Oxygen isotopes of fuel pellets from the fifth collaborative materials exercise and uranium oxides reference materials determined by continuous flow laser fluorination mass spectrometry for nuclear forensic applications

Journal of Radioanalytical and Nuclear Chemistry - Uranium oxides are essential materials in the production process of nuclear fuel for civilian or military applications. As such, identifying their...     

Photobleaching pathways in single-molecule FRET experiments

To acquire accurate structural and dynamical information on complex biomolecular machines using single-molecule fluorescence resonance energy transfer (sm-FRET), a large flux of donor and acceptor photons is needed. To achieve such fluxes, one may use higher laser excitation intensity; however, this induces increased rates of photobleaching. Anti-oxidant additives have been extensively used for reducing acceptor's photobleaching. Here we focus on deciphering the initial step along the photobleaching pathway. Utilizing an array of recently developed single-molecule and ensemble spectroscopies and doubly labeled Acyl-CoA binding protein and double-stranded DNA as model systems, we study these photobleaching pathways, which place fundamental limitations on sm-FRET experiments. We find that: (i) acceptor photobleaching scales with FRET efficiency, (ii) acceptor photobleaching is enhanced under picosecond-pulsed (vs continuous-wave) excitation, and (iii) acceptor photobleaching scales with the intensity of only the short wavelength (donor) excitation laser. We infer from these findings that the main pathway for acceptor's photobleaching is through absorption of a short wavelength photon from the acceptor's first excited singlet state and that donor's photobleaching is usually not a concern. We conclude by suggesting the use of short pulses for donor excitation, among other possible remedies, for reducing acceptor's photobleaching in sm-FRET measurements.     

Energy transfer between manganese(II) and thulium(III) in transition-metal fluoride glasses

Lead zinc gallium fluoride glass containing 2 mole% TmF₃ is shown to luminesce from ¹I₆ (35200 cm⁻¹), ¹D₂(28000), ¹G₄ (21500) and ³H₄ (12800) to several lower J levels. When the 24 mole% ZnF₂ is replaced by MnF₂ in the absence of thulium, the (broad band) quartet-to-sextet (ground state) emission at 16800cm⁻¹ has a mean lifetime of 1 ms. If 2 mole% TmF₃ is added to the latter glass, energy transfer decreases the lifetime to below 0.03 ms. The high critical radius R₀ (above 20 Å) is rationalized by Mn-Mn energy migration (concomitant with a lifetime shortened from 24 ms) to be compared with R₀≈ 8 Å for low Mn(II) concentration, almost identical with energy transfer between 1 mole% each of MnF₂ and NdF₃ previously studied in an analogous glass.     

Optimal Treatment with Cannabis Extracts Formulations Is Gained via Knowledge of Their Terpene Content and via Enrichment with Specifically Selected Monoterpenes and Monoterpenoids

Differences between therapeutic effects of medical cannabis inflorescences and those of their extracts are generally attributed to the differences in administration form and in the resultant pharmacokinetics. We hypothesized that difference may further extend to the composition of the actually consumed drug. Cannabinoid and terpene contents were compared between commercial cannabis inflorescences (n = 19) and decarboxylated extracts (n = 12), and between inflorescences and decarboxylated extracts produced from them (n = 10). While cannabinoid content was preserved in the extracts, a significant loss of terpenes was evident, mainly in the more volatile monoterpenes and monoterpenoids (representing a loss of about 90%). This loss changes the total terpene content, the proportion of monoterpenes out of the total terpenes, and the monoterpene/cannabinoid ratio. Terpene deficiency might impair extracts’ pharmacological efficacy and might contribute to the patients’ preference to inflorescences-smoking. This argues against the validity of terms such as “whole plant” and “full spectrum” extracts and creates a misleading assumption that extracts represent the pharmacological profile of the sourced inflorescences. Furthermore, it reduces the diversity in extracts, such as loss of differences between sativa-type and indica-type. Enriching cannabis extracts with selected terpenes may provide a suitable solution, generating a safe, precise, and reproducible drug with tailored cannabinoid and terpene contents. Careful selection of terpenes to be added enables tailor-made extracts, adjusted for various medicinal aims and for different populations.     

Electrospun polymer nanofibers: Mechanical and thermodynamic perspectives

This article reviews and discusses some open problems concerning polymer materials of reduced sizes and dimensions. Such objects exhibit exceptional physical properties when compared with their macroscopic counterparts. More specifically, abrupt increases in polymer nanofiber elastic modulus have been observed when diameters drop below a certain value. In addition, temperature dependence of elastic modulus is highly influenced by fiber diameter. Mechanical (macroscopic) analyses have failed to provide satisfactory explanations for the mechanisms ruling such features, calling for detailed microscopic examination of the systems in question. A hypothesis bridging the current knowledge gaps is presented. The key element of this hypothesis is based on confinement of the supermolecular microstructure of polymer nanofibers and its dominant role in the deformation process. This suggestion challenges the commonly held view suggesting that surface effects are the most significant parameters impacting mechanical and thermodynamic nanofiber behaviors. The review will focus on the mechanical and thermodynamic properties of electrospun polymer nanofibers, selected as representatives of nanoscale polymer objects. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Critical Ising on the Square Lattice Mixes in Polynomial Time

The Ising model is widely regarded as the most studied model of spin-systems in statistical physics. The focus of this paper is its dynamic (stochastic) version, the Glauber dynamics, introduced in 1963 and by now the most popular means of sampling the Ising measure. Intensive study throughout the last three decades has yielded a rigorous understanding of the spectral-gap of the dynamics on everywhere except at criticality. While the critical behavior of the Ising model has long been the focus for physicists, mathematicians have only recently developed an understanding of its critical geometry with the advent of SLE, CLE and new tools to study conformally invariant systems.     

The unramified computation of Rankin-Selberg integrals for SO2l × GLn

Let SO _2l be the special orthogonal group, either split or quasi-split over a number field, and 1 < l < n. We compute the local integral, where data are unramified, derived from the global Rankin-Selberg construction for SO _2l × GL _n . In the general case, the local integral is difficult to compute directly, so instead it is transformed to an integral related to a construction for SO _2n+1×GL _n , which carries a Bessel model on SO _2n+1. For the quasisplit case, when l = n − 1 we are able to compute the local integral, by a modification of our recently introduced approach using invariant theory. This leads to another proof of our result for 1 < l < n, as well as a new proof of a known result regarding the unramified Bessel function.     

Resolution enhancement in MRI

We consider the problem of super-resolution reconstruction (SRR) in MRI. Subpixel-shifted MR images were taken in several fields of view (FOVs) to reconstruct a high-resolution image. A novel algorithm is presented. The algorithm can be applied locally and guarantees perfect reconstruction in the absence of noise. Results that demonstrate resolution improvement are given for phantom studies (mathematical model) as well as for MRI studies of a phantom carried out with a GE clinical scanner. The method raises questions that are discussed in the last section of the paper. Open questions should be answered in order to apply this method for clinical purposes.     

Shot-noise limited single-molecule FRET histograms: comparison between theory and experiments

We describe a simple approach and present a straightforward numerical algorithm to compute the best fit shot-noise limited proximity ratio histogram (PRH) in single-molecule fluorescence resonant energy transfer diffusion experiments. The key ingredient is the use of the experimental burst size distribution, as obtained after burst search through the photon data streams. We show how the use of an alternated laser excitation scheme and a correspondingly optimized burst search algorithm eliminates several potential artifacts affecting the calculation of the best fit shot-noise limited PRH. This algorithm is tested extensively on simulations and simple experimental systems. We find that dsDNA data exhibit a wider PRH than expected from shot noise only and hypothetically account for it by assuming a small Gaussian distribution of distances with an average standard deviation of 1.6 A. Finally, we briefly mention the results of a future publication and illustrate them with a simple two-state model system (DNA hairpin), for which the kinetic transition rates between the open and closed conformations are extracted.