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We present a molecular dynamics study of the low-temperature polymorph of silver closo-borate α-Ag2B12H12, which is a promising ionic conductor. By means of 1H and 11B nuclear magnetic resonance spectroscopy, we identified two dynamic processes in the system that involve the movements of B12H122- cages: fast rotations with an activation energy of 308 meV and tumbling of the cages at lower temperatures with an activation energy of 67 meV. Fast rotations are known to facilitate the diffusion of Ag+ ions (the activation energy of 482 meV for ion jumps was determined from solid-state ionic conductivity measurements) while the tumbling motions are likely made possible by either impurities or local disorder, allowing for easier reorientations of the boron cages.

The photoluminescent properties of selected metal closo-boranes have been assessed. Group 3 elements Sc, Y, and La as well as Li, Na, and Eu-based B 10 H 10 2- and B 12 H 12 2- compounds displayed photoluminescence in the ultraviolet (emission λ max ≈ 350 nm) that was not visible to the human eye, in contrast to previous reports. We attribute these earlier results to arachno-borane impurities, which are more readily observed due to their longer wavelength emission spectra.

Efficient development of new solid-state electrolytes based on closo-borates, an emerging class of ion conductors, relies on a thorough understanding of these materials, including their crystal structures and derived trends thereof. Here we present the syntheses and crystal structures of the first anhydrous decahydro-closo-decaborates of divalent cations (Sr2+ and Mn2+), expanding the library of available closo-borate compounds. The structures of the two compounds are highly dissimilar, displaying a distorted bcc- or helical anion sublattice, as well as a tetrahedral or trigonal coordination environment of the cation in SrB10H10 and MnB10H10, respectively. Crystal structures of two hydrates of the compounds were also solved, SrB10H10∙4H2O and MnB10H10∙6H2O, and the (di)hydrogen bond network was investigated based on their structures optimized by density functional theory. The characterization of these new compounds, as well as the insight provided by comparing with previously reported closo-borates, are useful for designing new materials with high ionic conductivity or other desirable properties by revealing which compounds are likely to have compatible crystal structures, e.g. for aliovalent substitution.

Boron and hydrogen have a rich chemistry that has attracted a significant, but mainly academic, interest during the past century. However, research over the past decades has revealed new applications for metal boranes including their implementation as ‘energy materials’ and as neutron capture agents in cancer treatment. The energy applications involve the use of boron–hydrogen compounds as ion-conductors for batteries, as hydrogen storage materials, or even rocket fuels. The intensive research focus on metal borohydrides in the early 21st century has recently broadened to encompass higher metal boranes such as metal–B 10 H 10 's and B 12 H 12 's. This review summarizes the recent breakthroughs in the area of higher metal boranes in these last few years, in addition to highlighting core research from the mid-20th century.

Metal closo-boranes show remarkable thermal and chemical stabilities, making them appealing candidates for a wide range of applications, such as electrolytes in electrochemical batteries and ammonia storage for indirect hydrogen storage. Furthermore, owing to the large size and nonspherical geometry of the anion (e.g., B10H10 2- or B12H12 2-), metal closo-boranes display a rich structural diversity and thermal polymorphism. Here, we present the synthesis, characterization, and structural determination of the ammoniated metal closo-boranes, Na2B10H10·nNH3 (n = 1, 2). The thermal decomposition of Na2B10H10·2NH3 was investigated with synchrotron radiation in situ powder X-ray diffraction and simultaneous thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry, revealing a reversible ammonia storage capacity of 15 wt % below 150 °C. Additionally, ionic conductivities of 2.7 × 10-8 (RT) and 4.7 × 10-8 S/cm (30 °C) for Na2B10H10·2NH3 and Na2B10H10·NH3, respectively, were measured with electrochemical impedance spectroscopy. A lower Na+ conductivity compared to the parent compound, Na2B10H10, is explained by an anchoring effect of ammonia in the rigid framework of the B10H10 2--anions.

We investigated molecular dynamics in two ammonium borane systems from the group of promising ion conductors. The investigation was performed by means of H-1 and B-11 NMR spectroscopy and spin-lattice relaxation techniques. We identified two reorientational processes, the rotations of NH4 units that are present already at low temperatures and rotations of large boron cages, B10H10 or B12H12, which are thermally activated and become prominent above 250 K. Activation energies for these processes were determined. In addition, solid-state ion conductivity measurements were conducted to determine poor NH4+ conductivity of both systems.

The hydrogen absorption properties of metal closo-borate/metal hydride composites, M2B10H10-8MH and M2B12H12-10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H2) = 400 bar and by ex situ hydrogen absorption measurements at p(H2) = 526 to 998 bar. The in situ experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH4) are formed. On the contrary, the M2B12H12-10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H2) = 537 to 970 bar. 11B MAS NMR of the M2B10H10-8MH composites reveal that the molar ratio of LiBH4 or NaBH4 and the remaining B species is 1 : 0.63 and 1 : 0.21, respectively. Solution and solid-state 11B NMR spectra reveal new intermediates with a B : H ratio close to 1 : 1. Our results indicate that the M2B10H10 (M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B12H12]2- composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage.

Physical chemistry, chemical physics 20(23), 16266 - 16275 (2018). doi:10.1039/C7CP07776A, The hydrogen absorption properties of metal closo-borate/metal hydride composites, M$_2$B$_{10}$H$_{10}$–8MH and M$_2$B$_{12}$H$_{12}$–10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H$_2$) = 400 bar and by ex situ hydrogen absorption measurements at p(H$_2$) = 526 to 998 bar. The in situ experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH$_4$) are formed. On the contrary, the M$_2$B$_{12}$H$_{12}$–10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H$_2$) = 537 to 970 bar. 11B MAS NMR of the M$_2$B$_{10}$H$_{10}$–8MH composites reveal that the molar ratio of LiBH4 or NaBH$_4$ and the remaining B species is 1 : 0.63 and 1 : 0.21, respectively. Solution and solid-state 11B NMR spectra reveal new intermediates with a B : H ratio close to 1 : 1. Our results indicate that the M$_2$B$_{10}$H$_{10}$ (M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B$_{12}$H$_{12}$]$^{2-}$ composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage., Published by RSC Publ., Cambridge

A series of ammine metal-dodecahydro-closo-dodecaboranes, M

A series of ammine metal-dodecahydro-closo-dodecaboranes, MxB12H12·nNH3 (M = Li, Na, Ca) were synthesized and their structural and thermal properties studied with in situ time-resolved synchrotron radiation powder X-ray diffraction, thermal analysis, Fourier transformed infrared spectroscopy, and temperature-programmed photographic analysis. The synthesized compounds, Li2B12H12·7NH3, Na2B12H12·4NH3 and CaB12H12·6NH3, contain high amounts of NH3, 43.3, 26.6 and 35.9 wt% NH3, respectively, which can be released and absorbed reversibly at moderate conditions without decomposition, thereby making the closo-boranes favorable 'host' materials for ammonia or indirect hydrogen storage in the solid state. In this work, fifteen new ammine metal dodecahydro-closo-dodecaborane compounds are observed by powder X-ray diffraction, of which six are structurally characterized, Li2B12H12·4NH3, Li2B12H12·2NH3, Na2B12H12·4NH3, Na2B12H12·2NH3, CaB12H12·4NH3 and CaB12H12·3NH3. Li2B12H12·4NH3 and Na2B12H12·4NH3 are isostructural and monoclinic (P21/n) whereas Na2B12H12·2NH3 and CaB12H12·3NH3 are both trigonal with space groups P3m1 and R3c, respectively. Generally, coordination between the metal and the icosahedral closo-borane anion is diverse and includes point sharing, edge sharing, or face sharing, while coordination of ammonia always occurs via the lone pair on nitrogen to the metal. Furthermore, a liquid intermediate is observed during heating of Li2B12H12·7NH3. This work provides deeper insight into the structural, physical, and chemical properties related to thermal decomposition and possible ammonia and hydrogen storage.

Chemistry of materials 29(8), 3423 - 3430 (2017). doi:10.1021/acs.chemmater.6b04797, Sodium compounds containing large weakly coordinating anions are explored as ion conductors. The halogenated sodium-closo-dodecaboranes (Na$_2$B$_{12}$Cl$_{12}$, Na$_2$B$_{12}$Br$_{12}$, and Na$_2$B$_{12}$I$_{12}$) are all isostructural (Pa3̅) at room temperature. These compounds undergo an order–disorder polymorphic transition to Fm3̅m where Na+ partially occupies two crystallographic sites and [B$_{12}$X$_{2}$]$^{2–}$ anions undergo reorientational motion. These dynamic structural properties promote extreme Na+ ion conductivity up to 0.162 S/cm at elevated temperature (500 °C). The polymorphic transition temperatures increase down the halogen group (Cl < Br < I). These temperatures are much higher (475, 525, and 570 °C) than for Na$_{2}$B$_{12}$H$_{12}$ (266 °C), which could be related to increasing anion size, mass, and the anisotropic electron density in the covalently bound halogens (B–X). The halogens may form Na–X interactions with increasing strength and directionality, which restrict dynamic motion until high temperature. The halogenated sodium-closo-dodecaboranes demonstrate excellent thermal stabilities (up to 500 °C) and may facilitate the development of new high temperature ion conductors., Published by American Chemical Society, Washington, DC

X-ray diffraction is a superior technique for structural char- acterization of crystalline matter. Here we review the use of in situ powder X-ray diffraction (PXD) mainly for real-time studies of solid- gas reactions, data analysis and the extraction of valuable knowledge of structural, chemical and physical properties. Furthermore, the dif- fraction data may also provide knowledge on reaction mechanisms, kinetics and thermodynamic properties. Thus, in situ PXD simulta- neously provides properties as a function of pressure, temperature and/ or time at different length scales, i.e. nanoscale structural data and bulk

The detailed mechanism of hydrogen release in LiBH4–MgH2–Al composites of molar ratios 4 : 1 : 1 and 4 : 1 : 5 are investigated during multiple cycles of hydrogen release and uptake. This study combines information from several methods, i.e., in situ synchrotron radiation powder X-ray diffraction, 11B magic-angle spinning (MAS) NMR, Sievert's measurements, Fourier transform infrared spectroscopy and simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy. The composites of LiBH4–MgH2–Al are compared with the behavior of the LiBH4–Al and LiBH4–MgH2 systems. The decomposition pathway of the LiBH4–MgH2–Al system is different for the two investigated molar ratios, although it ultimately results in the formation of LiAl, MgxAl1−xB2 and Li2B12H12 in both cases. For the 4 : 1 : 1-molar ratio, Mg0.9Al0.1 and Mg17Al12 are observed as intermediates. However, only Mg is observed as an intermediate in the 4 : 1 : 5-sample, which may be due to an earlier formation of MgxAl1−xB2, reflecting the complex chemistry of Al–Mg phases. Hydrogen release and uptake reveals a decrease in the hydrogen storage capacity upon cycling. This loss reflects the formation of Li2B12H12 as observed by 11B NMR and infrared spectroscopy for the cycled samples. Furthermore, it is shown that the Li2B12H12 formation can be limited significantly by applying moderate hydrogen partial pressure during decomposition.

[1] Up to a few years ago, terrestrial gamma ray flashes (TGFs) were only observed by spaceborne instruments. The aircraft campaign ADELE was able to observe one TGF, and more attempts on aircraft observations are planned. There is also a planned campaign with stratospheric balloons, COBRAT. In this context an important question that arises is what count rates we can expect and how these estimates are affected by the initial properties of the TGFs. Based on simulations of photon propagation in air we find the photon fluence at different observation points at aircraft and balloon altitudes. The observed fluence is highly affected by the initial parameters of the simulated TGFs. One of the most important parameters is the number of initial photons in a TGF. In this paper, we give a semi-analytical approach to find the initial number of photons with an order of magnitude accuracy. The resulting number varies over several orders of magnitude, depending mostly on the production altitude of the TGF. The initial production altitude is also one of the main parameters in the simulations. Given the same number of initial photons, the fluence at aircraft and balloon altitude from a TGF produced at 10 km altitude is 2–3 orders of magnitude smaller then a TGF originating from 20 km altitude. Other important parameters are altitude distribution, angular distribution and amount of feedback. The differences in altitude, altitude distribution and amount of feedback are especially important for the fluence of photons observed at altitudes less than 20 km, and for instruments with a low-energy threshold larger than 100 keV. We find that the maximum radius of observation in 14 km for a TGF with the intensity of an average RHESSI TGF is smaller than the results reported by Smith et al. (2011), and our results support the conclusion in Gjesteland et al. (2012) and Ostgaard et al. (2012) that TGFs probably are a more common phenomenon than previously reported.

Maintenance of X-inactivation is achieved through a combination of different repressive mechanisms, thus perpetuating the silencing message through many cell generations. The second human X-Y pseudoautosomal region 2 (PAR2) is a useful model to explore the features and internal relationships of the epigenetic circuits involved in this phenomenon. Recently, we demonstrated that DNA methylation plays an essential role for the maintenance of X- and Y-inactivation of the PAR2 gene SYBL1; here we report that the silencing of the second repressed PAR2 gene, SPRY3, appears to be independent of DNA methylation. In contrast to SYBL1, the inactive X and Y alleles of SPRY3 are not reactivated in cells treated with a DNA methylation inhibitor and in cells from ICF (immunodeficiency, centromeric instability, facial anomalies) syndrome patients, which have mutations in the DNA methyltransferase gene DNMT3B. SPRY3 X- and Y-inactivation is associated with a differential enrichment of repressive histone modifications and the recruitment of Polycomb 2 group proteins compared to the active X allele. Another major factor in SPRY3 repression is late replication; the inactive X and Y alleles of SPRY3 have delayed replication relative to the active X allele, even in ICF syndrome cells where the closely linked SYBL1 gene is reactivated and advanced in replication. The relatively stable maintenance of SPRY3 silencing compared with SYBL1 suggests that genes without CpG islands may be less prone to reactivation than previously thought and that genes with CpG islands require promoter methylation as an additional layer of repression.

Eutectic melting, reactive hydride composites, and nanoconfinement have the potential to improve the reversible hydrogen storage properties in metal borohydrides. We study and compare the combined effect of all three methods on reversible hydrogen release and uptake of the eutectic melting lithium potassium borohydride system, 0.725LiBH4–0.275KBH4, with low melting temperature (Tm = 105 °C). The eutectic mixture and reactive hydride composites of the eutectic mixture with Mg or MgH2 are melt infiltrated into a CO2 activated nanoporous carbon scaffold, and their properties are compared to those of bulk samples. The decomposition of 0.725LiBH4–0.275KBH4 and the reactive hydride composites initiates simultaneously with the melting at 105 °C, but the decomposition remains slow until higher temperatures are reached (T > 300 °C). Eutectic melting appears to improve kinetics of hydrogen release and absorption, while nanoconfinement lowers the main hydrogen release temperature in the first cycle by up to 200 °C. ...

New sample environments and techniques specifically designed for in situ powder X-ray diffraction studies up to 1000 bar (1 bar = 105 Pa) gas pressure are reported and discussed. The cells can be utilized for multiple purposes in a range of research fields. Specifically, investigations of gas–solid reactions and sample handling under inert conditions are undertaken here. Sample containers allowing the introduction of gas from one or both ends are considered, enabling the possibility of flow-through studies. Various containment materials are evaluated, e.g. capillaries of single-crystal sapphire (Al2O3), quartz glass (SiO2), stainless steel (S316) and glassy carbon (Sigradur K), and burst pressures are calculated and tested for the different tube materials. In these studies, high hydrogen pressure is generated with a metal hydride hydrogen compressor mounted in a closed system, which allows reuse of the hydrogen gas. The advantages and design considerations of the in situ cells are discussed and their usage is illustrated by a case study.

Due to its thermodynamic properties and high reversibility, Ti doped sodium alanate is considered as a prototype hydrogen storage material. In this work we show how sodium alanate can be synthesized by reactive ball milling using aluminum particles obtained from recycled waste incineration slag. The synthesis was monitored with an in situ milling vial and characterized stepwise by PXD and DTA analyses. The sorption properties of the material were investigated using in situ synchrotron radiation PXD and volumetric analyses. A complete conversion of the starting reactants was obtained.

The detailed mechanism of hydrogen release in LiBH4-MgH2-Al composites of molar ratios 4 : 1 : 1 and 4 : 1 : 5 are investigated during multiple cycles of hydrogen release and uptake. This study combines information from several methods, i.e., in situ synchrotron radiation powder X-ray diffraction, (11)B magic-angle spinning (MAS) NMR, Sievert's measurements, Fourier transform infrared spectroscopy and simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy. The composites of LiBH4-MgH2-Al are compared with the behavior of the LiBH4-Al and LiBH4-MgH2 systems. The decomposition pathway of the LiBH4-MgH2-Al system is different for the two investigated molar ratios, although it ultimately results in the formation of LiAl, Mg(x)Al(1-x)B2 and Li2B12H12 in both cases. For the 4 : 1 : 1-molar ratio, Mg(0.9)Al(0.1) and Mg17Al12 are observed as intermediates. However, only Mg is observed as an intermediate in the 4 : 1 : 5-sample, which may be due to an earlier formation of Mg(x)Al(1-x)B2, reflecting the complex chemistry of Al-Mg phases. Hydrogen release and uptake reveals a decrease in the hydrogen storage capacity upon cycling. This loss reflects the formation of Li2B12H12 as observed by (11)B NMR and infrared spectroscopy for the cycled samples. Furthermore, it is shown that the Li2B12H12 formation can be limited significantly by applying moderate hydrogen partial pressure during decomposition.

The molecular basis for the cytogenetic appearance of chromosomal fragile sites is not yet understood. Late replication and further delay of replication at fragile sites expressing alleles has been observed for FRAXA, FRAXE and FRA3B fragile site loci. We analysed the timing of replication at the FRA10B and FRA16B loci to determine whether late replication is a feature which is shared by all fragile sites and, therefore, is a necessary condition for chromosomal fragile site expression. The FRA10B locus was located in a transitional region between early and late zones of replication. Fragile and non-fragile alleles exhibit a similar replication pattern proximal to the repeat, but fragile alleles are delayed relative to non-fragile ones on the distal side. Although fragility at FRA10B appears to be caused by expansion of an AT-rich repeat in the region, replication time near the repeat was similar in fragile and non-fragile alleles. The FRA16B locus was late replicating and appeared to replicate even later on fragile chromosomes. While these observations are compatible with the hypothesis that delayed replication may play a role in fragile site expression, they suggest that replication delay may not need to occur at the expanded repeat region itself in order to be permissive for fragility.

DNA methylation is an important regulator of genetic information in species ranging from bacteria to humans. DNA methylation appears to be critical for mammalian development because mice nullizygous for a targeted disruption of the DNMT1 DNA methyltransferase die at an early embryonic stage. No DNA methyltransferase mutations have been reported in humans until now. We describe here the first example of naturally occurring mutations in a mammalian DNA methyltransferase gene. These mutations occur in patients with a rare autosomal recessive disorder, which is termed the ICF syndrome, for immunodeficiency, centromeric instability, and facial anomalies. Centromeric instability of chromosomes 1, 9, and 16 is associated with abnormal hypomethylation of CpG sites in their pericentromeric satellite regions. We are able to complement this hypomethylation defect by somatic cell fusion to Chinese hamster ovary cells, suggesting that the ICF gene is conserved in the hamster and promotes de novo methylation. ICF has been localized to a 9-centimorgan region of chromosome 20 by homozygosity mapping. By searching for homologies to known DNA methyltransferases, we identified a genomic sequence in the ICF region that contains the homologue of the mouse Dnmt3b methyltransferase gene. Using the human sequence to screen ICF kindreds, we discovered mutations in four patients from three families. Mutations include two missense substitutions and a 3-aa insertion resulting from the creation of a novel 3′ splice acceptor. None of the mutations were found in over 200 normal chromosomes. We conclude that mutations in the DNMT3B are responsible for the ICF syndrome.

It is well established that the expression of simian virus 40 (SV40) early gene products causes oncogenic transformation of rodent cells. An important aspect of this process is the inactivation of the p53 and retinoblastoma (pRb) tumour suppressor proteins through interaction with the SV40 large tumour antigen (LT). In addition, the SV40 small tumour antigen (ST) may enhance LT induced transformation. Here we show that LT induces apoptotic cell death in rat embryo fibroblast (REF) cells and that ST functions to inhibit this effect by a mechanism which is different from other known anti-apoptotic proteins. Mutational analysis of LT indicates that mutants defective in the pRb-binding domain are unable to induce apoptosis whereas LT mutants defective in the p53-binding domain are still competent to induce apoptosis. Thus, interaction between LT and one or more pRb family members must occur for induction of apoptosis and that binding of p53 by LT is insufficient to inhibit LT induced apoptosis in REFs. The data presented herein suggest that the anti-apoptotic function of ST may explain, at least in part, how ST contributes to SV40 early region induced transformation of REF cells.

The XIST gene, expressed only from the inactive X chromosome, is a critical component of X inactivation. Although apparently unnecessary for maintenance of inactivation, XIST expression is thought to be sufficient for inactivation of genes in cis even when XIST is located abnormally on another chromosome. This repression appears to involve the association of XIST RNA with the chromosome from which it is expressed. Reactivated genes on the inactive X chromosome, however, maintain expression in several somatic cell hybrid lines with stable expression of XIST . We describe here another example of an XIST -expressing human–hamster hybrid that lacks X-linked gene repression in which the human XIST gene present on an active X chromosome was reactivated by treatment with 5-aza-2′-deoxycytidine. These data raise the possibility that human XIST RNA does not function properly in human–rodent somatic cell hybrids. As part of our approach to address this question, we reactivated the XIST gene in normal male fibroblasts and then compared their patterns of XIST RNA localization by subcellular fractionation and in situ hybridization with those of hybrid cells. Although XIST RNA is nuclear in all cell types, we found that the in situ signals are much more diffuse in hybrids than in human cells. These data suggest that hybrids lack components needed for XIST localization and, presumably, XIST-mediated gene repression.

The timing of DNA replication in the Xq27 portion of the human X chromosome was studied in cells derived from normal and fragile X males to further characterize the replication delay on fragile X chromosomes. By examining a number of sequence-tagged sites (STSs) that span several megabases of Xq27, we found this portion of the normal active X chromosome to be composed of two large zones with different replication times in fibroblasts, lymphocytes, and lymphoblastoid cells. The centromere-proximal zone replicates very late in S, whereas the distal zone normally replicates somewhat earlier and contains FMR1 , the gene responsible for fragile X syndrome when mutated. Our analysis of the region of delayed replication in fragile X cells indicates that it extends at least 400 kb 5′ of FMR1 and appears to merge with the normal zone of very late replication in proximal Xq27. The distal border of delayed replication varies among different fragile X males, thereby defining three replicon-sized domains that can be affected in fragile X syndrome. The distal boundary of the largest region of delayed replication is located between 350 and 600 kb 3′ of FMR1 . This example of variable spreading of late replication into multiple replicons in fragile X provides a model for the spread of inactivation associated with position-effect variegation or X chromosome inactivation.

A detailed investigation of the decomposition reactions and decay in the hydrogen storage capacity during repeated hydrogen release and uptake cycles for the reactive composite LiBH 4-Al (2:3) is presented. Furthermore, the influence of a titanium boride, TiB 2, additive is investigated. The study combines information from multiple techniques: in situ synchrotron radiation powder X-ray diffraction, Sieverts measurements, simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and Raman spectroscopy. The decomposition of LiBH 4-Al results in the formation of LiAl, AlB 2, and Li 2B 12H 12 via several reactions and intermediate compounds. The TiB 2 additive appears to have a limited effect on the decomposition pathway of the samples, but seems to facilitate formation of intermediate species at lower temperatures compared to the sample without additive. Solid solutions of Li xAl 1-xB 2 or Al 1-xB 2 are observed during decomposition and from Rietveld refinement the composition of the solid solution is estimated to be Li 0.22Al 0.78B 2. The intercalation of Li in the AlB 2 structure is further investigated by 11B and 27Al MAS NMR spectra of the LiH-AlB 2 and AlB 2 samples (presented in Supporting Information). Hydrogen release and uptake for LiBH 4-Al reveals a significant loss in the hydrogen storage capacity, that is, after four cycles a capacity of about 45% remains, and after 10 cycles, the capacity is degraded to approximately 15% of the theoretically available hydrogen content. This capacity loss may be due to the formation of Li 2B 12H 12, as observed by 11B MAS NMR and Raman spectroscopy. Formation of Li 2B 12H 12 has previously been observed during the decomposition of LiBH 4, but it has not been reported earlier in the LiBH 4-Al (2:3) system.

[1] In this paper we use the fluence distributions observed by two different instruments, RHESSI and Fermi GBM, corrected for the effects of their different orbits, combined with their different daily TGF detection rates and their relative sensitivities to make an estimate of the true fluence distribution of TGFs as measured at satellite altitudes. The estimate is then used to calculate the dead-time loss for an average TGF measured by RHESSI. An independent estimate of RHESSI dead-time loss and true fluence distribution is obtained from a Monte Carlo (MC) simulation in order to evaluate the consistency of our results. The two methods give RHESSI dead-time losses of 24–26% for average fluence of 33–35 counts. Assuming a sharp cut-off the true TGF fluence distribution is found to follow a power law with λ = 2.3 ± 0.2 down to ∼5/600 of the detection threshold of RHESSI. This corresponds to a lowest number of electrons produced in a TGF of ∼1014 and a global production rate within ±38° latitude of 50000 TGFs/day or about 35 TGFs every minute, which is 2% of all IC lightning. If a more realistic distribution with a roll-off below 1/3 (or higher) of the RHESSI lower detection threshold with a true distribution with λ ≤ 1.7 that corresponds to a source distribution with λ ≤ 1.3 is considered, we can not rule out that all discharges produce TGFs. In that case the lowest number of total electrons produced in a TGF is ∼1012.

Reactivation of the hypoxanthine phosphoribosyltransferase (HPRT) gene on an inactive human X chromosome in a somatic cell hybrid was analyzed following exposure to 5-aza-2'-deoxycytidine. Hemimethylation and chromatin hypersensitivity in the 5' CpG island appeared by 6 h after exposure and continued to increase for 24 h in an exponentially growing cell culture. These results imply that the conformation of inactive chromatin requires a symmetrically methylated 5' G+C-rich promoter region. In addition, quantitative analysis of the time course patterns suggest that chromatin sensitivity changes may depend on strand-specific demethylation. Symmetrically demethylated DNA was first detected at 24 h and continued to increase until 48 h. HPRT mRNA was first detected at 24 h and increased in a biphasic pattern until 48 h. These results suggest that hemimethylation permits nuclease attack but not transcription factor binding, which requires symmetrically demethylated DNA. We also show that in G1-arrested cells, 5-aza-2'-deoxycytidine has no effect on methylation, chromatin conformation, or transcription. We conclude that reactivation of the HPRT gene present on the inactive X chromosome of a somatic cell hybrid involves the initial events of DNA hemimethylation and chromatin hypersensitivity at the 5' CpG island, followed by symmetrical demethylation and transcriptional reactivation.

Reactivation of the hypoxanthine phosphoribosyltransferase (HPRT) gene on an inactive human X chromosome in a somatic cell hybrid was analyzed following exposure to 5-aza-2'-deoxycytidine. Hemimethylation and chromatin hypersensitivity in the 5' CpG island appeared by 6 h after exposure and continued to increase for 24 h in an exponentially growing cell culture. These results imply that the conformation of inactive chromatin requires a symmetrically methylated 5' G+C-rich promoter region. In addition, quantitative analysis of the time course patterns suggest that chromatin sensitivity changes may depend on strand-specific demethylation. Symmetrically demethylated DNA was first detected at 24 h and continued to increase until 48 h. HPRT mRNA was first detected at 24 h and increased in a biphasic pattern until 48 h. These results suggest that hemimethylation permits nuclease attack but not transcription factor binding, which requires symmetrically demethylated DNA. We also show that in G1-arrested cells, 5-aza-2'-deoxycytidine has no effect on methylation, chromatin conformation, or transcription. We conclude that reactivation of the HPRT gene present on the inactive X chromosome of a somatic cell hybrid involves the initial events of DNA hemimethylation and chromatin hypersensitivity at the 5' CpG island, followed by symmetrical demethylation and transcriptional reactivation.

The fragile-X syndrome of mental retardation is associated with an expansion in the number of CGG repeats present in the FMR1 gene. The repeat region is within sequences characteristic of a CpG island. Methylation of CpG dinucleotides that are 5' to the CGG repeat has been shown to occur on the inactive X chromosome of normal females and on the X chromosome of affected fragile-X males, and is correlated with silencing of the FMR1 gene. The methylation status of CpG sites 3' to the repeat and within the repeat itself has not previously been reported. We have used two methylation-sensitive restriction enzymes, AciI and Fnu4HI, to further characterize the methylation pattern of the FMR1 CpG island in normal individuals and in those carrying fragile-X mutations. Our results indicate that: (i) CpG dinucleotides on the 3' side of the CGG repeat are part of the CpG island that is methylated during inactivation of a normal X chromosome in females; (ii) the CGG repeats are also part of the CpG island and are extensively methylated as a result of normal X-chromosome inactivation; (iii) similar to normal males, unaffected fragile-X males with small CGG expansions are unmethylated in the CpG island; for affected males, the patterns of methylation are similar to those of a normal, inactive X chromosome; (iv) in contrast to the partial methylation observed for certain sites in lymphocyte DNA, complete methylation was observed in DNA from cell lines containing either a normal inactive X chromosome or a fragile-X chromosome from an affected male.(ABSTRACT TRUNCATED AT 250 WORDS)

The major techniques presently used for characterising solid surfaces are summarized. These techniques include interactions (or release) of photons, electrons and neutral or charged atoms and molecules, and other miscellaneous methods which may be applied to single crystal surfaces or polycrystalline material

The molecular basis for the cytogenetic appearance of chromosomal fragile sites is not yet understood. Late replication and further delay of replication at fragile sites expressing alleles has been observed for FRAXA, FRAXE and FRA3B fragile site loci. We analysed the timing of replication at the FRA10B and FRA16B loci to determine whether late replication is a feature which is shared by all fragile sites and, therefore, is a necessary condition for chromosomal fragile site expression. The FRA10B locus was located in a transitional region between early and late zones of replication. Fragile and non-fragile alleles exhibit a similar replication pattern proximal to the repeat but fragile alleles are delayed relative to non-fragile ones on the distal side. Although fragility at FRA10B appears to be caused by expansion of an AT-rich repeat in the region, replication time near the repeat was similar in fragile and non-fragile alleles. The FRA16B locus was late replicating and appeared to replicate even later on fragile chromosomes. While these observations are compatible with the hypothesis that delayed replication may play a role in fragile site expression, they suggest that replication delay may not need to occur at the expanded repeat region itself in order to be permissive for fragility.

ICF syndrome is a rare autosomal recessive immunoglobulin deficiency, sometimes combined with defective cellular immunity. Other features that are frequently observed in ICF syndrome patients include facial dysmorphism, developmental delay, and recurrent infections. The most diagnostic feature of ICF syndrome is the branching of chromosomes 1, 9, and 16 due to pericentromeric instability. Positional candidate cloning recently discovered the de novo DNA methyltransferase 3B (DNMT3B) as the responsible gene by identifying seven different mutations in nine ICF patients. DNMT3B specifically methylates repeat sequences adjacent to the centromeres of chromosome 1, 9, and 16. Our panel of 14 ICF patients was subjected to mutation analysis in the DNMT3B gene. Mutations in DNMT3B were discovered in only nine of our 14 ICF patients. Moreover, two ICF patients from consanguineous families who did not show autozygosity (i.e. homozygosity by descent) for the DNMT3B locus did not reveal DNMT3B mutations, suggesting genetic heterogeneity for this disease. Mutation analysis revealed 11 different mutations, including seven novel ones: eight different missense mutations, two different nonsense mutations, and a splice-site mutation leading to the insertion of three aa's. The missense mutations occurred in or near the catalytic domain of DNMT3B protein, indicating a possible interference with the normal functioning of the enzyme. However, none of the ICF patients was homozygous for a nonsense allele, suggesting that absence of this enzyme is not compatible with life. Compound heterozygosity for a missense and a nonsense mutation did not seem to correlate with a more severe phenotype.

p53 is known to suppress oncogenic cell transformation, inhibit cell growth, induce apoptosis and activate and repress gene transcription. To investigate the relationships between these functions, we have examined various mutant forms of p53 for their abilities to perform each activity. This study has shown that growth inhibition is not a prerequisite for apoptotic cell death as these two functions are separate and alternative activities of p53. Additionally, we have demonstrated that the ability of p53 to suppress transformation (by adenovirus E1a and activated Ras) correlates with its ability to induce apoptosis and not with its ability to inhibit cell growth. Although p53 is thought to inhibit growth through the transactivation of p21WAFI, our study has demonstrated that transcriptional activation and repression are neither sufficient nor necessary for growth inhibition. This indicates that p53 has more than one mechanism for inhibiting cell growth and that another type of biochemical function must be involved. Furthermore, we have shown that transcriptional activation and repression may each be necessary, and the combination of these activities may even be sufficient, for p53-dependent apoptosis. In summary, our results have provided new information about the cellular and biochemical mechanisms through which p53 acts as a tumor suppressor.

The timing of DNA replication appears to be an important epigenetic regulator of gene expression during development. Replication of active genes in expressing tissues occurs earlier than does replication of their inactive counterparts in nonexpressing tissues. This pattern is also observed for active and inactive alleles present in the same cell, as exemplified by genes subject to X chromosome inactivation in females. We find that the replication timing of the X-linked XIST gene in normal human fibroblasts provides a striking exception to this well-established pattern. Within the same cell, the expressed allele of XIST replicates late in S phase and the silent allele replicates early. This 'reverse' replication timing may have functional significance with respect to XIST or could be a passive consequence of the replication timing requirements of neighboring genes that are subject to X chromosome inactivation. Our finding of early replication for XIST in male fibroblasts contrasts with a report of late replication in such cells as determined by an in situ hybridization method [Torchia et al., (1994) Am. J. Hum. Genet. 55, 96-104]. We propose that our data and those obtained by the in situ method can be accommodated by the existence of structural features that differ between the silent and expressed alleles of XIST. Similar features may be important determinants of the replication asynchrony found by the in situ method for other genes subject to monoallelic expression.

The 5' region of the gene encoding human X chromosome-linked phosphoglycerate kinase 1 (PGK1) is a promoter-containing CpG island known to be methylated at 119 of 121 CpG dinucleotides in a 450-base-pair region on the inactive human X chromosome in the hamster-human cell line X8-6T2. Here we report the use of polymerase chain reaction-aided genomic sequencing to determine the complete methylation pattern of this region in clones derived from X8-6T2 cells after treatment with the methylation inhibitor 5-azacytidine. We find (i) a clone showing full expression of human phosphoglycerate kinase is fully unmethylated in this region; (ii) clones not expressing human phosphoglycerate kinase remain methylated at approximately 50% of CpG sites, with a pattern of interspersed methylated (M) and unmethylated (U) sites different for each clone; (iii) singles, defined as M-U-M or U-M-U, are common; and (iv) a few CpG sites are partially methylated. The data are interpreted according to a model of multiple, autonomous CpG sites, and estimates are made for two key parameters, maintenance efficiency (Em approximately 99.9% per site per generation) and de novo methylation efficiency (Ed approximately 5%). These parameter values and the hypothesis that several independent sites must be unmethylated for transcription can explain the stable maintenance of X chromosome inactivation. We also consider how the active region is kept free of methylation and suggest that transcription inhibits methylation by decreasing Em so that methylation cannot be maintained. Thus, multiple CpG sites, independent with respect to a dynamic methylation system, can stabilize two alternative states of methylation and transcription.