Reaction of boron compounds with hydroxyquinones: A colour test of diagnostic value

The reactions of esters of boric acid, the wilson reagent, the DIMROTH reagent and the modified DIMROTH reagent have been studied with a wide variety of hydioxy-carbonyl compounds. The modified DIMROTH reagent and triphenyl borate behave very similarly and these reagents have the advantage of convenience, sensitivity and selectivity over the original DIMROTH test. The wilson reagent has been found to react with suitable hydroxy and methoxy acridones and xanthones as well as flavonees, flavonols and chalkones.     

Spin coherence of holes in GaAs/(Al,Ga)As quantum wells

Carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a diluted hole gas is studied by picosecond pump-probe Kerr rotation. For resonant optical excitation of the positively charged exciton the spin precession shows two types of oscillations: Electron spin beats decaying with the charged exciton radiative lifetime of 50 ps, and long-lived hole spin beats with dephasing times up to 650 ps, which decrease with increasing temperature, underlining the importance of hole localization. The mechanism of hole spin coherence generation is discussed.     

Overexposure effects of a 1-kHz tone on the distortion product otoacoustic emission in humans

The effects of overexposure on the properties of distortion product otoacoustic emissions (DPOAEs) are investigated. In total, 39 normal-hearing humans were monaurally exposed to a 1-kHz tone lasting for 3 min at an equivalent threshold sound-pressure level of 105.5 dB. The effects of overexposure were studied in two experiments (1) on the broadband DPOAE and (2) on the DPOAE fine structure, measured using a higher frequency resolution in a narrower frequency range. The obtained DPOAE shifts were compared to temporary threshold shift (TTS) obtained after a similar exposure. Similarities between DPOAE shifts and TTS were found in the affected frequency range and the time course of recovery. The amount of TTS was higher in the early recovery time (1-4-min postexposure), but similar to the DPOAE shift (even in absolute terms) at later recovery times (5-20-min postexposure). The DPOAE fine structure was not systematically changed after the exposure.     

Subsecond spin relaxation times in quantum dots at zero applied magnetic field due to a strong electron-nuclear interaction

A key to ultralong electron spin memory in quantum dots (QDs) at zero magnetic field is the polarization of the nuclei, such that the electron spin is stabilized along the average nuclear magnetic field. We demonstrate that spin-polarized electrons in n-doped (In,Ga)As/GaAs QDs align the nuclear field via the hyperfine interaction. A feedback onto the electrons occurs, leading to stabilization of their polarization due to formation of a nuclear spin polaron [I. A. Merkulov, Phys. Solid State 40, 930 (1998)]. Spin depolarization of both systems is consequently greatly reduced, and spin memory of the coupled electron-nuclear spin system is retained over 0.3 sec at temperature of 2 K.     

Towards an exact treatment of exchange and correlation in materials: application to the "CO adsorption puzzle" and other systems

It is shown that the errors of present-day exchange-correlation (XC) functionals are rather short ranged. For extended systems, the correction can therefore be evaluated by analyzing properly chosen clusters and employing highest-quality quantum chemistry methods. The XC correction rapidly approaches a universal dependence with cluster size. The method is applicable to bulk systems as well as to defects in the bulk and at surfaces. It is demonstrated here for CO adsorption at transition-metal surfaces, where present-day XC functionals dramatically fail to predict the correct adsorption site, and for the crystal bulk cohesive energy.     

Design and Stereoselective Synthesis of ProM‐2: A Spirocyclic Diproline Mimetic with Polyproline Type II (PPII) Helix Conformation

With the aim of developing polyproline type II helix (PPII) secondary‐structure mimetics for the modulation of prolin‐rich‐mediated protein–protein interactions, the novel diproline mimetic ProM‐2 was designed by bridging the two pyrrolidine rings of a diproline (Pro–Pro) unit through a Z‐vinylidene moiety. This scaffold, which closely resembles a section of a PPII helix, was then stereoselectively synthesized by exploiting a ruthenium‐catalyzed ring‐closing metathesis (RCM) as a late key step. The required vinylproline building blocks, that is, (R)‐N‐Boc‐2‐vinylproline (Boc=tert‐butyloxycarbonyl) and (S,S)‐5‐vinylproline‐tert‐butyl ester, were prepared on a gram scale as pure stereoisomers. The difficult peptide coupling of the sterically demanding building blocks was achieved in good yield and without epimerization by using 2‐(1H‐7‐azabenzotriazol‐1‐yl)‐1,1,3,3‐tetramethyluronium hexafluorophosphate (HATU)/N,N‐diisopropylethylamine (DIPEA). The RCM proceeded smoothly in the presence of the Grubbs II catalyst. Stereostructural assignments for several intermediates were secured by X‐ray crystallography. As a proof of concept, it was shown that certain peptides containing ProM‐2 exhibited improved (canonical) binding towards the Ena/VASP homology 1 (EVH1) domain as a relevant protein interaction target.     

Analysis of pipe flow transition. Part II. Energy transfer

The present article describes the results from a study of nonlinear mechanisms at work during the process of transition to turbulence in pipe flows. Using an accurate hybrid finite-difference code for the simulation of unsteady incompressible pipe flow, we have performed a direct numerical simulation designed to model experiments performed by Han, Tumin and Wygnanski [12]. Based on these numerical data, we have conducted a meticulous investigation of the dynamic interactions of the structures and flow modes that can be observed during this process. Based on this study, we can paint a detailed picture of the dynamical interactions of flow structures during both the linear and nonlinear stages of pipe flow transition. While this picture does have some similarities to earlier proposed mechanisms, we find that even for the simple cases considered here the structure of the pertinent interactions is much richer than suggested by these earlier models.     

Long wavelength impact of izod fracture specimens: An experimental/numerical investigation

Elastic wave propagation fields for a stationary crack in an Izod fracture specimen have been investigated. Moiré interferometry was used to obtain the asymptotic cracktip displacement fields under Hopkinson bar loading at sampling rates on the order of 100 kHz. Mixed-mode stress-intensity factors as a function of time were extracted from the displacement data using local asymptotic extrapolation. Two-dimensional continuum finite elements were employed and agreement was found between actual experiment and two-dimensional computation.     

The high‐anti conformation of 8‐aza‐1,3‐dide­aza‐2′‐deoxy­adenosine

In the title compound, 4‐amino‐1‐(2‐deoxy‐β‐d ‐erythro‐pento­furan­osyl)‐1H‐benzotriazole, C₁₁H₁₄N₄O₃, the conformation of the N‐glycosidic bond is in the high‐anti range [χ = ?77.1 (4)°] and the 2′‐deoxy­ribo­furan­ose moiety adopts a 2′‐­endo (²E) sugar puckering. The 5′‐hydroxyl group is disordered and has conformations ap with γ = 171.1 (3)° [occupation of 61.4 (3)%] and +sc with γ = 52.4 (6)° [occupation of 38.6 (3)%]. The nucleobases are stacked in the crystal state.