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<jats:p> Nitrous oxide is a greenhouse gas that also plays a role in the cycling of stratospheric ozone. Air samples from the lower stratosphere exhibit <jats:sup>15</jats:sup> N/ <jats:sup>14</jats:sup> N and <jats:sup>18</jats:sup> O/ <jats:sup>16</jats:sup> O enrichment in nitrous oxide, which can be accounted for with a simple model describing an irreversible destruction process. The observed enrichments are quite large and incompatible with those determined for the main stratospheric nitrous oxide loss processes of photolysis and reaction with excited atomic oxygen. Thus, although no stratospheric source needs to be invoked, the data indicate that present understanding of stratospheric nitrous oxide chemistry is incomplete. </jats:p>

<jats:p> We propose an isotopic fractionation mechanism, based on photolytic destruction, to explain the <jats:sup>15</jats:sup> N/ <jats:sup>14</jats:sup> N and <jats:sup>18</jats:sup> O/ <jats:sup>16</jats:sup> O fractionation of stratospheric nitrous oxide (N <jats:sub>2</jats:sub> O) and reconcile laboratory experiments with atmospheric observations. The theory predicts that (i) the isotopomers <jats:sup>15</jats:sup> N <jats:sup>14</jats:sup> N <jats:sup>16</jats:sup> O and <jats:sup>14</jats:sup> N <jats:sup>15</jats:sup> N <jats:sup>16</jats:sup> O have very different isotopic fractionations in the stratosphere, and (ii) laboratory photolysis experiments conducted at 205 nanometers should better simulate the observed isotopic fractionation of stratospheric N <jats:sub>2</jats:sub> O. Modeling results indicate that there is no compelling reason to invoke a significant chemical source of N <jats:sub>2</jats:sub> O in the middle atmosphere and that individual N <jats:sub>2</jats:sub> O isotopomers might be useful tracers of stratospheric air parcel motion. </jats:p>

<jats:p> Because of dissolution of lighter elements such as sulfur, carbon, hydrogen, and oxygen, Earth's outer core is about 10 percent less dense than molten iron at the relevant pressure and temperature conditions. To determine whether hydrogen can account for a major part of the density deficit and is therefore an important constituent in the molten iron outer core, the hydrogen concentration in molten iron was measured at 7.5 gigapascals. From these measurements, the metal-silicate melt partitioning coefficient of hydrogen was determined as a function of temperature. If the magma ocean of primordial Earth was hydrous, more than 95 mole percent of H <jats:sub>2</jats:sub> O in this ocean should have reacted with iron to form FeH <jats:italic>x</jats:italic> , and about 60 percent of the density deficit is reconciled by adding hydrogen to the core. </jats:p>

<jats:p> The ground and excited state spectra of a semiconductor quantum dot with successive electron occupancy were studied with linear and nonlinear magnetoconductance measurements. A direct correlation was observed between the <jats:italic>m</jats:italic> th excited state of the <jats:italic>N</jats:italic> -electron system and the ground state of the ( <jats:italic>N</jats:italic> + <jats:italic>m</jats:italic> )-electron system for <jats:italic>m</jats:italic> up to 4. The results are consistent with a single-particle picture in which a fixed spectrum of energy levels is successively filled, except for a notable absence of spin degeneracy. Further departures from the single-particle picture due to electron-electron interaction were also observed. Magnetoconductance fluctuations of ground states show anticrossings where wave function characteristics are exchanged between adjacent levels. </jats:p>

<jats:p>Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow “atomic physics” experiments to be performed in magnetic field regimes not accessible for atoms.</jats:p>

<jats:p>Atomically precise quantum dots of mesoscopic size have been fabricated in the gallium arsenide–aluminum gallium arsenide material system by cleaved edge overgrowth, with a high degree of control over shape, composition, and position. The formation of bonding and antibonding states between two such “artificial atoms” was studied as a function of quantum dot separation by microscopic photoluminescence (PL) spectroscopy. The coupling strength within these “artificial molecules” is characterized by a systematic dependence of the separation of the bonding and antibonding levels, and of the PL linewidth, on the “interatomic” distance. This model system opens new insights into the physics of coupled quantum objects.</jats:p>

<jats:p> Organically modified aluminosilicate mesostructures were synthesized from two metal alkoxides with the use of poly(isoprene- <jats:italic>b</jats:italic> -ethyleneoxide) block copolymers (PI-b-PEO) as the structure-directing molecules. By increasing the fraction of the inorganic precursors with respect to the polymer, morphologies expected from the phase diagrams of diblock copolymers were obtained. The length scale of the microstructures and the state of alignment were varied using concepts known from the study of block copolymers. These results suggest that the use of higher molecular weight block copolymer mesophases instead of conventional low–molecular weight surfactants may provide a simple, easily controlled pathway for the preparation of various silica-type mesostructures that extends the accessible length scale of these structures by about an order of magnitude. </jats:p>

<jats:p>Experiments in Serengeti National Park, Tanzania, provide direct evidence that large, free-ranging mammalian grazers accelerate nutrient cycling in a natural ecosystem in a way that enhances their own carrying capacity. Both nitrogen and sodium were at considerably higher plant-available levels in soils of highly grazed sites than in soils of nearby areas where animal density is sparse. Fencing that uncoupled grazers and soils indicated that the animals promote nitrogen availability on soils of inherently similar fertility and select sites of higher sodium availability as well as enhancing that availability.</jats:p>

<jats:p> Sequence analysis of the 330-kilobase genome of the virus PBCV-1 that infects a chlorella-like green algae revealed an open reading frame, A98R, with similarity to several hyaluronan synthases. Hyaluronan is an essential polysaccharide found in higher animals as well as in a few pathogenic bacteria. Expression of the <jats:italic>A98R</jats:italic> gene product in <jats:italic>Escherichia coli</jats:italic> indicated that the recombinant protein is an authentic hyaluronan synthase. <jats:italic>A98R</jats:italic> is expressed early in PBCV-1 infection and hyaluronan is produced in infected algae. These results demonstrate that a virus can encode an enzyme capable of synthesizing a carbohydrate polymer and that hyaluronan exists outside of animals and their pathogens. </jats:p>

<jats:p>The signal transducer and activator of transcription–3 (Stat3) protein is activated by the interleukin 6 (IL-6) family of cytokines, epidermal growth factor, and leptin. A protein named PIAS3 (protein inhibitor of activated STAT) that binds to Stat3 was isolated and characterized. The association of PIAS3 with Stat3 in vivo was only observed in cells stimulated with ligands that cause the activation of Stat3. PIAS3 blocked the DNA-binding activity of Stat3 and inhibited Stat3-mediated gene activation. Although Stat1 is also phosphorylated in response to IL-6, PIAS3 did not interact with Stat1 or affect its DNA-binding or transcriptional activity. The results indicate that PIAS3 is a specific inhibitor of Stat3.</jats:p>