Catálogo de publicaciones - revistas

<jats:p> Structural and mechanistic studies show that when the selection criteria of the immune system are changed, catalytic antibodies that have the efficiency of natural enzymes evolve, but the catalytic antibodies are much more accepting of a wide range of substrates. The catalytic antibodies were prepared by reactive immunization, a process whereby the selection criteria of the immune system are changed from simple binding to chemical reactivity. This process yielded aldolase catalytic antibodies that approximated the rate acceleration of the natural enzyme used in glycolysis. Unlike the natural enzyme, however, the antibody aldolases catalyzed a variety of aldol reactions and decarboxylations. The crystal structure of one of these antibodies identified the reactive lysine residue that was selected in the immunization process. This lysine is deeply buried in a hydrophobic pocket at the base of the binding site, thereby accounting for its perturbed p <jats:italic>K</jats:italic> <jats:sub>a</jats:sub> . </jats:p>

<jats:p> The sequence of a promoter determines not only the efficiency with which it forms a complex with RNA polymerase, but also the concentration of nucleoside triphosphate (NTP) required for initiating transcription. <jats:italic>Escherichia coli</jats:italic> ribosomal RNA ( <jats:italic>rrn</jats:italic> P1) promoters require high initiating NTP concentrations for efficient transcription because they form unusually short-lived complexes with RNA polymerase; high initiating NTP concentrations [adenosine or guanosine triphosphate (ATP or GTP), depending on the <jats:italic>rrn</jats:italic> P1 promoter] are needed to bind to and stabilize the open complex. ATP and GTP concentrations, and therefore <jats:italic>rrn</jats:italic> P1 promoter activity, increase with growth rate. Because ribosomal RNA transcription determines the rate of ribosome synthesis, the control of ribosomal RNA transcription by NTP concentration provides a molecular explanation for the growth rate–dependent control and homeostatic regulation of ribosome synthesis. </jats:p>

<jats:p> High-pressure diamond-cell experiments indicate that the iron-magnesium partitioning between (Fe,Mg)SiO <jats:sub>3</jats:sub> -perovskite and magnesiowüstite in Earth's lower mantle depends on the pressure, temperature, bulk iron/magnesium ratio, and ferric iron content. The perovskite stability field expands with increasing pressure and temperature. The ferric iron component preferentially dissolves in perovskite and raises the apparent total iron content but had little effect on the partitioning of the ferrous iron. The ferrous iron depletes in perovskite at the top of the lower mantle and gradually increases at greater depth. These changes in iron-magnesium composition should affect geochemical and geophysical properties of the deep interior. </jats:p>

<jats:p>Catalytically active Langmuir-Blodgett films of a rhodium complex were prepared and characterized to determine the possible effect of the molecular order of metal complexes on catalytic activity. The hydrogenation of carbon-oxygen double bonds was used as a model reaction. The complex in solution exhibited low catalytic activity, whereas it was highly active in the film. The catalytic activity was found to be highly dependent on the orientation of the complex within the film. The reactions were also highly selective with regard to the substrate. These observations and the observed rate dependence on temperature strongly implicate the molecular order of a metal complex as an important dimension in catalysis.</jats:p>

<jats:p> Although in principle semiconductor-metal (Schottky) diodes should be tunable by changing the work function of the metal, such flexibility cannot be achieved in a single device and in practice is often limited by interfacial states that cause Fermi-level pinning. A tunable diode is reported based on a hybrid inorganic-organic, <jats:italic>n</jats:italic> -indium phosphide|poly- (pyrrole)|nonaqueous electrolyte architecture. By electrochemically manipulating the work function of the conjugated polymer poly(pyrrole), the turn-on voltage (more precisely, the forward bias potential required to pass a particular current) of the diode can be continuously and actively tuned by more than 0.6 volt. The work highlights a distinguishing feature of conjugated polymers relative to more traditional semiconductor materials, namely, the ability of dopant ions to permeate conjugated polymers, thereby enabling electrochemical manipulation. </jats:p>

<jats:p> The terminal navigation of the Near Earth Asteroid Rendezvous (NEAR) spacecraft during its close flyby of asteroid 253 Mathilde involved coordinated efforts to determine the heliocentric orbits of the spacecraft and Mathilde and then to determine the relative trajectory of the spacecraft with respect to Mathilde. The gravitational perturbation of Mathilde on the passing spacecraft was apparent in the spacecraft tracking data. As a result of the accurate targeting achieved, these data could be used to determine Mathilde's mass as 1.033 (± 0.044) × 10 <jats:sup>20</jats:sup> grams. Coupled with a volume estimate provided by the NEAR imaging team, this mass suggests a low bulk density for Mathilde of 1.3 grams per cubic centimeter. </jats:p>

<jats:p>On 27 June 1997, the Near Earth Asteroid Rendezvous (NEAR) spacecraft flew within 1212 kilometers of asteroid 253 Mathilde. Mathilde is an irregular, heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers. The asteroid's surface is dark (estimated albedo between 0.035 and 0.050) and similar in color to some CM carbonaceous chondrites. No albedo or color variations were detected. The volume derived from the images and the mass from Doppler tracking of the spacecraft yield a mean density of 1.3 ± 0.2 grams per cubic centimeter, about half that of CM chondrites, indicating a porous interior structure.</jats:p>

<jats:p> The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state (∼10 <jats:sup>5</jats:sup> cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices. </jats:p>

<jats:p> The threonine-glycine (Thr-Gly) encoding repeat within the clock gene <jats:italic>period</jats:italic> of <jats:italic>Drosophila melanogaster</jats:italic> is polymorphic in length. The two major variants <jats:italic>(Thr-Gly)17</jats:italic> and <jats:italic>(Thr-Gly)20</jats:italic> are distributed as a highly significant latitudinal cline in Europe and North Africa. Thr-Gly length variation from both wild-caught and transgenic individuals is related to the flies' ability to maintain a circadian period at different temperatures. This phenomenon provides a selective explanation for the geographical distribution of Thr-Gly lengths and gives a rare glimpse of the interplay between molecular polymorphism, behavior, population biology, and natural selection. </jats:p>

<jats:p> The <jats:italic>NPH1</jats:italic> (nonphototropic hypocotyl 1) gene encodes an essential component acting very early in the signal-transduction chain for phototropism. <jats:italic>Arabidopsis NPH1</jats:italic> contains a serine-threonine kinase domain and LOV1 and LOV2 repeats that share similarity (36 to 56 percent) with <jats:italic>Halobacterium salinarium</jats:italic> Bat, <jats:italic>Azotobacter vinelandii</jats:italic> NIFL, <jats:italic>Neurospora crassa</jats:italic> White Collar–1, <jats:italic>Escherichia coli</jats:italic> Aer, and the Eag family of potassium-channel proteins from <jats:italic>Drosophila</jats:italic> and mammals. Sequence similarity with a known (NIFL) and a suspected (Aer) flavoprotein suggests that NPH1 LOV1 and LOV2 may be flavin-binding domains that regulate kinase activity in response to blue light–induced redox changes. </jats:p>