Free-electron lasers: present status and future prospects

Free-electron lasers as scientific instruments are reviewed. The present status and future prospects are delineated with attention drawn to the size, complexity, availability, and performance capability of this new tool.     

Mechanism of Na+/H+ antiporting

Na+/H+ antiporters are central to cellular salt and pH homeostasis. The structure of Escherichia coli NhaA was recently determined, but its mechanisms of transport and pH regulation remain elusive. We performed molecular dynamics simulations of NhaA that, with existing experimental data, enabled us to propose an atomically detailed model of antiporter function. Three conserved aspartates are key to our proposed mechanism: Asp164 (D164) is the Na+-binding site, D163 controls the alternating accessibility of this binding site to the cytoplasm or periplasm, and D133 is crucial for pH regulation. Consistent with experimental stoichiometry, two protons are required to transport a single Na+ ion: D163 protonates to reveal the Na+-binding site to the periplasm, and subsequent protonation of D164 releases Na+. Additional mutagenesis experiments further validated the model.     

Cristobalitic stage in the diagenesis of diatomaceous shale

With increasing depth of burial, diagenetic cristobalite in the Monterey Shale of California shows a decrease in the d(101) spacing from 4.115 to 4.040 angstroms, indicative of a progressive change in its internal structure. The spacing is 0.004 to 0.015 angstrom smaller in porcellanite than in associated chert, probably because the cristobalite of porcellanite formed later than that of chert.     

Geochemistry and thermolysis of flavonoids

The thermal and pH stability of selected flavonoids has been determined under simulated geologic conditions. Thermolytic rates and products for various regimes, as determined by gas chromatography-mass spectroscopy, indicate the potential usefulness of flavonoids as thermometric indicators in sediments. The parametric factors affecting flavonoid stability are used to geochemically characterize angiosperm "green leaves" (36 to 25 x 10(6) years old) from Succor Creek and indicate that these sediments have not experienced temperatures higher than 80 degrees C or extreme pH shifts (beyond the range 6.3 to 7.2) during postdepositional maturation.     

Simulated wind pollination and airflow around ovules of some early seed plants

Scale models of various early seed plant ovules and cupules were analyzed both for their characteristic air disturbance patterns and for the frequency of impact with airborne models of pollen (pseudopollen). The fossils on which the models were based had been arranged in an evolutionary sequence purporting to show the origin of the integument by the acropetal fusion and reduction in length of a subtending truss of lobes. Wind tunnel analyses of scale models showing the various stages in the consolidation of these preintegumentary lobes indicated that turbulent flow increases and becomes localized around the nucellar apex (salpinx) with the syngenesis and length reduction of those lobes. Similarly, the frequency of windborne pseudopollen impact increased. Thus, the transition from the megasporangium to the fully integumented ovule appears to favor increased wind-mediated pollination.     

Space research in the era of the space station

With the continuing flights and increasing capabilities of the space shuttle, and with the design and development of a space station, there will be a 0 significant increase in our space research capabilities during the 1990's. Ways in which the nation's space science program may evolve in response to these developments are described.