4,284 research outputs found
Quark-Model Baryon-Baryon Interaction and its Applications to Hypernuclei
The quark-model baryon-baryon interaction fss2, proposed by the Kyoto-Niigata
group, is a unified model for the complete baryon octet (B_8=N, Lambda, Sigma
and Xi), which is formulated in a framework of the (3q)-(3q) resonating-group
method (RGM) using the spin-flavor SU_6 quark-model wave functions and
effective meson-exchange potentials at the quark level. Model parameters are
determined to reproduce properties of the nucleon-nucleon system and the
low-energy cross section data for the hyperon-nucleon scattering. Due to the
several improvements including the introduction of vector-meson exchange
potentials, fss2 has achieved very accurate description of the NN and YN
interactions, comparable to various one-boson exchange potentials. We review
the essential features of fss2 and our previous model FSS, and their
predictions to few-body systems in confrontation with the available
experimental data. Some characteristic features of the B_8 B_8 interactions
with the higher strangeness, S=-2, -3, -4, predicted by fss2 are discussed.
These quark-model interactions are now applied to realistic calculations of
few-body systems in a new three-cluster Faddeev formalism which uses
two-cluster RGM kernels. As for the few-body systems, we discuss the
three-nucleon bound states, the Lambda NN-Sigma NN system for the hypertriton,
the alpha alpha Lambda system for 9Be Lambda, and the Lambda Lambda alpha
system for 6He Lambda Lambda.Comment: 20 pages, 12 figures, 18th Nishinomiya Yukawa Memorial Symposium on
Strangeness in Nuclear Matter, 4 - 5 December 2003, Nishinomiya, Japan. (to
be published in Prog. Theor. Phys. Suppl.
Living with Oxygen
Work on the electronic structures of metalâoxo complexes began in Copenhagen over 50 years ago. This work led to the prediction that tetragonal multiply bonded transition metalâoxos would not be stable beyond the ironârutheniumâosmium oxo wall in the periodic table and that triply bonded metalâoxos could not be protonated, even in the strongest BrĂžnsted acids. In this theory, only double bonded metalâoxos could attract protons, with basicities being a function of the electron donating ability of ancillary ligands. Such correlations of electronic structure with reactivity have gained importance in recent years, most notably owing to the widespread recognition that high-valent ironâoxos are intermediates in biological reactions critical to life on Earth.
In this Account, we focus attention on the oxygenations of inert organic substrates by cytochromes P450, as these reactions involve multiply bonded ironâoxos. We emphasize that P450 ironâoxos are strong oxidants, so strong that they would destroy nearby amino acids if substrates are not oxygenated rapidly; it is our view that these high-valent ironâoxos are such dangerous reactive oxygen species that Nature surely found ways to disable them. Looking more deeply into this matter, mainly by examining many thousands of structures in the Protein Data Bank, we have found that P450s and other enzymes that require oxygen for function have chains of tyrosines and tryptophans that extend from active-site regions to protein surfaces. Tyrosines are near the heme active sites in bacterial P450s, whereas tryptophan is closest in most human enzymes. High-valent ironâoxo survival times taken from hole hopping maps range from a few nanoseconds to milliseconds, depending on the distance of the closest Trp or Tyr residue to the heme. In our proposed mechanism, multistep hole tunneling (hopping) through Tyr/Trp chains guides the damaging oxidizing hole to the protein surface, where it can be quenched by soluble protein or small molecule reductants. As the Earthâs oxygenic atmosphere is believed to have developed about 2.5 billion years ago, the increase in occurrence frequency of tyrosine and tryptophan since the last universal evolutionary ancestor may be in part a consequence of enzyme protective functions that developed to cope with the environmental toxin, O_2
Disk-Halo Model for Flat-Spectrum T Tauri Stars
We explore the origin of the flat spectrum seen in some T Tauri stars by
considering a three-component structure: a central star, a circumstellar disk,
and a dusty halo. The radiative energy transport is faithfully treated by
solving the angle- and frequency-dependent radiative transfer equation in two
space dimensions assuming axisymmetry, and hence the radiative equilibrium
temperature in the disk and halo is determined simultaneously. The disk is
effectively heated by the scattering and reprocessing of stellar radiation
through the halo. The large mid- to far-infrared excess originates from the
photosphere of the warmed disk, resulting in a flat spectrum, as observed. The
halo which we consider is observed as a compact reflection nebula, and is
discriminated from extended, disk-like envelopes around flat-spectrum T Tauri
stars. We show that the overall spectral shape of flat-spectrum T Tauri stars
can be reproduced by the present disk-halo model.Comment: 10 pages including 9 figure
Evaluation of Agricultural Statistics for ADAP
The Agricultural Development in the American Pacific (ADAP) Directors requested that the USDA, National Agricultural Statistics Service (NASS) extend its statistical program to the ADAP region: American Samoa, the Federated States of Micronesia (FSM), Palau, the Republic of the Marshall Islands (RMI), Guam, and the Commonwealth of the Northern Marianas (CNMI).This is the final report on the feasibility of, and our recommendations on establishing agricultural statistics in the region. The current section presents material that is generally applicable over the region, with separate sections containing relevant notes for each jurisdictionFunded through the US Department of Agriculture Cooperative Extension Service Grant Number 92-EXCA-1-0187
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