Ammonium fluoride's analogy to ice: Possibilities and limitations.

Ammonium fluoride, NH4F, is often seen as an analog to ice, with several of its solid phases closely resembling known ice phases. While its ionic and hydrogen-ordered nature puts topological constraints on the ice-like network structures it can form, it is not clear what consequences these constraints have for NH4F compound formation and evolution. Here, we explore computationally the reach and eventual limits of the ice analogy for ammonium fluoride. By combining data mining of known and hypothetical ice networks with crystal structure prediction and density functional calculations, we explore the high-pressure phase diagram of NH4F and host-guest compounds of its hydrides. Pure NH4F departs from ice-like behavior above 80 GPa with the emergence of close-packed ionic structures. The predicted stability of NH4F hydrides shows that NH4F can act as a host to small guest species, albeit in a topologically severely constraint configuration space. Finally, we explore the binary NH3-HF chemical space, where we find candidate structures for several unsolved polyfluoride phases; among them is the chemical analog to H2O2 dihydrate.

On a new nitrogen sX hydrate from ice XVII.

Recently, a new gas hydrate structure has been discovered. This structure, sX, is unique in a sense that it is so far the only gas hydrate with chiral channels. It is formed by hydrogen-water or carbon dioxide-water mixtures at pressures above 0.300 GPa, and it has been shown that it is the only clathrate hydrate that is refillable with hydrogen. This property makes it a possible storage material for gases. By analyzing neutron diffraction data and calculations based on density-functional theory, we show that sX is also refillable with nitrogen; the guest:host ratio will be shown to be 2.6(3). Furthermore, we report sX's decomposition behavior and give evidence that it undergoes several transitions into the exotic hydrates sH and sIII that have not been observed at these pressure and temperature conditions-before forming the stable nitrogen hydrate sII.

The role of joint engagement in the development of language in a community-derived sample of slow-to-talk children.

We explored whether supported (SJE) or coordinated joint engagement (CJE) between mothers recruited from the community and their 24-month-old children who were slow-to-talk at 18 months old were associated with child language scores at ages 24, 36, and 48 months (n = 197). We further explored whether SJE or CJE modified the concurrent positive associations between maternal responsive behaviours and language scores. Previous research has shown that SJE, maternal expansions, imitations, and responsive questions were associated with better language scores. Our main finding was that SJE but not CJE was consistently positively associated with 24- and 36-month-old expressive and receptive language scores, but not with 48-month-old language scores. SJE modified how expansions and imitations, but not responsive questions, were associated with language scores; the associations were evident in all but the highest levels of SJE. Further research is necessary to test these findings in other samples before clinical recommendations can be made.

Mutations of Arabidopsis thaliana that transform leaves into cotyledons.

We describe mutations of three genes in Arabidopsis thaliana-extra cotyledon1 (xtc1), extra cotyledon2 (xtc2), and altered meristem programming1 (amp1)-that transform leaves into cotyledons. In all three of these mutations, this transformation is associated with a change in the timing of events in embryogenesis. xtc1 and xtc2 delay the morphogenesis of the embryo proper at the globular-to-heart transition but permit the shoot apex to develop to an unusually advanced stage late in embryogenesis. Both mutations have little or no effect on seed maturation and do not affect the viability of the shoot or the rate of leaf initiation after germination. amp1 perturbs the pattern of cell division at an early globular stage, dramatically increases the size of the shoot apex and, like xtc1 and xtc2, produces enlarged leaf primordia during seed development. These unusual phenotypes suggest that these genes play important regulatory roles in embryogenesis and demonstrate that the development of the shoot apical meristem and the development of the embryo proper are regulated by independent processes that must be temporally coordinated to ensure normal organ identity.

Sulfur regulation of heparinase and sulfatases in Flavobacterium heparinum.

Sulfur regulation of heparinase synthesis and sulfatase synthesis was studied in Flavobacterium heparinum. Heparinase synthesis was strongly repressed by sulfate and L-cysteine, while the activity of this enzyme showed little or no inhibition by these compounds. Heparinase was synthesized in the absence of heparin when L-methionine was used as the sole sulfur source. The sulfatases produced by F. heparinum, which include the sulfatases involved in heparin catabolism, were also studied. At least some of the sulfatase activity was regulated by sulfur compounds in a manner similar to heparinase regulation. L-Cysteic acid and taurine were not suitable sulfur sources to support the growth of F. heparinum.