Identity and search in social networks

Social networks have the surprising property of being "searchable": Ordinary people are capable of directing messages through their network of acquaintances to reach a specific but distant target person in only a few steps. We present a model that offers an explanation of social network searchability in terms of recognizable personal identities: sets of characteristics measured along a number of social dimensions. Our model defines a class of searchable networks and a method for searching them that may be applicable to many network search problems, including the location of data files in peer-to-peer networks, pages on the World Wide Web, and information in distributed databases.     

Resolution of lung inflammation by CD44

Successful repair after tissue injury and inflammation requires resolution of the inflammatory response and removal of extracellular matrix breakdown products. We have examined whether the cell-surface adhesion molecule and hyaluronan receptor CD44 plays a role in resolving lung inflammation. CD44-deficient mice succumb to unremitting inflammation following noninfectious lung injury, characterized by impaired clearance of apoptotic neutrophils, persistent accumulation of hyaluronan fragments at the site of tissue injury, and impaired activation of transforming growth factor-beta1. This phenotype was partially reversed by reconstitution with CD44+ cells, thus demonstrating a critical role for this receptor in resolving lung inflammation.     

Probing high-barrier pathways of surface reactions by scanning tunneling microscopy

The ability of scanning tunneling microscopy to probe the pathways of thermally activated high-barrier surface processes is frequently limited by competing low-barrier processes that can confuse measurement of the true initial and final configuration. We introduce an approach to circumvent this difficulty by driving the surface process with nanosecond laser heating. The method is applied to determine the pathway of recombinative desorption in the H/Si(001) system. The observed configuration of dangling bonds after laser heating reveals that the desorbed hydrogen molecules are not formed on single dimers, but rather from neighboring silicon dimers via an interdimer reaction pathway.     

Molecular recognition by self-assembled monolayers of cavitand receptors

It is shown by angle-resolved x-ray photoelectron spectroscopy that cavitands derived from resorcin[4]arenes provided with four dialkylsulfide chains form stable monolayers on gold surfaces that are well organized by self-assembly. The cavitand headgroups at the surface of the resorcin[4]arene monolayer act as molecular recognition sites for small organic molecules with remarkable selectivity for perchloroethylene (C(2)Cl(4)). Comparative thermal desorption experiments indicate binding sites with high interaction energies of C(2)Cl(4) at the surface of the resorcin[4]arene monolayers. Fast and reversible "host-guest" interactions were found by the monitoring of extremely small mass changes (in the nanogram range) with a quartz microbalance oscillator provided with gold electrodes coated by resorcin[4]arene monolayers.     

Molecular and neuronal substrate for the selective attenuation of anxiety

Benzodiazepine tranquilizers are used in the treatment of anxiety disorders. To identify the molecular and neuronal target mediating the anxiolytic action of benzodiazepines, we generated and analyzed two mouse lines in which the alpha2 or alpha3 GABAA (gamma-aminobutyric acid type A) receptors, respectively, were rendered insensitive to diazepam by a knock-in point mutation. The anxiolytic action of diazepam was absent in mice with the alpha2(H101R) point mutation but present in mice with the alpha3(H126R) point mutation. These findings indicate that the anxiolytic effect of benzodiazepine drugs is mediated by alpha2 GABAA receptors, which are largely expressed in the limbic system, but not by alpha3 GABAA receptors, which predominate in the reticular activating system.     

Ethanol-induced apoptotic neurodegeneration and fetal alcohol syndrome

The deleterious effects of ethanol on the developing human brain are poorly understood. Here it is reported that ethanol, acting by a dual mechanism [blockade of N-methyl-D-aspartate (NMDA) glutamate receptors and excessive activation of GABA(A) receptors], triggers widespread apoptotic neurodegeneration in the developing rat forebrain. Vulnerability coincides with the period of synaptogenesis, which in humans extends from the sixth month of gestation to several years after birth. During this period, transient ethanol exposure can delete millions of neurons from the developing brain. This can explain the reduced brain mass and neurobehavioral disturbances associated with human fetal alcohol syndrome.     

A mercury-catalyzed, high-yield system for the oxidation of methane to methanol

A homogeneous system for the selective, catalytic oxidation of methane to methanol via methyl bisulfate is reported. The net reaction catalyzed by mercuric ions, Hg(II), is the oxidation of methane by concentrated sulfuric acid to produce methyl bisulfate, water, and sulfur dioxide. The reaction is efficient. At a methane conversion of 50 percent, 85 percent selectivity to methyl bisulfate ( approximately 43 percent yield; the major side product is carbon dioxide) was achieved at a molar productivity of 10(-7) mole per cubic centimeter per second and Hg(II) turnover frequency of 10(-3) per second. Separate hydrolysis of methyl bisulfate and reoxidation of the sulfur dioxide with air provides a potentially practical scheme for the oxidation of methane to methanol with molecular oxygen. The primary steps of the Hg(II)-catalyzed reaction were individually examined and the essential elements of the mechanism were identified. The Hg(II) ion reacts with methane by an electrophilic displacement mechanism to produce an observable species, CH(3)HgOSO(3)H, 1. Under the reaction conditions, 1 readily decomposes to CH(3)OSO(3)H and the reduced mercurous species, Hg(2)(2+) The catalytic cycle is completed by the reoxidation of Hg(2)(2+) with H(2)SO(4) to regenerate Hg(II) and byproducts SO(2) and H(2)O. Thallium(III), palladium(II), and the cations of platinum and gold also oxidize methane to methyl bisulfate in sulfuric acid.     

A look inside the San Andreas Fault at Parkfield through vertical seismic profiling

The San Andreas Fault Observatory at Depth pilot hole is located on the southwestern side of the Parkfield San Andreas fault. This observatory includes a vertical seismic profiling (VSP) array. VSP seismograms from nearby microearthquakes contain signals between the P and S waves. These signals may be P and S waves scattered by the local geologic structure. The collected scattering points form planar surfaces that we interpret as the San Andreas fault and four other secondary faults. The scattering process includes conversions between P and S waves, the strengths of which suggest large contrasts in material properties, possibly indicating the presence of cracks or fluids.     

Wax esters in marine copepods

Two pelagic copepods, Calanus helgolandicus and Gaussia princeps, contained wax esters with 28 to 44 carbon atoms as major lipid constituents. In laboratory cultures of the former species, changes in nutrition (amount or species of diatoms fed) affected both the amount of total lipid and the composition of the wax esters. Thus, the wax esters serve as a reserve energy store in this organism.