Catálogo de publicaciones - revistas

<jats:p>The soft x-ray spectra of the Virgo and Coma clusters cannot be modeled by emission from the well-known hot intracluster medium attenuated by photoelectric absorption along the line of sight in our galaxy. If the excess soft emission is due to thermal emission in the clusters from gas at about 1 million kelvin, then the gas should be rapidly cooling. The high rate of cooling poses problems for the origin of the gas and its sink. A model in which the component is due to turbulent mixing layers around clouds scattered throughout the intracluster medium may explain the excess emission. The gas in the mixing layer is reheated after cooling, so that the total mass remains relatively small.</jats:p>

<jats:p>The instability resulting from the relative motion of newly picked-up cometary photoions and the solar wind generates lower hybrid waves that are capable of accelerating electrons to the keV range of energies. These electrons may be responsible for the x-rays from comet C/Hyakutake 1996 B2 seen by the Röntgen X-ray Satellite. The inferred x-ray photon power depends on the electron energy, with keV electrons providing values of photon power two orders of magnitude greater than 100 eV electrons. These observations and in particular, spectral resolution of the x-rays, should provide more insight into the composition of the comet.</jats:p>

<jats:p>A general method for combining existing algorithms into new programs that are unequivocally preferable to any of the component algorithms is presented. This method, based on notions of risk in economics, offers a computational portfolio design procedure that can be used for a wide range of problems. Tested by solving a canonical NP-complete problem, the method can be used for problems ranging from the combinatorics of DNA sequencing to the completion of tasks in environments with resource contention, such as the World Wide Web.</jats:p>

<jats:p>Time-resolved absorption spectroscopy on the femtosecond time scale has been used to monitor the earliest events associated with excited-state relaxation in tris-(2,2′-bipyridine)ruthenium(II). The data reveal dynamics associated with the temporal evolution of the Franck-Condon state to the lowest energy excited state of this molecule. The process is essentially complete in ∼300 femtoseconds after the initial excitation. This result is discussed with regard to reformulating long-held notions about excited-state relaxation, as well as its implication for the importance of non-equilibrium excited-state processes in understanding and designing molecular-based electron transfer, artificial photosynthetic, and photovoltaic assemblies in which compounds of this class are currently playing a key role.</jats:p>

<jats:p>Photopolymerizable diacrylate monomers dissolved in fluid-layer smectic A and smectic C liquid crystal (LC) hosts exhibited significant spatial segregation and orientation that depend strongly on monomer structure. Small, flexible monomers such as 1,6-hexanediol diacrylate (HDDA) oriented parallel to the smectic layers and intercalated, whereas rod-shaped mesogen-like monomers such as 1,4-di-(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylbenzene (C6M) oriented normal to the smectic layers and collected within them. Such spatial segregation caused by the smectic layering dramatically enhanced photopolymerization rates; for HDDA, termination rates were reduced, whereas for C6M, both the termination and propagation rates were increased. These polymerization precursor structures suggest novel materials-design paradigms for gel LCs and nanophase-separated polymer systems.</jats:p>

<jats:p> The reaction of tetrakis(tri- <jats:italic>tert</jats:italic> -butylsilyl)cyclotrigermene with trityl tetraphenylborate in benzene produces tris(tri- <jats:italic>tert</jats:italic> -butylsilyl)cyclotrigermenium tetraphenylborate [( <jats:italic>tert</jats:italic> -Bu <jats:sub>3</jats:sub> SiGe) <jats:sub>3</jats:sub> <jats:sup>+</jats:sup> BPh <jats:sub>4</jats:sub> <jats:sup>−</jats:sup> ], which can be isolated as a yellow solid that is stable in the absence of air. The crystal structure of the cyclotrigermenium ion reveals a free germyl cation with a 2π <!--<inline-graphic xlink:href="pi_002" xlink:type="simple"/>--> -electron system. The three germanium atoms form an equilateral triangle similar to the carbon analog, the cyclopropenium ion. </jats:p>

<jats:p> In situ neutron diffraction experiments show that at pressures above 2 kilobars, cubic zirconium tungstate (ZrW <jats:sub>2</jats:sub> O <jats:sub>8</jats:sub> ) undergoes a quenchable phase transition to an orthorhombic phase, the structure of which has been solved from powder diffraction data. This phase transition can be reversed by heating at 393 kelvin and 1 atmosphere and involves the migration of oxygen atoms in the lattice. The high-pressure phase shows negative thermal expansion from 20 to 300 kelvin. The relative thermal expansion and compressibilities of the cubic and orthorhombic forms can be explained in terms of the “cross-bracing” between polyhedra that occurs as a result of the phase transition. </jats:p>

<jats:p>A basic issue in ecology is the relation between extinction and population size. One of the clearest manifestations of a population threshold for extinction is the critical community size below which infections like measles do not persist. The current generation of stochastic models overestimates the observed critical community size for measles, generating much less persistence of infection than is observed. The inclusion of a more biologically realistic model for the duration of infection produced a much closer fit to the actual critical community size and explains previously undescribed high-frequency oscillations in measles incidence.</jats:p>

<jats:p> The controversial question of how thiamine diphosphate, the biologically active form of vitamin B <jats:sub>1</jats:sub> , is activated in different enzymes has been addressed. Activation of the coenzyme was studied by measuring thermodynamics and kinetics of deprotonation at the carbon in the 2-position (C2) of thiamine diphosphate in the enzymes pyruvate decarboxylase and transketolase by use of nuclear magnetic resonance spectroscopy, proton/deuterium exchange, coenzyme analogs, and site-specific mutant enzymes. Interaction of a glutamate with the nitrogen in the 1′-position in the pyrimidine ring activated the 4′-amino group to act as an efficient proton acceptor for the C2 proton. The protein component accelerated the deprotonation of the C2 atom by several orders of magnitude, beyond the rate of the overall enzyme reaction. Therefore, the earlier proposed concerted mechanism or stabilization of a C2 carbanion can be excluded. </jats:p>

<jats:p> In <jats:italic>Drosophila melanogaster</jats:italic> , a testis-specific β-tubulin (β2) is required for spermatogenesis. A sequence motif was identified in carboxyl termini of axonemal β-tubulins in diverse taxa. As a test of whether orthologous β-tubulins from different species are functionally equivalent, the moth <jats:italic>Heliothis virescens</jats:italic> β2 homolog was expressed in <jats:italic>Drosophila</jats:italic> testes. When coexpressed with β2, the moth isoform imposed the 16-protofilament structure characteristic of that found in the moth on the corresponding subset of <jats:italic>Drosophila</jats:italic> microtubules, which normally contain only 13-protofilament microtubules. Thus, the architecture of the microtubule cytoskeleton can be directed by a component β-tubulin. </jats:p>