A correlation for burn time of aluminum particles in the transition regime

A study of the combustion times for aluminum particles in the size range of 3–11 μm with oxygen, carbon dioxide, and water vapor oxidizers at high temperatures (>2400 K), high pressures (4–25 atm), and oxidizer composition (15–70% by volume in inert diluent) in a heterogeneous shock tube has generated a correlation valid in the transition regime. The deviation from diffusion limited behavior and burn times that could otherwise be accurately predicted by the widely accepted Beckstead correlation is seen, for example, in particles below 20 μm, and is evidenced by the lowering of the diameter dependence on the burn time, a dependence on pressure, and a reversal of the relative oxidizer strengths of carbon dioxide and water vapor. The strong dependence on temperature of burn time that is seen in nano-Al is not observed in these micron-sized particles. The burning rates of aluminum in these oxidizers can be added to predict an overall mixture burnout time adequately. This correlation should extend the ability of modelers to predict combustion rates of particles in solid rocket motor environments down to particle diameters of a few microns.     

Structural characterization of ß-amyloid oligomer-aggregates by ion mobility mass spectrometry and electron spin resonance spectroscopy

Formation and accumulation of fibrillar plaques and aggregates of ß-amyloid peptide (Aß) in brain have been recognized as characteristics of Alzheimer’s disease (AD). Oligomeric aggregates of Aß are considered critical intermediates leading to progressive neurodegeneration; however, molecular details of the oligomerization and aggregation pathway and the structures of Aß-oligomers are hitherto unclear. Using an in vitro fibril formation procedure of Aß(1–40), ß-amyloid aggregates were prepared and insoluble aggregates separated from soluble products by centrifugation. In this study, ion mobility mass spectrometry (IM-MS) was applied in combination with electron paramagnetic resonance spectroscopy (EPR) to the identification of the components of Aß-oligomers, and to their structural and topographical characterization. The formation of Aß-oligomers and aggregates was monitored by gel electrophoresis, and Aß-oligomer bands were identified by in-gel tryptic digestion and matrix-assisted laser desorption ionization–mass spectrometry (MALDI-MS) to consist predominantly of Aß(1–40) peptide. First, ion mobility-MS studies of soluble Aß-aggregates prepared by incubation for 5 days were performed on a quadrupole time-of-flight mass spectrometer and revealed (1) the presence of at least two different conformational states, and (2), the formation of Met-35 oxidized products. For estimation of the size of Aß-aggregates using EPR spectroscopy, a modified Aß(1–40) peptide containing an additional N-terminal cysteine residue was prepared, and a 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy radical spin label derivative (IPSL) was coupled by S-alkylation. The EPR spectra of the spin-labeled Cys-Aß(1–40) oligomers were matched with spectra simulations using a multi-component simulation strategy, resulting in complete agreement with the gel electrophoresis results.     

Giant excitonic Zeeman splittings in colloidal Co2+ -doped ZnSe quantum dots

Colloidal Co(2+):ZnSe diluted magnetic semiconductor quantum dots (DMS-QDs) were prepared by the hot injection method and studied spectroscopically. Ligand-field electronic absorption and magnetic circular dichroism (MCD) spectra confirm homogeneous substitutional speciation of Co(2+) in the ZnSe QDs. Absorption spectra collected at various times throughout the syntheses reveal that dopants are absent from the central cores of the QDs but are incorporated at a constant concentration during nanocrystal growth. The undoped cores are associated with dopant exclusion from the ZnSe critical nuclei. Analysis of low-temperature electronic absorption and MCD spectra revealed excitonic Zeeman splitting energies (DeltaE(Zeeman)) of these Co(2+):ZnSe QDs that were substantially smaller than anticipated from bulk Co(2+):ZnSe data. This reduction in DeltaE(Zeeman) is explained quantitatively by the absence of dopants from the QD cores, where dopant-exciton overlap would be greatest. Since dopant exclusion from nucleation appears to be a general phenomenon for DMS-QDs grown by direct chemical methods, we propose that DeltaE(Zeeman) will always be smaller in colloidal DMS-QDs grown by such methods than in the corresponding bulk materials.     

Characterization of a single molecule DNA switch in free solution

We have studied a donor-acceptor fluorophore-labeled DNA switch where the acceptor is Alexa-647, a carbocyanine dye, in solution at the single molecule level to elucidate the fluorescence switching mechanism. The acceptor, which is in an initial high fluorescence trans state, undergoes a photoisomerization reaction resulting in two additional states during its sub-millisecond transit across the probe volume. These two states are assigned to a nonfluorescent triplet trans state that strongly quenches the donor emission and a singlet cis state that blocks the fluorescence resonance energy transfer (FRET) pathway and gives rise to donor-only fluorescence. The formation of these states is faster than the transit time, so that all three states are approximately equally populated under our experimental conditions. The acceptor dye can stick to the DNA in all these states, with the rate of unsticking determining the rate of isomerization into the other states. Measurement of the rate of change of the FRET signal therefore provides information about the fluorophore-DNA intramolecular dynamics. These results explain the large zero peak in the proximity ratio, often seen in single molecule FRET experiments, and suggest that photoinduced effects may be important in single molecule FRET experiments using carbocyanine dyes. They also suggest that for fast photoinduced switching the interactions of the acceptor dye with the DNA and other surfaces should be prevented.     

Structural and molecular evolutionary analysis of Agouti and Agouti-related proteins

Agouti (ASIP) and Agouti-related protein (AgRP) are endogenous antagonists of melanocortin receptors that play critical roles in the regulation of pigmentation and energy balance, respectively, and which arose from a common ancestral gene early in vertebrate evolution. The N-terminal domain of ASIP facilitates antagonism by binding to an accessory receptor, but here we show that the N-terminal domain of AgRP has the opposite effect and acts as a prodomain that negatively regulates antagonist function. Computational analysis reveals similar patterns of evolutionary constraint in the ASIP and AgRP C-terminal domains, but fundamental differences between the N-terminal domains. These studies shed light on the relationships between regulation of pigmentation and body weight, and they illustrate how evolutionary structure function analysis can reveal both unique and common mechanisms of action for paralogous gene products.     

Protonolysis of the Hg-C bond of chloromethylmercury and dimethylmercury. A DFT and QTAIM study

Possible mechanisms for degrading chloromethylmercury (CH(3)HgCl) and dimethylmercury [(CH3)2Hg] involving thiol and ammonium residues were investigated by DFT and atoms-in-molecules (QTAIM) calculations. Using H2S and HS- as models for thiol and thiolate groups RSH and RS-, respectively, we obtained transition states and energy barriers for possible decomposition routes to Hg(SH)2 based on a model proposed by Moore and Pitts (Moore, M. J.; Distefano, M. D.; Zydowsky, L. D.; Cummings, R. T.; Walsh, C. T. Acc. Chem. Res. 1990, 23, 301. Pitts, K. E.; Summers, A. O. Biochemistry 2002, 41, 10287). Demethylation was found to be a multistep process that involved initial substitution of Cl- by RS-. We found that successive coordination of Hg with thiolates leads to increased negative charge on the methyl group and facilitates the protonolysis of the Hg-C bond by H-SH. This was also found to be the case for (CH3)2Hg. We found that NH4(+) readily protonolyzes the Hg-C bond of these thiolate complexes, suggesting that ammonium residues of protonated amino acids might also act as effective proton donors.     

Thiamine models and perspectives on the mechanism of action of thiamine-dependent enzymes

Thiamine dependent enzymes catalyze ligase and lyase reactions near a carbonyl moiety. Chemical models for these reactions serve as useful tools to substantiate a detailed mechanism of action. This tutorial review covers all such studies performed thus far, emphasizing the role of each part around the active site and the conformation of the cofactor during catalysis.     

SmartPhantom--an fMRI simulator

Many informatics tools have emerged to process the voluminous and complex data generated by functional magnetic resonance imaging (fMRI). The interpretation of fMRI exams is largely determined by these tools. However, their performance is hard to evaluate because there is no independent means of calibration. A novel fMRI calibration system called SmartPhantom has been developed to simulate functional blood oxygen level dependent (BOLD) imaging. SmartPhantom contains a quadrature radio frequency coil, comprising two perpendicular planar loops that can be externally activated or deactivated. The system is able to produce reasonably uniform signal enhancements in a calibration sample surrounded by the two loops during an MRI scan. The enhancement is controlled well in both magnitude and predefined timing and produces BOLD-like signals. Characteristics of SmartPhantom are discussed in detail, followed by a comparison of fMRI informatics tools. Two fMRI data sets are acquired with the SmartPhantom. One with high signal-to-noise ratio provides the calibration. Another with lower SNR is input into three software packages (BrainVoyager, FSL and Statistical Parametric Mapping 2) for data preprocessing and statistical analysis. Results from the three packages are compared in both sensitivity of detecting the activation and correlation between the predicted activation and calibration.     

Factors Affecting the Production of Aromatic Immonium Ions in MALDI 157 nm Photodissociation Studies

Immonium ions are commonly observed in the high energy fragmentation of peptide ions. In a MALDI-TOF/TOF mass spectrometer, singly charged peptides photofragmented with 157 nm VUV light yield a copious abundance of immonium ions, especially those from aromatic residues. However, their intensities may vary from one peptide to another. In this work, the effect of varying amino acid position, peptide length, and peptide composition on immonium ion yield is investigated. Internal immonium ions are found to have the strongest intensity, whereas immonium ions arising from C-terminal residues are the weakest. Peptide length and competition among residues also strongly influence the immonium ion production. Quantum calculations provide insights about immonium ion structures and the fragment ion conformations that promote or inhibit immonium ion formation.

Open image in new window

Graphical Abstract ?

     

Use of CT scans and treatment planning software for validation of the dose component of food irradiation protocols

The challenging problem of estimating the dose delivered to heterogeneous products by radiation modalities of limited penetration can be readily handled by using technologies developed for, and widely used in, radiation therapy applications. In particular, combining CT scanning with radiation treatment planning programs can simulate radiation processing with either photons or electrons, and can provide detailed, high resolution and accurate dose maps for any arbitrary product and package configuration. Such dose maps are an essential part of process validation. Comparison of the simulated dose distributions with measured dose maps verifies the soundness of this approach. The present communication presents results obtained with the simulation technique for a variety of common food items which are likely candidates for radiation processing.