Field-induced microwave absorption in Fe3O4 nanoparticles and Fe3O4/polyaniline composites synthesized by different methods

Three kinds of nanoscale powders containing Fe₃O₄ nanoparticles (NPs) have been studied by ferromagnetic resonance (FMR): (i) Fe₃O₄ NPs grown and then covered with polyaniline (PANI), (ii) unclad Fe₃O₄ NPs, and (iii) Fe₃O₄ NPs grown “in situ” with the PANI. In every case, there is no low field microwave absorption, rather a single FMR line is observed. However, the half-power widths are of order of 1 kOe presumably due to a distribution of internal fields. For type I particles with a low concentration (below 40%) of Fe₃O₄, the observed resonance fields (H_r) are close to those expected for spheres with negligible magnetocrystalline anisotropy. For all other cases, H_r values are significantly lower. Such shortfalls can be roughly understood by invoking dipolar interactions between the grains, stresses frozen in grains during manufacture (method III), as well as anisotropy fields when the specimens are prepared in an aligning field.     

Complexes of 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (4,13-diaza-18-crown-6) with 2,5-pyridinedicarboxylic and 2,2′-dithiosalicylic acids

A 1:2:2 complex of 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane with 2,5-pyridinedicarboxylic acid and water (1) and its 1:1:1.75 complex with 2,2-dithiosalicylic acid and water (2) have been obtained and characterized by x-ray diffraction. C₂₆H₄₀N₄O₁₄ (1), triclinic, , a = 9.574(1) Å, b = 9.632(1) Å, c = 9.723(1) Å, = 112.70(1)°, = 91.07(1)°, = 115.45(1)°, V = 728.35(13) ų, Z = 1. C₂₆H_39.5N₂O_9.75S₂ (2), M = 600.22, triclinic, , a = 10.492(1) Å, b = 10.945(1) Å, c = 14.535(2) Å, = 102.74(1)°, = 109.08(1)°, = 90.68(1)°, V = 1532.2(3) ų, Z = 2. In complex 1, both N-atoms of the macrocyclic ring are protonated. The following types of H-bonding have been found: (1) between protonated aza groups of the macrocycle and ionized carboxylic groups; (2) between the protonated aza groups and N-atoms of the pyridine nuclei (D–A distance slightly exceeds 3 Å); (3) between water molecules and C=O groups of the non-ionized carboxylic groups; and (4) between the nonionized and ionized groups of the carboxylic acid. The above interactions give rise to the formation of a developed supramolecular network in the crystals of 1. In complex 2, despite the presence of several types of hydrogen bonds involving the aza crown, 2,2-dithiosalicylic acid and water, the aromatic anions are H-bound only to the two other components, and not to each other. The H-bonds found in complex 2 are between (1) one of the protonated aza groups and water, (2) protonated aza groups and O-atoms of the ionized carboxylic groups, (3) water molecule and carboxylic O-atom, (4) water molecule and sulfur atom, and (5) water molecule and O-atom of diaza-18-crown-6.     

Wavelet estimation of conditional density with truncated, censored and dependent data

In this paper we define a new nonlinear wavelet-based estimator of conditional density function for a random left truncation and right censoring model. We provide an asymptotic expression for the mean integrated squared error (MISE) of the estimator. It is assumed that the lifetime observations form a stationary α-mixing sequence. Unlike for kernel estimators, the MISE expression of the wavelet-based estimators is not affected by the presence of discontinuities in the curves. Also, asymptotic normality of the estimator is established.     

Dissociation of deprotonated microcystin variants by collision-induced dissociation following electrospray ionization

Microcystins (MC) are a family of hepatotoxic cyclic heptapeptides produced by a number of different cyanobacterial species. Considering the recent advances in the characterization of deprotonated peptides by mass spectrometry, the fragmentation behavior of four structurally related microcystin compounds was investigated using collision-induced dissociation (CID) experiments on an orbitrap mass spectrometer. It is demonstrated in this study that significant structural information can be obtained from the CID spectra of deprotonated microcystins. A predominant ring-opening reaction at the isoMeAsp residue, as well as two major complementary fragmentation pathways, was observed, reducing the complexity of the product ion spectra in comparison with spectra observed from protonated species. This proposed fragmentation behavior was applied to characterize [Leu(1)]MC-LR from a cyanobacterial cell extract. In conclusion, CID spectra of microcystins in the negative ion mode provide rich structurally informative mass spectra which greatly enhance confidence in structural assignments, in particular when combined with complementary positive ion CID spectra.     

Effects of spatial competition on the diversity of a quasispecies

The diversity harbored by populations of RNA viruses results from high mutation rates, as well as from the characteristics of the environment where they evolve. By means of a simple model for structured quasispecies, we quantify how competition for space among phenotypic types shapes their distribution at the mutation-selection equilibrium. We introduce a general framework to treat this problem and relate mutation rate and competition strength to the quasispecies composition. For diffusion limited competition, diversity typically increases and the asymptotic growth rate of the population diminishes as diffusion decreases. Limited mobility confers a relative advantage to worse competitors. The stationary state is characterized by an over-production of viral particles. Empirical data allow an estimation of mutation rates compatible with the diversity observed in viral populations infecting cellular monolayers.