Persistence of the Isotopic Signature of Pentavalent Uranium in Magnetite

Uranium isotopic signatures can be harnessed to monitor the reductive remediation of subsurface contamination or to reconstruct paleo-redox environments. However, the mechanistic underpinnings of the isotope fractionation associated with U reduction remain poorly understood. Here, we present a coprecipitation study, in which hexavalent U (U(VI)) was reduced during the synthesis of magnetite and pentavalent U (U(V)) was the dominant species. The measured δ$^{238}$U values for unreduced U(VI) (∼−1.0‰), incorporated U (96 ± 2% U(V), ∼−0.1‰), and extracted surface U (mostly U(IV), ∼0.3‰) suggested the preferential accumulation of the heavy isotope in reduced species. Upon exposure of the U-magnetite coprecipitate to air, U(V) was partially reoxidized to U(VI) with no significant change in the δ$^{238}$U value. In contrast, anoxic amendment of a heavy isotope-doped U(VI) solution resulted in an increase in the δ$^{238}$U of the incorporated U species over time, suggesting an exchange between incorporated and surface/aqueous U. Overall, the results support the presence of persistent U(V) with a light isotope signature and suggest that the mineral dynamics of iron oxides may allow overprinting of the isotopic signature of incorporated U species. This work furthers the understanding of the isotope fractionation of U associated with iron oxides in both modern and paleo-environments

Heterochromatin Protein 1β (HP1β) has distinct functions and distinct nuclear distribution in pluripotent versus differentiated cells

Background: Pluripotent embryonic stem cells (ESCs) have the unique ability to differentiate into every cell type and to self-renew. These characteristics correlate with a distinct nuclear architecture, epigenetic signatures enriched for active chromatin marks and hyperdynamic binding of structural chromatin proteins. Recently, several chromatin-related proteins have been shown to regulate ESC pluripotency and/or differentiation, yet the role of the major heterochromatin proteins in pluripotency is unknown. Results: Here we identify Heterochromatin Protein 1β (HP1β) as an essential protein for proper differentiation, and, unexpectedly, for the maintenance of pluripotency in ESCs. In pluripotent and differentiated cells HP1β is differentially localized and differentially associated with chromatin. Deletion of HP1β, but not HP1aα, in ESCs provokes a loss of the morphological and proliferative characteristics of embryonic pluripotent cells, reduces expression of pluripotency factors and causes aberrant differentiation. However, in differentiated cells, loss of HP1β has the opposite effect, perturbing maintenance of the differentiation state and facilitating reprogramming to an induced pluripotent state. Microscopy, biochemical fractionation and chromatin immunoprecipitation reveal a diffuse nucleoplasmic distribution, weak association with chromatin and high expression levels for HP1β in ESCs. The minor fraction of HP1β that is chromatin-bound in ESCs is enriched within exons, unlike the situation in differentiated cells, where it binds heterochromatic satellite repeats and chromocenters. Conclusions: We demonstrate an unexpected duality in the role of HP1β: it is essential in ESCs for maintaining pluripotency, while it is required for proper differentiation in differentiated cells. Thus, HP1β function both depends on, and regulates, the pluripotent state

Structural, optical and crystal field analyses of undoped and Mn2+-doped ZnS nanoparticles synthesized via reverse micelle route

Zinc sulfide, both as a bulk material and in nanocrystalline form, is a valuable luminescent material with important applications. Doped ZnS nanoparticles of around 5 nm are the material of choice for optoelectronic applications running in the UV region owing to their significant quantum size effect. This paper concerns detailed structural, spectroscopic and crystal field studies of ZnS nanoparticles, both pure and doped with Mn2+ ions, successfully synthesized at room temperature using a simple reverse micelle technique in the Triton X-100/cyclohexane medium. The resulting ZnS sphalerite phase smallsize nanoparticles (3-5 nm) have a much larger energy band gap ( similar to 4.7 eV) than that reported for the bulk ZnS (3.6 eV), thus confirming a pronounced quantum confinement effect. The electron paramagnetic resonance data provided evidence for the existence of two distinct environments for Mn2+ ions: the interior (core) and near the surface of the nanoparticles. The presence of an Mn2+-characteristic orange emission centered at 600 nm confirmed that our sample's were properly doped with Mn2+ ions, as the T-4(1)->(6)A(1) radiation transition could arise only on the basis of Mn2+ ions incorporated into the ZnS nanoparticles. To the best of our knowledge, our finding include the longest decay time component for the orange emission ever observed, timed at about 3.3 ms. The experimental excitation spectra were analyzed and the transitions assigned using the exchange charge model of theory of crystal field, which allowed to calculate the energy level scheme of the Mn2+ ions. The results presented in this paper provide us with detailed information about the ZnS sphalerite nanocrystals studied and can be readily applied to other similar systems. (C) 2013 Elsevier B.V. All rights reserved

Increased Default Mode Network Connectivity In Individuals At High Familial Risk For Depression

Research into the pathophysiology of major depressive disorder (MDD) has focused largely on individuals already affected by MDD. Studies have thus been limited in their ability to disentangle effects that arise as a result of MDD from precursors of the disorder. By studying individuals at high familial risk for MDD, we aimed to identify potential biomarkers indexing risk for developing MDD, a critical step toward advancing prevention and early intervention. Using resting-state functional connectivity MRI (rs-fcMRI) and diffusion MRI (tractography), we examined connectivity within the default mode network (DMN) and between the DMN and the central executive network (CEN) in 111 individuals, aged 11–60 years, at high and low familial risk for depression. Study participants were part of a three-generation longitudinal, cohort study of familial depression. Based on rs-fcMRI, individuals at high vs low familial risk for depression showed increased DMN connectivity, as well as decreased DMN-CEN-negative connectivity. These findings remained significant after excluding individuals with a current or lifetime history of depression. Diffusion MRI measures based on tractography supported the findings of decreased DMN-CEN-negative connectivity. Path analyses indicated that decreased DMN-CEN-negative connectivity mediated a relationship between familial risk and a neuropsychological measure of impulsivity. Our findings suggest that DMN and DMN-CEN connectivity differ in those at high vs low risk for depression and thus suggest potential biomarkers for identifying individuals at risk for developing MDD