bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced Ethylene and Ethane Production with Free-Radical Cracking Catalysts
Authors: Kolts John H. , Delzer Gary A.
Published: October 2006

A series of free-radical catalysts have been discovered that increase the yield of highly valuable olefins from the cracking of low molecular weight paraffins. For example, catalytic cracking of
n
-butane, isobutane, and propane over manganese or iron supported on magnesium oxide (MgO) gave product distributions different from those given by thermal (free-radical) cracking or cracking over traditional acid catalysts. With
n
-butane and propane feeds, the products from catalytic cracking included large amounts of ethylene and ethane; with isobutane feed, propylene was the major product. Physical characterization of the MgO-supported catalyst showed the manganese to be in a 2+ oxidation state in the reduced catalyst and a 4+ oxidation state in the fully oxidized catalyst. Manganese was also shown to be uniformly distributed in the support material with very little enrichment at the surface. Matrix isolation of the gas-phase radicals from
n
-butane feed showed that ethyl and methyl radicals were produced over the active catalysts. In the thermal process, only methyl radicals were produced. The mechanism of the catalytic reaction appears to be selective formation of primary carbanions at the catalyst surface followed by electron transfer and release of primary hydrocarbon radicals to the gas phase.
Biotechnology in Europe
Authors: Dibner Mark D.
Published: October 2006

The countries of the European Economic Community have recently mounted considerable efforts to commercialize biotechnology. Together, these efforts approach the same number of companies and level of government spending as those in the United States. In Europe there is more government emphasis on support for industry-university collaborations and industrial projects than in the United States, where basic research is emphasized. European efforts are often not easily delineated from those in the United States; many European companies have extensive U.S. operations and many U.S. companies have involvement in Europe. Strategies and efforts in European biotechnology are examined and compared to those in the United States.
Overview of the Shuttle Imaging Radar-B Preliminary Scientific Results
Authors: Elachi Charles , Cimino JoBea , Settle M.
Published: October 2006

The Shuttle Imaging Radar-B experiment consisted of a large number of scientific investigations in the earth sciences. Nine oceanographic experiments were conducted to study the generation and propagation of surface waves, the dynamics of internal waves, oil slick detection, and the properties of southern polar ice. Stereo imaging from space allowed three-dimensional viewing of surface features. Geologic experiments were conducted to study subsurface penetration, structural mapping, and lithologic classification. Imaging radar angular scatterometry was used in the vegetation cover, forest type, and urban areas classification experiments. This article provides an overview of the scientific results, some of which are also presented in this issue.
Internal Mixture of Sea Salt, Silicates, and Excess Sulfate in Marine Aerosols
Authors: Andreae Meinrat O. , Charlson Robert J. , Bruynseels Frank , Storms Hedwig , Van Grieken Rene , Maenhaut Willy
Published: October 2006

Individual aerosol particles from the remote marine atmosphere were investigated by scanning electron microscopy and electron microprobe analysis. A large fraction of the silicate mineral component of the aerosol was found to be internally mixed with sea-salt aerosol particles. This observation explains the unexpected similarity in the size distributions of silicates and sea salt that has been observed in remote marine aerosols. Reentrainment of dust particles previously deposited onto the sea surface and collision between aerosol particles can be excluded as possible source mechanisms for these internally mixed aerosols. The internal mixing could be produced by processes within clouds, including droplet coalescence. Cloud processes may also be responsible for the observed enrichment of excess (nonsea-salt) sulfate on sea-salt particles.
A New Member of the Leucine Zipper Class of Proteins that Binds to the HLA DRα Promoter
Authors: Liou Hsiou-Chi , Boothby Mark R. , Finn Patricia W. , Davidon Ruth , Nabavi Nasrin , Zeleznik-Le Nancy J. , Ting Jenny P.-Y. , Glimcher Laurie H.
Published: October 2006

Several mutants derived from transformed human B cell lines are defective in expressing major histocompatibility complex (MHC) class II genes. The failure to express a class II gene in at least one such mutant line has been mapped to the MHC class II X box, a conserved transcriptional element in the promoter region. A complementary DNA encoding a DNA-binding protein (human X box binding protein, hXBP-1) whose target is the human DRα X box and the 3′ flanking region has now been cloned. This complementary DNA encoded a protein with structural similarities to the c-
jun
proto-oncogene product, and its target sequence was closely related to the palindromic target sequence of c-
jun
. Mutation of the hXBP-1 DNA target sequence decreased DRα promoter activity in vivo. These studies suggest that the hXBP-1 protein acts as a transcription factor in B cells.
The Direct Methods of X-ray Crystallography
Authors: Hauptman Herbert
Published: October 2006

The electron density function ρ(r) in a crystal determines its diffraction pattern, that is, both the magnitudes and phases of its x-ray diffraction maxima, and conversely. If, however, as is always the case, only magnitudes are available from the diffraction experiment, then the density function ρ(r) cannot be recovered. If one invokes prior structural knowledge, usually that the crystal is composed of discrete atoms of known atomic numbers, then the observed magnitudes are, in general, sufficient to determine the positions of the atoms, that is, the crystal structure.
A 40-Million-Year Lake Record of Early Mesozoic Orbital Climatic Forcing
Authors: Olsen Paul E.
Published: October 2006

Sediments of the early Mesozoic Newark Supergroup of eastern North America consist largely of sedimentary cycles produced by the rise and fall of very large lakes that responded to periodic climate changes controlled by variations in the earth's orbit. Fourier analysis of long sections of the Late Triassic Lockatong and Passaic formations of the Newark Basin show periods in thickness of 5.9, 10.5, 25.2, 32.0, and 96.0 meters corresponding to periodicities in time of roughly 25,000, 44,000, 100,000, 133,000 and 400,000 years, as judged by radiometric time scales and varve-calibrated sedimentation rates. The ratios of the shortest cycle with longer cycles correspond closely to the ratios of the present periods of the main orbital terms that appear to influence climate. Similar long sequences of sedimentary cycles occur through most of the rest of the Newark Supergroup spanning a period of more than 40 million years. This is strong evidence of orbital forcing of climate in the ice-free early Mesozoic and indicates that the main periods of the orbital cycles were not very different 200 million years ago from those today.
Band-Gap Engineering: From Physics and Materials to New Semiconductor Devices
Authors: Capasso Federico
Published: October 2006

Band-gap engineering is a powerful technique for the design of new semiconductor materials and devices. Heterojunctions and modern growth techniques, such as molecular beam epitaxy, allow band diagrams with nearly arbitrary and continuous band-gap variations to be made. The transport properties of electrons and holes can be independently and continuously tuned for a given application. A new generation of devices with unique capabilities, ranging from solid-state photomultipliers to resonant tunneling transistors, is emerging from this approach.
Chronology of Fluctuating Sea Levels Since the Triassic
Authors: Haq Bilal U. , Hardenbol Jan , Vail Peter R.
Published: October 2006

Advances in sequence stratigraphy and the development of depositional models have helped explain the origin of genetically related sedimentary packages during sea level cycles. These concepts have provided the basis for the recognition of sea level events in subsurface data and in outcrops of marine sediments around the world. Knowledge of these events has led to a new generation of Mesozoic and Cenozoic global cycle charts that chronicle the history of sea level fluctuations during the past 250 million years in greater detail than was possible from seismic-stratigraphic data alone. An effort has been made to develop a realistic and accurate time scale and widely applicable chronostratigraphy and to integrate depositional sequences documented in public domain outcrop sections from various basins with this chronostratigraphic framework. A description of this approach and an account of the results, illustrated by sea level cycle charts of the Cenozoic, Cretaceous, Jurassic, and Triassic intervals, are presented.
The Resonating Valence Bond State in La
2
CuO
4
and Superconductivity
Authors: Anderson P. W.
Published: October 2006

The oxide superconductors, particularly those recently discovered that are based on La
2
CuO
4
, have a set of peculiarities that suggest a common, unique mechanism: they tend in every case to occur near a metal-insulator transition into an odd-electron insulator with peculiar magnetic properties. This insulating phase is proposed to be the long-sought "resonating-valence-bond" state or "quantum spin liquid" hypothesized in 1973. This insulating magnetic phase is favored by low spin, low dimensionality, and magnetic frustration. The preexisting magnetic singlet pairs of the insulating state become charged superconducting pairs when the insulator is doped sufficiently strongly. The mechanism for superconductivity is hence predominantly electronic and magnetic, although weak phonon interactions may favor the state. Many unusual properties are predicted, especially of the insulating state.
Correlation of Volcanic Activity with Sulfur Oxyanion Speciation in a Crater Lake
Authors: Takano Bokuichiro
Published: October 2006

The Yugama crater lake at Kusatsu-Shirane volcano, Japan, contains nearly 2200 tons (2800 parts per million) of polythionate ions (S

n

O
6
2-
, where
n
= 4 to 9). Analytical data on lake water sampled before and during eruptions in 1982 showed that the concentrations of polythionates decreased and sulfate increased in response to the preeruption activities of the subaqueous fumaroles. These changes were observed 2 months before the first phreatic explosion on 26 October 1982. The monitoring of polythionates and sulfate in crater lake water is a promising means of anticipating potential volcanic eruption hazards.
Asymmetries in Mating Preferences Between Species: Female Swordtails Prefer Heterospecific Males
Authors: Ryan Michael J. , Wagner William E.
Published: October 2006

In male swordtails (
Xiphophorus nigrensis
) there are three size classes that derive from allelic variation at the pituitary locus on the Y chromosome. Progeny analysis and preference tests suggest that females prefer to mate with larger males. In the closely related
X. pygmaeus
, there is no allelic variation at this locus; this species consists of males similar in size only to smaller
X. nigrensis
males. In addition to being smaller than most
X. nigrensis
males, these
X. pygmaeus
males also lack both the swordtail and a major component of the courtship display common in most
X. nigrensis
males. Usually, female
X. pygmaeus
prefer to mate with heterospecific males rather than conspecifics, regardless of body size and the presence of a swordtail. However, the smallest
X. nigrensis
males lack the same courtship component as do the
X. pygmaeus
males, and in this comparison female
X. pygmaeus
show no preference. Although sexual selection, through its action on divergence of courtship displays, has been implicated as a factor leading to speciation, in this case sexual selection could lead to the congealing of gene pools between heterospecifics.
Can the U.S. Oil and Gas Resource Base Support Sustained Production?
Authors: Fisher William L.
Published: October 2006

Aggressive drilling for oil and gas in the lower 48 states of the United States over the past decade yielded reserve additions sufficient to arrest decline and to stabilize levels of production. Such positive response from a maturely explored and developed oil resource base was unpredicted and largely unanticipated. Two elements of the recent experience—maintenance of stable rates of finding and substantial levels of conventional reserve growth in older fields—indicate that the capability of the resource base to sustain production is yet considerable. The volume of domestic oil and gas production that is necessary in the national interest and the extent to which the resource base should be pursued are the central issues of public energy policy.
RAG-1 and RAG-2, Adjacent Genes That Synergistically Activate V(D)J Recombination
Authors: Oettinger Marjorie A. , Schatz David G. , Gorka Carolyn , Baltimore David
Published: October 2006

The vast repertoire of immunoglobulins and T cell receptors is generated, in part, by V(D)J recombination, a series of genomic rearrangements that occur specifically in developing lymphocytes. The recombination activating gene, RAG-1, which is a gene expressed exclusively in maturing lymphoid cells, was previously isolated. RAG-1 inefficiently induced V(D)J recombinase activity when transfected into fibroblasts, but cotransfection with an adjacent gene, RAG-2, has resulted in at least a 1000-fold increase in the frequency of recombination. The 2.1-kilobase RAG-2 complementary DNA encodes a putative protein of 527 amino acids whose sequence is unrelated to that of RAG-1. Like RAG-1, RAG-2 is conserved between species that carry out V(D)J recombination, and its expression pattern correlates precisely with that of V(D)J recombinase activity. In addition to being located just 8 kilobases apart, these convergently transcribed genes are unusual in that most, if not all, of their coding and 3′ untranslated sequences are contained in single exons. RAG-1 and RAG-2 might activate the expression of the V(D)J recombinase but, more likely, they directly participate in the recombination reaction.
Bedform Alignment in Directionally Varying Flows
Authors: Rubin David M. , Hunter Ralph E.
Published: October 2006

Many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport. For this reason, the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport. Such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand-covered board in steady winds. Transverse, oblique, and longitudinal bedforms were created by changing only two parameters: the angle between the two winds and the proportions of sand transported in the two directions. Regardless of whether the experimental bedforms were transverse, oblique, or longitudinal (as defined by the bedform trend relative to the resultant transport direction), they all had trends that yielded the maximum gross transport across the bedforms. The fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment.
Bacterial Methanogenesis and Growth from CO
2
with Elemental Iron as the Sole Source of Electrons
Authors: Daniels Lacy , Belay Negash , Rajagopal Basavapatna S. , Weimer Paul J.
Published: October 2006

Previous studies of anaerobic biocorrosion have suggested that microbial sulfur and phosphorus products as well as cathodic hydrogen consumption may accelerate anaerobic metal oxidation. Methanogenic bacteria, which normally use molecular hydrogen (H
2
) and carbon dioxide (CO
2
) to produce methane (CH
4
) and which are major inhabitants of most anaerobic ecosystems, use either pure elemental iron (Fe
0
) or iron in mild steel as a source of electrons in the reduction of CO
2
to CH
4
. These bacteria use Fe
0
oxidation for energy generation and growth. The mechanism of Fe
0
oxidation is cathodic depolarization, in which electrons from Fe
0
and H
+
from water produce H
2
, which is then released for use by the methanogens; thermodynamic calculations show that significant Fe
0
oxidation will not occur in the absence of H
2
consumption by the methanogens. The data suggest that methanogens can be significant contributors to the corrosion of iron-containing materials in anaerobic environments.
Structural Mutations of the T Cell Receptor ζ Chain and Its Role in T Cell Activation
Authors: Frank Stuart J. , Niklinska Barbara B. , Orloff David G. , Merćep Mladen , Ashwell Jonathan D. , Klausner Richard D.
Published: October 2006

T cell hybridomas that express ζζ, but not ζη, dimers in their T cell receptors (TCRs) produce interleukin-2 (IL-2) and undergo an inhibition of spontaneous growth when activated by antigen, antibodies to the receptor, or antibodies to Thy-1. Hybridomas without ζ and η were reconstituted with mutated ζ chains. Cytoplasmic truncations of up to 40% of the ζ molecule reconstituted normal surface assembly of TCRs, but antigen-induced IL-2 secretion and growth inhibition were lost. In contrast, cross-linking antibodies to the TCR activated these cells. A point mutation conferred the same signaling phenotype as did the truncations and caused defective antigen-induced tyrosine kinase activation. Thus, ζ allows the binding of antigen/major histocompatibility complex (MHC) to αβ to effect TCR signaling.
A Substantial Bias in Nonparametric Tests for Periodicity in Geophysical Data
Authors: Stigler Stephen M. , Wagner Melissa J.
Published: October 2006

A nonparametric test that has been used to conclude that extinction rates are periodic with a period of 26 million years is shown to be substantially biased toward this conclusion, regardless of whether or not the data are periodic in origin (and, indeed, regardless of the actual period if they are in fact periodic). The test is shown to be sensitive to measurement error of a type expected with these data (early recording of extinctions due to missing fossil specimens, the "Signor-Lipps effect"), and it is shown that because of the unequal spacing in time, such models may be expected to produce statistically significant but artifactual periods of (in this case) exactly 26 million years over the span of time actually used.
Evidence for Divergent Plate-Boundary Characteristics and Crustal Spreading on Venus
Authors: Head James W. , Crumpler L. S.
Published: October 2006

Detailed examination of the topography and morphology of western Aphrodite Terra reveals numerous features that are similar to terrestrial divergent plate-boundary characteristics. Individual domains between fracture-zone-like discontinuities contain a variety of bilaterally symmetrical topographic elements that suggest that topographic features have been created at rise crests, rifted and separated, and moved laterally to their present symmetrical positions. The topographic and morphologic similarities, together with strikingly similar mirror-image map patterns on both sides of the rise axis, suggest that western Aphrodite Terra shares the characteristics of oceanic divergent plate boundaries, and is the site of crustal spreading on Venus. Topographic profiles are consistent with spreading rates of the order of several centimeters per year.
Star Formation in W49A: Gravitational Collapse of the Molecular Cloud Core Toward a Ring of Massive Stars
Authors: Welch Wm. J. , Dreher J. W. , Jackson J. M. , Terebey S. , Vogel S. N.
Published: October 2006

High-resolution molecular line and continuum radio images from the Hat Creek Radio Observatory and the Very Large Array suggest that the core of the W49A star-forming region is undergoing gravitational collapse. The radio continuum shows a 2-parsec ring of at least ten distinct ultracompact H-II regions, each associated with at least one O star. The ring is a region of large-scale, organized massive star formation. Recombination line velocities and HCO
+
excitation requirements indicate that the ring is rotating around 50,000 solar masses of material. Because the HCO
+
(1-0) line shows red-shifted absorption but blue-shifted emission, the molecular cloud core is believed to be collapsing toward the center of the ring. The HCO
+
radial velocities, as well as H-I, H
2
CO, and magnetic-field measurements, fit a simple model of inside-out gravitational collapse of a once magnetically supported cloud.
Tropical Forests and the Global Carbon Cycle
Authors: Detwiler R. P. , Hall Charles A. S.
Published: October 2006

New data on the three major determinants of the carbon release from tropical forest clearing are used in a computer model that simulates land use change and its effects on the carbon content of vegetation and soil in order to calculate the net flux of carbon dioxide between tropical ecosystems and the atmosphere. The model also permits testing the sensitivity of the calculated flux to uncertainties in these data. The tropics were a net source of at least 0.4 × 10
15
grams but not more than 1.6 × 10
15
grams of carbon in 1980, considerably less than previous estimates. Decreases in soil organic matter were responsible for 0.1 × 10
15
to 0.3 × 10
15
grams of the release, while the burning and decay of cleared vegetation accounted for 0.3 × 10
15
to 1.3 × 10
15
grams. These estimates are lower than many previous ones because lower biomass estimates and slightly lower land clearing rates were used and because ecosystem recovery processes were included. These new estimates of the biotic release allow for the possibility of a balanced global budget given the large remaining uncertainties in the marine, terrestrial, and fossil fuel components of the carbon cycle.
Cavitation and the Interaction Between Macroscopic Hydrophobic Surfaces
Authors: Christenson Hugo K. , Claesson Per M.
Published: October 2006

The interaction in water of neutral hydrocarbon and fluorocarbon surfaces, prepared by Langmuir-Blodgett deposition of surfactant monolayers, has been investigated. The attraction between these hydrophobic surfaces can be measured at separations of 70 to 90 nanometers and thus is of considerably greater range than previously found. Spontaneous cavitation occurred as soon as the fluorocarbon surfaces were brought into contact but occurred between the hydrocarbon surfaces only after separation from contact. The very long range forces measured are a consequence of the metastability of water films between macroscopic hydrophobic surfaces. Thus the hydrophobic interaction between macroscopic surfaces may not be related to water structure in the same way that the hydrophobic effect between nonpolar molecules is related to water structure.
Gas Bubbles in Fossil Amber as Possible Indicators of the Major Gas Composition of Ancient Air
Authors: Berner Robert A. , Landis Gary P.
Published: October 2006

Gases trapped in Miocene to Upper Cretaceous amber were released by gently crushing the amber under vacuum and were analyzed by quadrupole mass spectrometry. After discounting the possibility that the major gases N
2
, O
2
, and CO
2
underwent appreciable diffusion and diagenetic exchange with their surroundings or reaction with the amber, it has been concluded that in primary bubbles (gas released during initial breakage) these gases represent mainly original ancient air modified by the aerobic respiration of microorganisms. Values of N
2
/(CO
2
+ O
2
) for each time period give consistent results despite varying O
2
/CO
2
ratios that presumably were due to varying degrees of respiration. This allows calculation of original oxygen concentrations, which, on the basis of these preliminary results, appear to have changed from greater than 30 percent O
2
during one part of the Late Cretaceous (between 75 and 95 million years ago) to 21 percent during the Eocene-Oligocene and for present-day samples, with possibly lower values during the Oligocene-Early Miocene. Variable O
2
levels over time in general confirm theoretical isotope-mass balance calculations and suggest that the atmosphere has evolved over Phanerozoic time.
A Network Model of Catecholamine Effects: Gain, Signal-to-Noise Ratio, and Behavior
Authors: Servan-Schreiber David , Printz Harry , Cohen Jonathan D.
Published: October 2006

At the level of individual neurons, catecholamine release increases the responsivity of cells to excitatory and inhibitory inputs. A model of catecholamine effects in a network of neural-like elements is presented, which shows that (i) changes in the responsivity of individual elements do not affect their ability to detect a signal and ignore noise but (ii) the same changes in cell responsivity in a network of such elements do improve the signal detection performance of the network as a whole. The second result is used in a computer simulation based on principles of parallel distributed processing to account for the effect of central nervous system stimulants on the signal detection performance of human subjects.
Abrupt Climate Change and Extinction Events in Earth History
Authors: Crowley Thomas J. , North Gerald R.
Published: October 2006

Slowly changing boundary conditions can sometimes cause discontinuous responses in climate models and result in relatively rapid transitions between different climate states. Such terrestrially induced abrupt climate transitions could have contributed to biotic crises in earth history. Ancillary events associated with transitions could disperse unstable climate behavior over a longer but still geologically brief interval and account for the stepwise nature of some extinction events. There is a growing body of theoretical and empirical support for the concept of abrupt climate change, and a comparison of paleoclimate data with the Phanerozoic extinction record indicates that climate and biotic transitions often coincide. However, more stratigraphic information is needed to precisely assess phase relations between the two types of transitions. The climate-life comparison also suggests that, if climate change is significantly contributing to biotic turnover, ecosystems may be more sensitive to forcing during the early stages of evolution from an ice-free to a glaciated state. Our analysis suggests that a terrestrially induced climate instability is a viable mechanism for causing rapid environmental change and biotic turnover in earth history, but the relation is not so strong that other sources of variance can be excluded.
Low Lake Stands in Lakes Malawi and Tanganyika, East Africa, Delineated with Multifold Seismic Data
Authors: Scholz C. A. , Rosendahl B. R.
Published: October 2006

Seismic data reveal that water level in Lake Malawi, East Africa, was 250 to 500 meters lower before about 25,000 years ago. Water levels in Lake Tanganyika at that time were more than 600 meters below the current lake level. A drier climate appears to have caused these low stands, but tectonic tilting may also have been a contributing factor in Lake Malawi. High-angle discordances associated with shallow sequence boundaries suggest that these low stands probably lasted many tens of thousands of years. Because of its basement topography, the Lake Tanganyika basin had three separate paleolakes, whereas the Lake Malawi basin had only one. The different geographies of these paleolakes may be responsible in part for the differences in the endemic fish populations in these lakes.
Inhibition of Serum- and Ras-Stimulated DNA Synthesis by Antibodies to Phospholipase C
Authors: Smith Mark R. , Liu Ya-Lun , Kim Hyun , Rhee Sue Goo , Kung Hsiang-Fu
Published: October 2006

Several immunologically distinct isozymes of inositol phospholipid-specific phospholipase C (PLC) have been purified from bovine brain. Murine NIH 3T3 fibroblasts were found to express PLC-γ, but the expression of PLC-β was barely detectable by radioimmunoassay or protein immunoblot. A mixture of monoclonal antibodies was identified that neutralizes the biological activity of both endogenous and injected purified PLC-γ. When co-injected with oncogenic Ras protein or PLC-γ, this mixture of antibodies inhibited the induction of DNA synthesis that characteristically results from the injection of these proteins into quiescent 3T3 cells. However, when oncogenic Ras protein or PLC-γ was co-injected with a neutralizing monoclonal antibody to Ras, only the DNA synthesis induced by the Ras protein was inhibited—that induced by PLC was unaffected. These results suggest that the Ras protein is an upstream effector of PLC activity in phosphoinositide-specific signal transduction and that PLC-γ activity is necessary for Ras-mediated induction of DNA synthesis.
Angiogenesis Induced by Degradation Products of Hyaluronic Acid
Authors: West D. C. , Hampson I. N. , Arnold F. , Kumar S.
Published: October 2006

Partial degradation products of sodium hyaluronate produced by the action of testicular hyaluronidase induced an angiogenic response (formation of new blood vessels) on the chick chlorioallantoic membrane. Neither macromolecular hyaluronate nor exhaustively digested material had any angiogenic potential. Fractionation of the digestion products established that the activity was restricted to hyaluronate fragments between 4 and 25 disaccharides in length.
Topographically Controlled Fronts in the Ocean and Their Biological Influence
Authors: Wolanski Eric , Hamner William M.
Published: October 2006

Headlands, islands, and reefs generate complex three-dimensional secondary flows that result in physical and biological fronts. Mixing and diffusion processes near these reefs and headlands are quite different from these processes in the open sea, and classical advection-diffusion models that were developed for the open sea are not valid near shore. Topographically generated fronts affect the distribution of sediments, and they aggregate waterborne eggs, larvae, and plankton. This aggregation influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators.
High Temperatures in the Early Solar Nebula
Authors: Boss Alan P.
Published: October 2006

One fundamental controversy about terrestrial planet and asteroid formation is the discrepancy between meteoritical evidence for high temperatures (1500 K to 2000 K) in the inner solar nebula, and much lower theoretical temperature predictions on the basis of models of viscous accretion disks that neglect compressional heating of infalling gas. It is shown here that rigorous numerical calculations of the collapse of a rotating, three-dimensional presolar nebula are capable of producing temperatures on the order of 1500 K in the asteroid region (2.5 astronomical units), in either nearly axisymmetric or strongly nonaxisymmetric nebula models. The latter models may permit significant thermal cycling of solid components in the early inner solar nebula.
Effect of a Coordination Change on the Strength of Amorphous SiO
2
Authors: Meade Charles , Jeanloz Raymond
Published: October 2006

Measurements of the yield strength of SiO
2
glass to pressures as high as 81 gigapascals at room temperature show that the strength of amorphous silica decreases significantly as it is compressed to denser structures with higher coordination. Above 27 gigapascals, as the silicon in amorphous SiO
2
is continuously transformed from fourfold to sixfold coordination, the strength of the glass decreases by more than an order of magnitude. These data confirm theoretical predictions that the mechanical properties of polymerized amorphous silicates are sensitive to pressure-induced structural transformations and suggest that the viscosity of silica-rich liquids decreases significantly at high pressures. Such a change in melt rheology could enhance the processes of chemical differentiation with depth in the Earth's mantle.
Detection of C
3
in the Circumstellar Shell of IRC+10216
Authors: Hinkle Kenneth W. , Keady John J. , Bernath Peter F.
Published: October 2006

Vibration-rotation lines of C
3
have been identified in the circumstellar spectrum of the obscured carbon star IRC+10216. This molecule is of interest in both the chemistry of flames, where it may be involved in the formation of soot, and in astrophysics, where it is a potential building block for carbonaceous grains. This high-resolution infrared detection of the pure carbon chain molecule C
3
allows the estimation of the equilibrium C-C bond length, 1.297 angstroms. Possible astrophysical formation and destruction mechanisms for C
3
are reviewed, including the relationship between C
3
and carbon clusters.
How Many Species Are There on Earth?
Authors: May Robert M.
Published: October 2006

This article surveys current answers to the factual question posed in the title and reviews the kinds of information that are needed to make these answers more precise. Various factors affecting diversity are also reviewed. These include the structure of food webs, the relative abundance of species, the number of species and of individuals in different categories of body size, along with other determinants of the commonness and rarity of organisms.
Intrinsic and Extrinsic Factors in Protein Antigenic Structure
Authors: Berzofsky Jay A.
Published: October 2006

Recent advances in the preparation of synthetic peptide vaccines and the use of synthetic peptides as probes of antigenic structure and function have led to renewed interest in the prediction of antigenic sites recognized by antibodies and T cells. This review focuses on antibodies. Features intrinsic to the antigen, such as hydrophilicity and mobility, may be useful in the selection of amino acid sequences of the native protein that will elicit antibodies cross-reacting with peptides, or sequences which, as peptides, will be more likely to elicit antibodies cross-reactive with the native protein. Structural mobility may also contribute to protein-protein interactions in general. However, the entire accessible surface of a protein is likely to be detectable by a large enough panel of antibodies. Which of these antibodies are made in any individual depends on factors extrinsic to the antigen molecule, host factors such as self-tolerance, immune response genes, idiotype networks, and the immunoglobulin structural gene repertoire.
Amino Acid Homology Between the Encephalitogenic Site of Myelin Basic Protein and Virus: Mechanism for Autoimmunity
Authors: Fujinami Robert S. , Oldstone Michael B. A.
Published: October 2006

Amino acid sequence homology was found between viral and host encephalitogenic protein. Immune responses were then generated in rabbits by using the viral peptide that cross-reacts with the self protein. Mononuclear cell infiltration was observed in the central nervous systems of animals immunized with the viral peptide. Myelin basic protein (MBP) is a host protein whose encephalitogenic site of ten amino acids induces experimental allergic encephalomyelitis. By computer analysis, hepatitis B virus polymerase (HBVP) was found to share six consecutive amino acids with the encephalitogenic site of rabbit MBP. Rabbits given injections of a selected eight- or ten-amino acid peptide from HBVP made antibody that reacted with the predetermined sequences of HBVP and also with native MBP. Peripheral blood mononuclear cells from the immunized rabbits proliferated when incubated with either MBP or HBVP. Central nervous system tissue taken from these rabbits had a histologic picture reminiscent of experimental allergic encephalomyelitis. Thus, viral infection may trigger the production of antibodies and mononuclear cells that cross-react with self proteins by a mechanism termed molecular mimicry. Tissue injury from the resultant autoallergic event can take place in the absence of the infectious virus that initiated the immune response.
Antigens Induced on Erythrocytes by
P. falciparum
: Expression of Diverse and Conserved Determinants
Authors: Marsh Kevin , Howard Russell J.
Published: October 2006

Red blood cells that are infected with the malaria parasite
Plasmodium falciparum
express new antigens on their surface. In a study of these antigens in the erythrocytes of naturally infected children in the Gambia, an antibody-mediated agglutination assay revealed an extreme degree of antigenic diversity. Serum samples from each of ten children in the convalescent stage of malaria infection reacted with infected cells from the same child but generally not with infected cells from the other children. The Gambian children's erythrocytes also expressed shared determinants: sera from Gambian adults often reacted with the surface of infected cells from all of the children and were shown by adsorption and elution experiments to contain antibodies that recognized several isolates. Conserved determinants exposed on infected erythrocytes may be important for development of antimalarial immunity either naturally or through vaccination.
Putative Reverse Transcriptase Intermediates of Human Hepatitis B Virus in Primary Liver Carcinomas
Authors: Will H. , Salfeld J. , Pfaff E. , Manso C. , Theilmann L. , Schaler H.
Published: October 2006

Nucleocapsid-pol fusion proteins have been detected by serological screening hepatocellular carcinoma tissues that contain hepatitis B virus (HBV) DNA. The existence of these fusion proteins suggests that HBV may synthesize its reverse transcriptase in a fashion analogous to the way that retroviruses synthesize and process a precursor. The accumulation of HBV reverse transcriptase intermediates in tumorous tissues and not in other tissues may be related to the absence of viral core particles and possibly contributes to tumor development.
Genetic Relatedness in Colonies of Tropical Wasps with Multiple Queens
Authors: Queller David C. , Strassmann Joan E. , Hughes Colin R.
Published: October 2006

The evolution of worker behavior in the social insects is usually explained by kin selection: although workers do not produce offspring, they do reproduce their genes by aiding the reproduction of relatives. The most difficult case for kin selection theory would be species in which workers are fully capable of reproducing but instead opt to rear brood of low relatedness. These conditions are perhaps best fulfilled by the swarm-founding wasps because they have little caste differentiation and their colonies usually have multiple queens, which should lower relatedness. Estimates of within-colony relatedness for three species in this group confirm that it is sometimes (but not always) very low. Inbreeding is negligible in these species, so the hypothesis that inbreeding may raise relatedness is not supported. The maintenance of worker behavior in such species is a significant challenge for kin selection theory.
A Poliovirus Neutralization Epitope Expressed on Hybrid Hepatitis B Surface Antigen Particles
Authors: Delpeyroux F. , Chenciner N. , Lim A. , Malpièce Y. , Blondel B. , Crainic R. , van der Werf S. , Streeck R. E.
Published: October 2006

The hepatitis B virus (HBV) envelope protein carrying the surface antigen (HBsAg) is assembled with cellular lipids in mammalian cells into empty viral envelopes. In a study to evaluate the capacity of such particles to present foreign peptide sequences in a biologically active form, in-phase insertions were created in the S gene encoding the major envelope protein. One of the sequences inserted was a synthetic DNA fragment encoding a poliovirus neutralization epitope. Mammalian cells expressing the modified gene secreted hybrid particles closely resembling authentic 22-nanometer HBsAg particles. These particles reacted with a poliovirus-specific monoclonal antibody and induced neutralizing antibodies against poliovirus. The results indicate that empty viral envelopes of HBV may provide a means for the presentation of peptide sequences and for their export from mammalian cells.
Ergodic Theory, Randomness, and "Chaos"
Authors: Ornstein D. S.
Published: October 2006

Ergodic theory is the theory of the long-term statistical behavior of dynamical systems. The baker's transformation is an object of ergodic theory that provides a paradigm for the possibility of deterministic chaos. It can now be shown that this connection is more than an analogy and that at some level of abstraction a large number of systems governed by Newton's laws are the same as the baker's transformation. Going to this level of abstraction helps to organize the possible kinds of random behavior. The theory also gives new concrete results. For example, one can show that the same process could be produced by a mechanism governed by Newton's laws or by a mechanism governed by coin tossing. It also gives a statistical analog of structural stability.
A Relation to Describe Rate-Dependent Material Failure
Authors: Voight Barry
Published: October 2006

The simple relation ω
-α
ω -
A
= 0, where ω is a measurable quantity such as strain and
A
and α are empirical constants, describes the behavior of materials in terminal stages of failure under conditions of approximately constant stress and temperature. Applicable to metals and alloys, ice, concrete, polymers, rock, and soil, the relation may be extended to conditions of variable and multiaxial stress and may be used to predict time to failure.
Evidence for Sediment Eruption on Deep Sea Floor, Gulf of Mexico
Authors: Prior David B. , Doyle Earl H. , Kaluza Michael J.
Published: October 2006

A large crater has been discovered on the sea floor, Gulf of Mexico, in a water depth of 2176 meters. Deep-tow high-resolution imagery shows that the crater is cut into a low hill surrounded by near-surface concentric faults. Approximately 2 million cubic meters of ejected sediment forms a peripheral debris field. The low hill and faults may be related to mud diapirism or intrusion of gas hydrates into near-surface sediments. A recent eruption evacuated sediments from the crater, apparently because of release of overpressured petrogenic gas.
Combining Cognitive and Statistical Approaches to Survey Design
Authors: Fienberg Stephen E. , Tanur Judith M.
Published: October 2006

Sample surveys provide data for academic research, government policy-making, the media, and business. Statistical research aims to improve survey data by reducing extraneous sources of variability and thus increasing accuracy. Researchers have begun to use paradigms adapted from the cognitive sciences to study those sources of variability associated with the processes that the respondent undertakes in understanding questions, remembering, judging and estimating, and formulating answers. To generalize laboratory-based findings, researchers must begin to embed designed experiments that vary the questionnaire content into sample surveys of broad populations. Issues associated with the design of and statistical inference from such embedded experiments are examined and illustrated with an example on the effects of context questions on responses in attitude surveys.
Polymers, Fractals, and Ceramic Materials
Authors: Schaefer Dale W.
Published: October 2006

Concepts borrowed from polymer science have been applied to tailor the properties of inorganic materials, especially those derived from amorphous precursors. Fractal geometry can be used to characterize macromolecular precursors and to relate their structures to kinetic growth processes. Within the silica system, for example, it is possible to manipulate the conditions of solution polymerization to yield a variety of macromolecules from branched chains to smooth colloidal particles.
Dendrites, Viscous Fingers, and the Theory of Pattern Formation
Authors: Langer J. S.
Published: October 2006

There has emerged recently a new theoretical picture of the way in which patterns are formed in dendritic crystal growth and in the closely analogous phenomenon of viscous fingering in fluids. Some interesting questions that arise in connection with this theory include: How broad is its range of validity? How do we understand dynamic stability in systems of this kind? What is the origin of sidebranches? Can weak noise, or even microscopic thermal fluctuations, play a role in determining the macroscopic features of these systems?
Multiple Calcium Channels and Neuronal Function
Authors: Miller Richard J.
Published: October 2006

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.
Ribavirin Antagonizes the Effect of Azidothymidine on HIV Replication
Authors: Vogt Markus W. , Hartshorn Kevan L. , Furman Phillip A. , Chou Ting-Chao , Fyfe James A. , Coleman Leslie A. , Crumpacker Clyde , Schooley Robert T. , Hirsch Martin S.
Published: October 2006

Azidothymidine and ribavirin both inhibit replication of human immunodeficiency virus in vitro and show promise of clinical utility in patients infected with this virus. In this study, the possible interactions of these drugs were examined in vitro, and a reproducible antagonism between azidothymidine and ribavirin was found to occur under a variety of experimental conditions. The mechanism responsible for this antagonism appeared to be inhibition of azidothymidine phosphorylation by ribavirin. Because similar effects may occur in vivo, clinical trials of these two drugs in combination must be performed only under carefully controlled conditions.
Expression and Processing of the AIDS Virus Reverse Transcriptase in
Escherichia coli
Authors: Farmerie William G. , Loeb Daniel D. , Casavant N. Carol , Hutchison Clyde A. , Edgell Marshall H. , Swanstrom Ronald
Published: October 2006

The ability to express the genes of pathogenic human viruses, such as the acquired immune deficiency syndrome (AIDS) virus (also called human immunodeficiency virus) in bacterial cells affords the opportunity to study proteins that are ordinarily difficult or inconvenient to obtain in amounts sufficient for detailed analysis. A segment of the AIDS virus
pol
gene was expressed in
Escherichia coli
. Expression resulted in the appearance of reverse transcriptase activity in the bacterial cell extracts. The extracts contained two virus-related polypeptides that have the same apparent molecular weights as the two processed forms of virion-derived reverse transcriptase (p66 and p51). The formation of these two polypeptides depended on the coexpression of sequences located near the 5′ end of the
pol
gene, a region that is thought to encode a viral protease. This bacterial system appears to generate mature forms of the AIDS virus reverse transcriptase by a proteolytic pathway equivalent to that which occurs during virus infection of human cells.
The Basis for the Immunoregulatory Role of Macrophages and Other Accessory Cells
Authors: Unanue Emil R. , Allen Paul M.
Published: October 2006

Macrophages handle extracellular proteins and secrete diverse bioactive molecules and, therefore, influence the physiology of many tissues. They also have an important immunoregulatory role. The immune response to proteins involves the activation of the T helper subset of lymphocytes. The T helper cell is activated only when it interacts with the protein displayed on the surface of a macrophage or other accessory cell. This interaction involves restrictive proteins encoded in the major histocompatibility gene complex as well as growth-differentiating proteins.