Metabolites of pyrenomycetes XIII: Structure of (+) hypothemycin,an antibiotic macrolide from Hypomyces trichothecoides

(+) Hypothemycin, an antibiotic metabolite of Hypomyces trichothecoides, has been assigned the macrolide structure I, based on chemical evidence and spectroscopic data.     

A stable least-squares finite element method for non-linear hyperbolic problems

A class of stable least-square finite element methods for non-linear hyperbolic problems is developed and some exploratory studies made. The methods are based on modifying the L²-norm of the. residual and a related approximation to the H¹-norm of the residual. The effect of the additional terms in these residual functionals is to introduce a dissipative effect proportional to the solution gradient. This acts to stabilize the solution for non-linear hyperbolic problems which generate shocks. Numerical results for a one-dimensional nozzle and shock tube problem demonstrate the accuracy and stability of the method. Results are for an implicit scheme and calculations for linear, quadratic and cubic elements are given.     

The massless Thirring model: Positivity of Klaiber'sn-point functions

We present a simple solution to the problem of proving positivity of Klaiber'sn-point functions for the massless Thirring model. The corresponding fields are obtained as strong limits of explicitly given approximate fields, obviating reconstruction. By invoking recent results on the boson-fermion correspondence it is shown how the model can be formulated on the charged fermion Fock space. It is pointed out that the question of cyclicity of the vacuum is open, and that an affirmative answer is necessary to confirm the superselection sector picture of the model.     

Steinberg symbols modulo the trace class, holonomy, and limit theorems for Toeplitz determinants

Suppose that where and , and the Toeplitz operator is invertible. Let be the determinant of the Toeplitz matrix where . Let be the orthogonal projection onto where ; set , let denote the Hankel operator associated to , and set for . For the Wiener-Hopf factorization where and , put , Theorem A.    

Let be a decomposition into invariant subspaces, and , so that restricted to is invertible, is finite dimensional, and restricted to is nilpotent. Let be the basis for the null space of , and let be the top vector in a Jordan root vector chain of length lying over , i.e., where . Theorem B.     , the holonomy of a Deligne bundle with connection defined by the factorization . Note that the generalizations of the Szegö limit theorem for which have appeared in the literature with instead of have the defect that the limit of does not exist in general. An example is given with yet for infinitely many .

     

Comparison of water-soluble CdTe nanoparticles synthesized in air and in nitrogen

It is commonly believed that high-quality CdTe nanoparticles with strong luminescence can only be prepared under the protection of an inert gas such as nitrogen or argon. Here, we report the preparation of highly luminescent CdTe nanoparticles in air and compare their luminescence properties with CdTe nanoparticles made in nitrogen. We find that both water-soluble CdTe nanoparticles made in air and in nitrogen exhibit strong photoluminescence as well as upconversion luminescence at room temperature. However, differences do exist between the particles made in air and those made in nitrogen. In particular, the particles prepared in air display a faster growth rate, grow to larger sizes, and display stronger electron coupling relative to the particles prepared in nitrogen. X-ray photoelectron spectroscopy analysis indicates that the oxygen content in the nanoparticles synthesized in air is higher that that in particles synthesized in N(2), likely resulting in a higher availability of excess free cadmium. Cytotoxicity measurements reveal that the particles made in air appear slightly more toxic, possibly due to the excess of free cadmium.     

Time-resolved imaging of spin transfer switching: beyond the macrospin concept

Time-resolved images of the magnetization switching process in a spin transfer structure, obtained by ultrafast x-ray microscopy, reveal the limitations of the macrospin model. Instead of a coherent magnetization reversal, we observe switching by lateral motion of a magnetic vortex across a nanoscale element. Our measurements reveal the fundamental roles played independently by the torques due to charge and spin currents in breaking the magnetic symmetry on picosecond time scales.     

A Deterministic Method for Robust Estimation of Multivariate Location and Shape

Abstract

The existence of outliers in a data set and how to deal with them is an important problem in statistics. The minimum volume ellipsoid (MVE) estimator is a robust estimator of location and covariate structure; however its use has been limited because there are few computationally attractive methods. Determining the MVE consists of two parts—finding the subset of points to be used in the estimate and finding the ellipsoid that covers this set. This article addresses the first problem. Our method will also allow us to compute the minimum covariance determinant (MCD) estimator. The proposed method of subset selection is called the effective independence distribution (EID) method, which chooses the subset by minimizing determinants of matrices containing the data. This method is deterministic, yielding reproducible estimates of location and scatter for a given data set. The EID method of finding the MVE is applied to several regression data sets where the true estimate is known. Results show that the EID method, when applied to these data sets, produces the subset of data more quickly than conventional procedures and that there is less than 6% relative error in the estimates. We also give timing results illustrating the feasibility of our method for larger data sets. For the case of 10,000 points in 10 dimensions, the compute time is under 25 minutes.     

Direct observation of spin-torque driven magnetization reversal through nonuniform modes

We present time-resolved x-ray images with 30 nm spatial and 70 ps temporal resolution, which reveal details of the spatially resolved magnetization evolution in nanoscale samples of various dimensions during reversible spin-torque switching processes. Our data in conjunction with micromagnetic simulations suggest a simple unified picture of magnetic switching based on the motion of a magnetic vortex. With decreasing size of the magnetic element the path of the vortex core moves from inside to outside of the nanoelement, and the switching process evolves from a curled nonuniform to an increasingly uniform mode.     

Fluorescence and Rotational Dynamics of Dityrosine

We have examined the lifetimes and rotational correlation times of dityrosine emission by time-correlated single-photon counting. We first noticed dityrosine fluorescence in samples of tyrosine and tyrosine dipeptides by its characteristic red-shifted emission at 400 to 430 nm. The longer rotational correlation time relative to tyrosine proved that this fluorescence emanated from a distinct species. Comparison with the fluorescence properties of synthesized dityrosine established the identity of the emitting species. Fluorescence intensity decays of dityrosine are generally characterized by two decay components, one with a lifetime in the range of 150 to 800 ps and another between 2.5 and 4.5 ns. We found no evidence for an excited-state reaction, since a rising phase (negative-amplitude component) was not observed. In the pH range from 4 to 10, two ground-state species exist in equilibrium with pK _a 7. Both species exhibit two fluorescence decays. The average fluorescence lifetime increases gradually with pH over the pH range from 4 to 10 and decreases at pH 2. Anisotropy decays were measured for dityrosine and the alanine–dityrosine–alanine and leucine–dityrosine–leucine dipeptides. The rotational correlation times of dityrosine and dityrosine dipeptides increase linearly with van der Waals volumes. The slope indicates a stronger solute–solvent interaction than predicted with stick boundary conditions. It is suggested that these interactions result from the presence of two zwitterionic pairs.     

Dynamics of molecular diffusion of rhodamine 6G in silica nanochannels

We describe a method to study diffusion of rhodamine 6G dye in single silica nanochannels using arrays of silica nanochannels. Dynamics of the molecules inside single nanochannel is found from the change of the dye concentration in solution with time. A 10(8) decrease in the dye diffusion coefficient relative to water was observed. In comparison to single fluorescent molecule studies, the presented method does not require fluorescence of the diffusing molecules.