Great metalloclusters in enzymology

Metallocluster-containing enzymes catalyze some of the most basic redox transformations in the biosphere. The reactions catalyzed by these enzymes typically involve small molecules such as N2, CO, and H2 that are used to generate both chemical building blocks and energy for metabolic purposes. During the past decade, structures have been established for the iron-sulfur-based metalloclusters present in the molybdenum nitrogenase, the iron-only hydrogenase, and the nickel-carbon monoxide dehydrogenase, and for the copper-sulfide-based cluster in nitrous oxide reductase. Although these clusters are built from interactions observed in simpler metalloproteins, they contain novel features that may be relevant for their catalytic function. The mechanisms of metallocluster-containing enzymes are still poorly defined, and represent substantial and continuing challenges to biochemists, biophysicists, and synthetic chemists. These proteins also provide a window into the union of the biological and inorganic worlds that may have been relevant to the early evolution of biochemical catalysis

Limits from rapid TeV variability of Mrk 421

The extreme variability event in the TeV emission of Mrk 421, recently reported by the Whipple team, imposes the tightest limits on the typical size of the TeV emitting regions in Active Galactic Nuclei (AGN). We examine the consequences that this imposes on the bulk Lorentz factor of the emitting plasma and on the radiation fields present in the central region of this Active Nucleus. No strong evidence is found for extreme Lorentz factors. However, energetics arguments suggest that any accretion in Mrk 421 has to take place at small rates, compatible with an advection-dominated regime.Comment: 5 pages (Latex MNRAS style), revised version, submitted to MNRA

Neural correlates of consciousness are not pictorial representations

O'Regan & Noe (O&N) are pessimistic about the prospects for discovering the neural correlates of consciousness. They argue that there can be no one-to-one correspondence between awareness and patterns of neural activity in the brain, so a project attempting to identify the neural correlates of consciousness is doomed to failure. We believe that this degree of pessimism may be overstated; recent empirical data show some convergence in describing consistent patterns of neural activity associated with visual consciousness

Fitting Pulsar Wind Tori. II. Error Analysis and Applications

We have applied the torus fitting procedure described in Ng & Romani (2004) to PWNe observations in the Chandra data archive. This study provides quantitative measurement of the PWN geometry and we characterize the uncertainties in the fits, with statistical errors coming from the fit uncertainties and systematic errors estimated by varying the assumed fitting model. The symmetry axis $\Psi$ of the PWN are generally well determined, and highly model-independent. We often derive a robust value for the spin inclination $\zeta$. We briefly discuss the utility of these results in comparison with new radio and high energy pulse measurementsComment: 15 pages, 3 figures, ApJ in pres

Implications of very rapid TeV variability in blazars

We discuss the implications of rapid (few-minute) variability in the TeV flux of blazars, which has been observed recently with the HESS and MAGIC telescopes. The variability timescales seen in PKS 2155-304 and Mrk 501 are much shorter than inferred light-crossing times at the black hole horizon, suggesting that the variability involves enhanced emission in a small region within an outflowing jet. The enhancement could be triggered by dissipation in part of the black hole's magnetosphere at the base of the outflow, or else by instabilities in the jet itself. By considering the energetics of the observed flares, along with the requirement that TeV photons escape without producing pairs, we deduce that the bulk Lorentz factors in the jets must be >50. The distance of the emission region from the central black hole is less well-constrained. We discuss possible consequences for multi-wavelength observations.Comment: 5 pages, no figures, accepted for publication in Monthly Notices of the Royal Astronomical Society Letter

The matter content of the jet in M87: evidence for an electron-positron jet

Recent observations have allowed the geometry and kinematics of the M87 jet to be tightly constrained. We combine these constraints with historical Very Long Baseline Interferometry (VLBI) results and the theory of synchrotron self-absorbed radio cores in order to investigate the physical properties of the jet. Our results strongly suggest the jet to be dominated by an electron-positron (pair) plasma. Although our conservative constraints cannot conclusively dismiss an electron-proton plasma, the viability of this solution is extremely vulnerable to further tightening of VLBI surface brightness limits. The arguments presented, coupled with future high-resolution multi-frequency VLBI studies of the jet core, will be able to firmly distinguish these two possibilities.Comment: 8 pages, 1 ps figure. Revised and accepted for publication in MNRA

How many metals does it take to fix N2? A mechanistic overview of biological nitrogen fixation

During the process of biological nitrogen fixation, the enzyme nitrogenase catalyzes the ATP-dependent reduction of dinitrogen to ammonia. Nitrogenase consists of two component metalloproteins, the iron (Fe) protein and the molybdenum-iron (MoFe) protein; the Fe protein mediates the coupling of ATP hydrolysis to interprotein electron transfer, whereas the active site of the MoFe protein contains the polynuclear FeMo cofactor, a species composed of seven iron atoms, one molybdenum atom, nine sulfur atoms, an interstitial light atom, and one homocitrate molecule. This Perspective provides an overview of biological nitrogen fixation and introduces three contributions to this special feature that address central aspects of the mechanism and assembly of nitrogenase

Avalanche noise characteristics of single Al/sub x/Ga/sub 1-x/As(0.3

Avalanche multiplication and excess noise have been measured on a series of Al/sub x/Ga/sub 1-x/As-GaAs and GaAs-Al/sub x/Ga/sub 1-x/As (x=0.3,0.45, and 0.6) single heterojunction p/sup +/-i-n/sup +/ diodes. In some devices excess noise is lower than in equivalent homojunction devices with avalanche regions composed of either of the constituent materials, the heterojunction with x=0.3 showing the greatest improvement. Excess noise deteriorates with higher values of x because of the associated increase in hole ionization in the Al/sub x/Ga/sub 1-x/As layer. It also depends critically upon the carrier injection conditions and Monte Carlo simulations show that this dependence results from the variation in the degree of noisy feedback processes on the position of the injected carriers

Indicator systems - resource use in organic systems

A balanced use of resources within organic farming systems is required to maintain sustainable systems. Hence, it is essential to have tools that can assess the use of resources within the farming system and their impact on the environment. The range of tools that have been developed include those assessing local farm-scale issues together with those that assess impacts at the global scale. At the global scale assessments are usually made on the basis of a unit of product whereas at the local scale assessments can also be made on an area basis. In addition, the tools also assess a variety of issues, e.g. biodiversity, pollution potential, energy and water use. The level of detail required for the different assessment tools differs substantially; nevertheless it is essential that the indicator systems developed are based on sound knowledge, are acceptable to the farmers and can guide their future actions