49 research outputs found
Pion-Nucleus Scattering at Medium Energies with Densities from Chiral Effective Field Theories
Recently developed chiral effective field theory models provide excellent
descriptions of the bulk characteristics of finite nuclei, but have not been
tested with other observables. In this work, densities from both relativistic
point-coupling models and mean-field meson models are used in the analysis of
meson-nucleus scattering at medium energies. Elastic scattering observables for
790
MeV/ on Pb are calculated in a relativistic impulse
approximation, using the Kemmer-Duffin-Petiau formalism to calculate the
nucleus optical potential.Comment: 9 page
Resolving subunit degeneracy with nonsymmetric pseudocontact shifts
Protein Science (2002), 11:2464–2470Desulfovibrio gigas desulforedoxin (Dx) consists of two identical peptides, each containing one [Fe-4S]center per monomer. Variants with different iron and zinc metal compositions arise when desulforedoxin is
produced recombinantly from Escherichia coli. The three forms of the protein, the two homodimers
[Fe(III)/Fe(III)]Dx and [Zn(II)/Zn(II)]Dx, and the heterodimer [Fe(III)/Zn(II)]Dx, can be separated by ion exchange chromatography on the basis of their charge differences. Once separated, the desulforedoxins
containing iron can be reduced with added dithionite. For NMR studies, different protein samples were prepared labeled with 15N or 15N + 13C. Spectral assignments were determined for [Fe(II)/Fe(II)]Dx and
[Fe(II)/Zn(II)]Dx from 3D 15N TOCSY-HSQC and NOESY-HSQC data, and compared with those reported previously for [Zn(II)/Zn(II)]Dx. Assignments for the 13C shifts were obtained from an HNCA experiment.
Comparison of 1H–15N HSQC spectra of [Zn(II)/Zn(II)]Dx, [Fe(II)/Fe(II)]Dx and [Fe(II)/Zn(II)]Dx revealed that the pseudocontact shifts in [Fe(II)/Zn(II)]Dx can be decomposed into inter- and intramonomer components, which, when summed, accurately predict the observed pseudocontact shifts observed for
[Fe(II)/Fe(II)]Dx. The degree of linearity observed in the pseudocontact shifts for residues 8.5 Å from the metal center indicates that the replacement of Fe(II) by Zn(II) produces little or no change in the structure of Dx. The results suggest a general strategy for the analysis of NMR spectra of homo-oligomeric proteins
in which a paramagnetic center introduced into a single subunit is used to break the magnetic symmetry and make it possible to obtain distance constraints (both pseudocontact and NOE) between subunits
Relativistic Point-Coupling Models as Effective Theories of Nuclei
Recent studies have shown that concepts of effective field theory such as
naturalness can be profitably applied to relativistic mean-field models of
nuclei. Here the analysis by Friar, Madland, and Lynn of naturalness in a
relativistic point-coupling model is extended. Fits to experimental nuclear
data support naive dimensional analysis as a useful principle and imply a
mean-field expansion analogous to that found for mean-field meson models.Comment: 26 pages, REVTeX 3.0 with epsf.sty, plus 5 figure
The Axial-Vector Current in Nuclear Many-Body Physics
Weak-interaction currents are studied in a recently proposed effective field
theory of the nuclear many-body problem. The Lorentz-invariant effective field
theory contains nucleons, pions, isoscalar scalar () and vector
() fields, and isovector vector () fields. The theory exhibits a
nonlinear realization of chiral symmetry and has three
desirable features: it uses the same degrees of freedom to describe the
axial-vector current and the strong-interaction dynamics, it satisfies the
symmetries of the underlying theory of quantum chromodynamics, and its
parameters can be calibrated using strong-interaction phenomena, like hadron
scattering or the empirical properties of finite nuclei. Moreover, it has
recently been verified that for normal nuclear systems, it is possible to
systematically expand the effective lagrangian in powers of the meson fields
(and their derivatives) and to reliably truncate the expansion after the first
few orders. Here it is shown that the expressions for the axial-vector current,
evaluated through the first few orders in the field expansion, satisfy both
PCAC and the Goldberger--Treiman relation, and it is verified that the
corresponding vector and axial-vector charges satisfy the familiar chiral
charge algebra. Explicit results are derived for the Lorentz-covariant,
axial-vector, two-nucleon amplitudes, from which axial-vector meson-exchange
currents can be deduced.Comment: 32 pages, REVTeX 4.0 with 12pt.rtx, aps.rtx, revsymb.sty,
revtex4.cls, plus 14 figures; two sentences added in Summary; two references
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Carboxylic ester hydrolases from hyperthermophiles
Carboxylic ester hydrolyzing enzymes constitute a large group of enzymes that are able to catalyze the hydrolysis, synthesis or transesterification of an ester bond. They can be found in all three domains of life, including the group of hyperthermophilic bacteria and archaea. Esterases from the latter group often exhibit a high intrinsic stability, which makes them of interest them for various biotechnological applications. In this review, we aim to give an overview of all characterized carboxylic ester hydrolases from hyperthermophilic microorganisms and provide details on their substrate specificity, kinetics, optimal catalytic conditions, and stability. Approaches for the discovery of new carboxylic ester hydrolases are described. Special attention is given to the currently characterized hyperthermophilic enzymes with respect to their biochemical properties, 3D structure, and classification