Structure of rabbit liver fructose 1,6-bisphosphatase at 2.3 Å resolution

The three-dimensional structure of the R form of rabbit liver fructose 1,6-bisphosphatase (Fru-1,6-Pase; E.C. 3.1.3.11) has been determined by a combination of heavy-atom and molecular-replacement methods. A model, which includes 2394 protein atoms and 86 water molecules, has been refined at 2.3 Å resolution to a crystallographic R factor of 0.177. The root-mean-square deviations of bond distances and angles from standard geometry are 0.012 Å and 1.7°, respectively. This structural result, in conjunction with recently redetermined amino-acid sequence data, unequivocally establishes that the rabbit liver enzyme is not an aberrant bisphosphatase as once believed, but is indeed homologous to other Fru-1,6-Pases. The root-mean-square deviation of the C atoms in the rabbit liver structure from the homologous atoms in the pig kidney structure complexed with the product, fructose 6-phosphate, is 0.7 Å. Fru-1,6-Pases are homotetramers, and the rabbit liver protein crystallizes in space group I222 with one monomer in the asymmetric unit. The structure contains a single endogenous Mg<sup>2+</sup> ion coordinated by Glu97, Asp118, Asp121 and Glu280 at the site designated metal site 1 in pig kidney Fru-1,6-Pase R-form complexes. In addition, two sulfate ions, which are found at the positions normally occupied by the 6-phosphate group of the substrate, as well as the phosphate of the allosteric inhibitor AMP appear to provide stability. Met177, which has hydrophobic contacts with the adenine moiety of AMP in pig kidney T-form complexes, is replaced by glycine. Binding of a non-hydrolyzable substrate analog,<sup> β</sup>-methyl-fructose 1,6-bisphosphate, at the catalytic site is also examined

Symmetrization and enhancement of the continuous Morlet transform

The forward and inverse wavelet transform using the continuous Morlet basis may be symmetrized by using an appropriate normalization factor. The loss of response due to wavelet truncation is addressed through a renormalization of the wavelet based on power. The spectral density has physical units which may be related to the squared amplitude of the signal, as do its margins the mean wavelet power and the integrated instant power, giving a quantitative estimate of the power density with temporal resolution. Deconvolution with the wavelet response matrix reduces the spectral leakage and produces an enhanced wavelet spectrum providing maximum resolution of the harmonic content of a signal. Applications to data analysis are discussed.Comment: 12 pages, 8 figures, 2 tables, minor revision, final versio

Subsurface tropical Pacific nitrogen isotopic composition of nitrate: Biogeochemical signals and their transport

We report measurements of the nitrogen isotopic composition of nitrate (the δ15N of NO3−) across the equatorial Pacific, for zonal transects from 165°E to 95°W and meridional transects across 95° and 110°W. The δ15N of NO3− is similar in the equatorial thermocline (≈100 m) and intermediate depth waters (≈150 to 600 m), averaging (7.1 ± 0.3)‰ and (7.1 ± 0.1)‰, respectively. These values are more than 2‰ higher than subthermocline waters of the Southern and Atlantic Oceans and are ≈1‰ higher than putative source waters in the high latitude South Pacific (Subantarctic Mode Water, SAMW). The combined constraints of nitrate concentration and δ15N of NO3−in the equatorial Pacific require (1) lateral exchange between the high‐latitude source waters and the zones of denitrification in the eastern tropical Pacific and (2) the accumulation of remineralized nutrients at depth. The zonal uniformity of the subsurface equatorial Pacificδ15N of NO3− indicates rapid transport within the equatorial zone, which works to homogenize the δ15N of NO3− across the Pacific basin. Against this backdrop of high δ15N of NO3−in the tropical Pacific, we find a discrete off‐equatorial core of lowerδ15N of NO3− (5.5 ± 0.3)‰ concentrated at 5°S and 150 to 200 m along the 110° and 95°W transects and in apparent association with the Southern Subsurface Counter Current (SSCC). We propose that the remineralized products of nitrogen fixation, at the source of the SSCC in the western south Pacific, are the origin of the low δ15N of NO3− in these waters

Effective Fokker-Planck Equation for Birhythmic Modified van der Pol Oscillator

We present an explicit solution based on the phase-amplitude approximation of the Fokker-Planck equation associated with the Langevin equation of the birhythmic modified van der Pol system. The solution enables us to derive probability distributions analytically as well as the activation energies associated to switching between the coexisting different attractors that characterize the birhythmic system. Comparing analytical and numerical results we find good agreement when the frequencies of both attractors are equal, while the predictions of the analytic estimates deteriorate when the two frequencies depart. Under the effect of noise the two states that characterize the birhythmic system can merge, inasmuch as the parameter plane of the birhythmic solutions is found to shrink when the noise intensity increases. The solution of the Fokker-Planck equation shows that in the birhythmic region, the two attractors are characterized by very different probabilities of finding the system in such a state. The probability becomes comparable only for a narrow range of the control parameters, thus the two limit cycles have properties in close analogy with the thermodynamic phases

Lower critical field measurements in YBa2Cu3O(6+x) single crystals

The temperature dependence of the lower critical field in YBa2Cu3O(6+x) single crystals was determined by magnetization measurements with the applied field parallel and perpendicular to the c-axis. Results are compared with data from the literature and fitted to Ginzberg-Landau equations by assuming a linear dependence of the parameter kappa on temperature. A value of 7 plus or minus 2 kOe was estimated for the thermodynamic critical field at T = O by comparison of calculated H (sub c2) values with experimental data from the literature

Measurement of H(sub c1) in a single crystal of YBa2Cu3O7 with low pinning

The measurement of H(sub c1) in barium yttrium copper oxide (BYCO) is often ambiguous because the presence of large pinning forces makes it difficult to discern exactly where the first deviation from linearity occurs. In addition there are complications because demagnetizing factors are often not well known. By utilizing a single crystal of YBCO with a nearly cubic shape, the uncertainty in the demagnetizing factor was minimized. In addition, the crystal used exhibited a very small amount of pinning with H applied perpendicular to the c axis, and a sharp break in the initial magnetization vs. field curve could be observed over a wide range of temperature. This allowed a precise determination of H(sub c1). The measured values of H(sub c1) could be well described by the Abrikosov relation with a Ginzburg-Landau parameter which varied linearly with temperature

Dark-Matter Decays and Self-Gravitating Halos

We consider models in which a dark-matter particle decays to a slightly less massive daughter particle and a noninteracting massless particle. The decay gives the daughter particle a small velocity kick. Self-gravitating dark-matter halos that have a virial velocity smaller than this velocity kick may be disrupted by these particle decays, while those with larger virial velocities will be heated. We use numerical simulations to follow the detailed evolution of the total mass and density profile of self-gravitating systems composed of particles that undergo such velocity kicks as a function of the kick speed (relative to the virial velocity) and the decay time (relative to the dynamical time). We show how these decays will affect the halo mass-concentration relation and mass function. Using measurements of the halo mass-concentration relation and galaxy-cluster mass function to constrain the lifetime--kick-velocity parameter space for decaying dark matter, we find roughly that the observations rule out the combination of kick velocities greater than 100 km/s and decay times less than a few times the age of the Universe.Comment: 17 pages, 10 figures, replaced with published versio

Galaxy-Galaxy Lensing in the Hubble Deep Field: The Halo Tully-Fisher Relation at Intermediate Redshift

A tangential distortion of background source galaxies around foreground lens galaxies in the Hubble Deep Field is detected at the 99.3% confidence level. An important element of our analysis is the use of photometric redshifts to determine distances of lens and source galaxies and rest-frame B-band luminosities of the lens galaxies. The lens galaxy halos obey a Tully-Fisher relation between halo circular velocity and luminosity; the typical lens galaxy, at a redshift z = 0.6, has a circular velocity of 210 +/-40 km/s at M_B = -18.5, if q_0 = 0.5. Control tests, in which lens and source positions and source ellipticities are randomized, confirm the significance level of the detection quoted above. Furthermore, a marginal signal is also detected from an independent, fainter sample of source galaxies without photometric redshifts. Potential systematic effects, such as contamination by aligned satellite galaxies, the distortion of source shapes by the light of the foreground galaxies, PSF anisotropies, and contributions from mass distributed on the scale of galaxy groups are shown to be negligible. A comparison of our result with the local Tully-Fisher relation indicates that intermediate-redshift galaxies are fainter than local spirals by 1.0 +/- 0.6 B mag at a fixed circular velocity. This is consistent with some spectroscopic studies of the rotation curves of intermediate-redshift galaxies. This result suggests that the strong increase in the global luminosity density with redshift is dominated by evolution in the galaxy number density.Comment: Revised version with minor changes. 13 pages, 7 figures, LaTeX2e, uses emulateapj and multicol styles (included). Accepted by Ap

Detecting the Earliest Galaxies Through Two New Sources of 21cm Fluctuations

The first galaxies that formed at a redshift ~20-30 emitted continuum photons with energies between the Lyman-alpha and Lyman limit wavelengths of hydrogen, to which the neutral universe was transparent except at the Lyman-series resonances. As these photons redshifted or scattered into the Lyman-alpha resonance they coupled the spin temperature of the 21cm transition of hydrogen to the gas temperature, allowing it to deviate from the microwave background temperature. We show that the fluctuations in the radiation emitted by the first galaxies produced strong fluctuations in the 21cm flux before the Lyman-alpha coupling became saturated. The fluctuations were caused by biased inhomogeneities in the density of galaxies, along with Poisson fluctuations in the number of galaxies. Observing the power-spectra of these two sources would probe the number density of the earliest galaxies and the typical mass of their host dark matter halos. The enhanced amplitude of the 21cm fluctuations from the era of Lyman-alpha coupling improves considerably the practical prospects for their detection.Comment: 11 pages, 7 figures, ApJ, published. Normalization fixed in top panels of Figures 4-