSB model provides excellent agreement with the experimental results while producing realistic protein wall concentrations. In addition, the pH dependence of the limiting flux is shown to correlate to the pH dependency of the specific protein diffusion coefficient.     

QSAR modeling using automatically updating correction libraries: application to a human plasma protein binding model

It is assumed that compounds occupying the same region of model space will be subject to similar errors in prediction, and hence, where these errors are known, they can be applied to predictions. Thus, any available measured data can be used to refine predictions of query compounds. This study describes the application of a correction library to a human plasma protein binding model. Compounds that have been measured since the model was built are entered into the library to improve predictions of current compounds. Time-series simulations were conducted to measure the time dependence of the correction library. This study demonstrates significant improvements in predictions where a library is applied, compared with both a static model and an updating model that includes recently measured data.     

Noncovalent interactions of Cu+ with N-donor ligands (pyridine, 4,4-dipyridyl, 2,2-dipyridyl, and 1,10-phenanthroline): collision-induced dissociation and theoretical studies

Collision-induced dissociation of complexes of Cu+ bound to a variety of N-donor ligands (N-L) with Xe is studied using guided ion beam tandem mass spectrometry. The N-L ligands examined include pyridine, 4,4-dipyridyl, 2,2-dipyridyl, and 1,10-phenanthroline. In all cases, the primary and lowest-energy dissociation channel observed corresponds to the endothermic loss of a single intact N-L ligand. Sequential dissociation of additional N-L ligands is observed at elevated energies for the pyridine and 4,4-dipyridyl complexes containing more than one ligand. Ligand exchange processes to produce Cu+Xe are also observed as minor reaction pathways in several systems. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level are performed to obtain model structures, vibrational frequencies, and rotational constants for the neutral N-L ligands and the Cu+(N-L)x complexes. The relative stabilities of the various conformations of these N-L ligands and Cu+(N-L)x complexes as well as theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) level of theory using B3LYP/6-31G* optimized geometries. Excellent agreement between theory and experiment is observed for all complexes containing one or two N-L ligands, while theory systematically underestimates the strength of binding for complexes containing more than two N-L ligands. The ground-state structures of these complexes and the trends in the sequential BDEs are explained in terms of stabilization gained from sd-hybridization and repulsive ligand-ligand interactions. The nature of the binding interactions in the Cu+(N-L)x complexes are examined via natural bond orbital analyses.     

Axial mode locking in a harmonic-multiplying, inverted gyrotwystron

When in microwave sources, an open waveguide operating at frequencies close to cutoff is used as a resonator, a number of modes with the same transverse structure, but different axial distribution can be excited by an electron beam. The width of the resonance curves of these modes broadens as the number of axial variations grows. This leads to overlapping of these curves at large axial indexes. As a result, phase locking of such modes may occur. A theory describing axial mode locking in gyrodevices is developed. The results of theoretical analysis are compared to a harmonic-multiplying, inverted gyrotwystron experiment. It is shown that such phase-locked operation in a set of modes with overlapping resonance curves can significantly enlarge the bandwidth of gyrodevices. Furthermore, this technique may be broadly applicable to other devices, which employ cavities with overlapping modes     

FLASH PHOTOLYSIS STUDIES OF HEMATO-AND COPRO-PORPHYRINS IN HOMOGENEOUS AND MICROHETEROGENEOUS AQUEOUS DISPERSIONS

This paper describes an experimental study of the photo properties of the triplet (T,) states of hematoporphyrin (HP) and coproporphyrin (CP), particularly in relation to their medium dependence and reactivity towards oxygen. Triplet-triplet absorption spectra of HP and CP have been determined in aqueous buffer at pH = 7.4 and in water-methanol and water-formamide mixtures. The spectra corrected for ground state contributions show major absorption peaks near 400 nm and lesser peaks near 500 nm. Extinction coefficient measurements have been made and their dependences on solvent composition investigated. Natural lifetimes of the T₁ states of HP and CP and the bimolecular quenching constants with oxygen have been determined. The quantum yields of T₁ formation are ca. 0.6 in buffer rising to 0.8 and higher in predominantly organic media. Incorporation of the porphyrins into micellar phases similarly causes φ_T, to increase. Quantum efficiencies of O^?₂ and O₂(^lΔ_g) formation have been determined for HP in buffer, some binary mixtures and micellar dispersions. Superoxide yields are low and may result from photo-ionization processes. O₂(^lΔ_g) yields are large but appear to have an unexpected dependence on the medium.     

2,4-Dihydroxy and O2 Protonated Tautomers of dThd and Thd Coexist in the Gas Phase: Methylation Alters Protonation Preferences versus dUrd and Urd

The gas-phase structures of protonated thymidine, [dThd + H]⁺, and its modified form, protonated 5-methyluridine, [Thd + H]⁺, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy combined with electronic structure calculations. IRMPD action spectra are measured over the ranges extending from ~600 to 1900 cm^–1 and ~2800 to 3800 cm^–1 using the FELIX free electron laser and an optical parametric oscillator/amplifier (OPO/OPA) laser system, respectively. Comparisons between the B3LYP/6-311+G(d,p) linear IR spectra calculated for the stable low-energy conformers and the measured IRMPD spectra are used to determine the most favorable tautomeric conformations of [dThd + H]⁺ and [Thd + H]⁺ and to identify those populated in the experiments. Both B3LYP and MP2 levels of theory predict a minor 2,4-dihydroxy tautomer as the ground-state conformer of [dThd + H]⁺ and [Thd + H]⁺ indicating that the 2'-hydroxyl substituent of Thd does not exert a significant impact on the structural features. [dThd + H]⁺ and [Thd + H]⁺ share parallel IRMPD spectral profiles and yields in both the FELIX and OPO regions. Comparisons between the measured IRMPD and calculated IR spectra suggest that minor 2,4-dihydroxy tautomers and O2 protonated conformers of [dThd + H]⁺ and [Thd + H]⁺ are populated in the experiments. Comparison of this work to our previous IRMPD spectroscopy study of protonated 2'-deoxyuridine and uridine suggests that the 5-methyl substituent alters the preferences of O2 versus O4 protonation.

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Stable isotope incorporation triples the upper mass limit for determination of elemental composition by accurate mass measurement

By comparing electrospray ionization Fourier-transform ion cyclotron resonance (FT-ICR) mass spectra and collision-induced dissociation (CID) FT-ICR mass spectra of a phospholipid (851 Da) extracted from natural abundance and 99% 13C bacterial growth media, we are able to reduce its number of possible elemental compositions (based on +/-10 ppm externally calibrated mass accuracy and biologically relevant compositional constraints) from 394 to 1. The basic idea is simply that the mass of a molecule containing N carbon atoms increases by N Da when 12C is replaced by 13C. Once the number of carbons is known, the number of possible combinations of other atoms in the molecule is greatly reduced. We demonstrate the method for a stored-waveform inverse Fourier transform-isolated phospholipid from an extract of membrane lipids from Rhodococcus rhodochrous hydrocarbon-degrading bacteria grown on either natural abundance or 99% 13C-enriched mixtures of n-hexadecane and n-octadecane. We project that this method raises the upper mass limit for unique determination of elemental composition from accurate mass measurement by a factor of at least 3, thereby extending "chemical formula" determination to identification and sequencing of larger synthetic and bio-polymers: phospholipids, oligopeptides of more than three to four amino acids, DNA or RNA of more than two nucleotides, oligosaccharides of more than three sugars, etc. The method can also be extended to determination of the number of other atoms for which heavy isotopes are available (e.g., 15N, 34S, 18O, etc.).