Sources of variability in distortion product otoacoustic emissions

The goal of this study was to determine the extent to which the variability seen in distortion product otoacoustic emissions (DPOAEs), among ears with normal hearing, could be accounted for. Several factors were selected for investigation, including behavioral threshold, differences in middle-ear transmission characteristics either in the forward or the reverse direction, and differences in contributions from the distortion and reflection sources. These variables were assessed after optimizing stimulus parameters for individual ears at each frequency. A multiple-linear regression was performed to identify whether the selected variables, either individually or in combination, explained significant portions of variability in DPOAE responses. Behavioral threshold at the f(2) frequency and behavioral threshold squared at that same frequency explained the largest amount of variability in DPOAE level, compared to the other variables. The combined model explained a small, but significant, amount of variance in DPOAE level at five frequencies. A large amount of residual variability remained, even at frequencies where the model accounted for significant amounts of variance.     

2-Aminopurine flipped into the active site of the adenine-specific DNA methyltransferase M.TaqI: crystal structures and time-resolved fluorescence

We report the crystal structure of the DNA adenine-N6 methyltransferase, M.TaqI, complexed with DNA, showing the fluorescent adenine analog, 2-aminopurine, flipped out of the DNA helix and occupying virtually the same position in the active site as the natural target adenine. Time-resolved fluorescence spectroscopy of the crystalline complex faithfully reports this state: base flipping is accompanied by the loss of the very short ( approximately 50 ps) lifetime component associated with fully base-stacked 2-aminopurine in DNA, and 2-aminopurine is subject to considerable quenching by pi-stacking interactions with Tyr108 in the catalytic motif IV (NPPY). This proves 2-aminopurine to be an excellent probe for studying base flipping by M.TaqI and suggests similar quenching in the active sites of DNA and RNA adenine-N6 as well as DNA cytosine-N4 methyltransferases sharing the conserved motif IV. In solution, the same distinctive fluorescence response confirms complete destacking from DNA and is also observed when the proposed key residue for base flipping by M.TaqI, the target base partner thymine, is substituted by an abasic site analog. The corresponding cocrystal structure shows 2-aminopurine in the active site of M.TaqI, demonstrating that the partner thymine is not essential for base flipping. However, in this structure, a shift of the 3' neighbor of the target base into the vacancy left after base flipping is observed, apparently replicating a stabilizing role of the missing partner thymine. Time-resolved fluorescence and acrylamide quenching measurements of M.TaqI complexes in solution provide evidence for an alternative binding site for the extra-helical target base within M.TaqI and suggest that the partner thymine assists in delivering the target base into the active site.     

ChemInform Abstract: X-Ray Structural Studies of the Polymorphic Elpasolites K2LiAlF6 and Rb2LiGaF6.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

High brightness keV harmonics from relativistically oscillating plasma surfaces

X-ray harmonic radiation extending to 3.3 Å, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to ~3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy hν > 1 keV is found to be into a cone angle < 4°, significantly less than that of the incident laser cone (20°).     

Fast liquid chromatography separation and multiple‐reaction monitoring mass spectrometric detection of neurotransmitters

We describe here the fast LC‐MS/MS separation of a mixture of neurotransmitters consisting of dopamine, epinephrine, norepinephrine, 3,4‐dihydroxybenzylamine (DHBA), salsolinol, serotonin, and γ‐aminobutyric acid (GABA). The new UltiMate® 3000 Rapid Separation system (RSLC) was successfully coupled to the 4000 QTRAP mass spectrometer operating in multiple‐reaction monitoring (MRM) mode. The separation was attained using a 100 mm length, 2.2 μm particle size Acclaim column at a flow rate of 0.5 mL/min. The column back pressure was 350 bar, while the total run time including column re‐equilibration was 5.2 min. The peak resolution was minimally affected by the fast separation. The RSLC‐MRM separation was found to have a precision range based on peak area for 50 replicate runs of 2–5% CV for all analytes, and the reproducibility of the retention time for all analytes was found to range from 0–2% CV. The described method represents an almost seven times shorter analysis time of neurotransmitters using LC/MRM which is very useful in screening large quantities of biological samples for various neurotransmitters.     

Laser-driven shock experiments on precompressed water: Implications for "icy" giant planets

Laser-driven shock compression of samples precompressed to 1 GPa produces high-pressure-temperature conditions inducing two significant changes in the optical properties of water: the onset of opacity followed by enhanced reflectivity in the initially transparent water. The onset of reflectivity at infrared wavelengths can be interpreted as a semiconductor<-->electronic conductor transition in water, and is found at pressures above approximately 130 GPa for single-shocked samples precompressed to 1 GPa. Our results indicate that conductivity in the deep interior of "icy" giant planets is greater than realized previously because of an additional contribution from electrons.     

Structure and morphology of the reaction fronts during the formation of MgAl2O4 thin films by solid state reaction between R-cut sapphire substrates and MgO films

The thin-film spinel-forming solid state reaction between Al₂O₃ and MgO has been studied under initially non-coherent conditions. MgO films in (001) orientation on -cut sapphire single crystals were heated at 1100°C for 30 min or 1h. The film/substrate reaction proceeds via cation counterdiffusion as was revealed by a marker experiment. The MgAl₂O₄ films formed were predominantly (001) oriented, with an additional systematic tilt of about 5° of the spinel lattice around the [010] axis. The structure of the Al₂O₃ /MgAl₂O₄(001) and MgAl₂O₄(001)/MgO(001) reaction fronts has been investigated on cross section samples by high-resolution electron microscopy. It appeared that after starting from an incoherent interface, the Al₂O₃ /MgAl₂O₄(001) front assumes an almost fully coherent structure during the reaction. As a result the lattice misfit is reduced to 1%, and interfacial ledges are formed. The latter most probably play an active role in the necessary c.p.h. f.c.c. reconstruction of the oxygen sublattice. The MgAl₂O₄(001)/MgO(001) reaction front consists of coherent regions divided by misfit dislocations. During the reaction the former run ahead whereas the latter lag behind. As a result the morphology of the reaction front is bowed. The results confirm earlier observations of Carter and Schmalzried of the semicoherent Al₂O₃(00.1)/CoAl₂O₄(111) interface, thus strongly supporting the conclusion of a fundamental new phase transformation mechanism specific to oxide systems.Presented at the workshop on High-Voltage and High-Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

Optimal Recovery of the Derivative of Bounded Analytic Functions

Given (–1, 0), n N, we discuss the optimal recoveryof (), for analytic and bounded in < 1, from the knowledge of the values of at n points z1,.zm[0,l),where these points are chosen to produce the least possibleintrinsic error. The optimal algorithms are explicitly determined.     

Computer program for neutron activation analysis with the SLOWPOKE reactor

A FORTRAN computer program for automatic neutron activation analysis is presented. The program locates and identifies peaks in a gamma-ray spectrum, calculates peak areas and the concentrations of the elements of interest in the sample. This program was specifically designed for the SLOWPOKE reactor, it uses a semi-absolute method and does not need standards or flux monitors. The program was written so as to minimize the computation time, and a typical 4096-channel spectrum is processed in five seconds by an IBM 360/75 computer.