8,276 research outputs found
Self-referential Monte Carlo method for calculating the free energy of crystalline solids
A self-referential Monte Carlo method is described for calculating the free energy of crystalline solids. All Monte Carlo methods for the free energy of classical crystalline solids calculate the free-energy difference between a state whose free energy can be calculated relatively easily and the state of interest. Previously published methods employ either a simple model crystal, such as the Einstein crystal, or a fluid as the reference state. The self-referential method employs a radically different reference state; it is the crystalline solid of interest but with a different number of unit cells. So it calculates the free-energy difference between two crystals, differing only in their size. The aim of this work is to demonstrate this approach by application to some simple systems, namely, the face centered cubic hard sphere and Lennard-Jones crystals. However, it can potentially be applied to arbitrary crystals in both bulk and confined environments, and ultimately it could also be very efficient
Anomalous Hall Effect in Ferromagnetic Semiconductors in the Hopping Transport Regime
We present a theory of the Anomalous Hall Effect (AHE) in ferromagnetic
(Ga,Mn)As in the regime when conduction is due to phonon-assisted hopping of
holes between localized states in the impurity band. We show that the
microscopic origin of the anomalous Hall conductivity in this system can be
attributed to a phase that a hole gains when hopping around closed-loop paths
in the presence of spin-orbit interactions and background magnetization of the
localized Mn moments. Mapping the problem to a random resistor network, we
derive an analytic expression for the macroscopic anomalous Hall conductivity
. We show that is proportional to the
first derivative of the density of states and thus can be
expected to change sign as a function of impurity band filling. We also show
that depends on temperature as the longitudinal conductivity
within logarithmic accuracy.Comment: 4 pages, 1 eps figure, final versio
Incorporation in vitro of labeled amino acids into bone marrow cell proteins
Nearly all experiments on the incorporation of labeled amino acids into tissue proteins in vitro have been done on tissues whose cell structure has been partially or completely disintegrated, e.g. tissue slices, segments, or homogenates. Since cell destruction reduces or abolishes the uptake of labeled amino acids (1), it seemed worth while to carry out studies on intact cells in vitro. Bone marrow cells were found to be suitable for this purpose. The labeled amino acids used were glycine-1-C14, L-leucine-1-C14, L-lysine-1-C14, and L-lysine-6-C14
Isolation of a peptide in guinea pig liver homogenate and its turnover of leucine
Leucine was synthesized with C14 in the carboxyl group. 10 mg. of the radioactive amino acid (DL) and 0.66 gm. (wet weight) of guinea pig liver homogenate were added to a reaction mixture containing 1.3 per cent of an amino acid mixture corresponding to the composition of casein and 0.005 M fumarate, all in a final volume of 4 ml. of isotonic saline solution(1) at pH 7.4. The reaction was carried out under oxygen for 6 hours at 38°
Alpha-aminoadipic acid: A product of lysine metabolism
As part of a study of protein and peptide metabolism lysine was synthesized with C14 in the ε position and resolved into the L and D isomers. 10 mg. of labeled lysine dihydrochloride (either L- or D-) and 0.66 gm. (wet weight) of guinea pig liver homogenate were added to a reaction mixture containing 1.3 per cent of an amino acid mixture corresponding to the composition of casein except for lysine and 0.01 M α-ketoglutarate, all in a final volume of 4 ml. of isotonic saline solution.(1) The reaction was carried out under oxygen for 6 hours at 38°
The incorporation of labeled lysine into the proteins of guinea pig liver homogenate
When C14-labeled lysine is incubated with guinea pig liver homogenate, α-aminoadipic, α-ketoadipic, and glutaric acids are formed from the lysine (1). These transformations were established by finding the radioactivity of the C14 tracer in the metabolic products. The homogenate proteins coagulated by boiling at pH 5 also contained radioactivity. The counts given by the proteins corresponded to about 0.02 to 0.03 per cent of that added as lysine; the extent of lysine incorporation into the proteins was of the same order of magnitude as Melchior and Tarver (2) had found after incubating S35-labeled methionine and Winnick et al. (3, 4) C14-labeled glycine with rat tissue homogenates. Yet we could not satisfy ourselves that the radioactivity remaining in the proteins in our experiments, although it persisted through exhaustive extraction, did not come from traces of adsorbed radioactive lysine. Some counts were found in the protein when the homogenate was boiled prior to incubation with isotopic lysine
The degradation of L-lysine in guinea pig liver homogenate: formation of alpha-aminoadipic acid
A summary of the little that is known of the metabolism of lysine in animals is as follows: it is indispensable in the diet, its α-amino group does not participate in reversible transamination reaction in vivo (2), neither the L nor D form is attacked by the appropriate amino acid oxidase, certain ε-nitrogen-substituted derivatives can replace lysine in the diet and their α-amino groups are oxidized by amino acid oxidases (3, 4), no α-nitrogen-substituted derivatives yet prepared can substitute for lysine in the diet (4-6)
Incorporation in vitro of labeled amino acids into proteins of rabbit reticuloytes
Continuing our work on the incorporation of labeled amino acids into proteins (1), we have begun a study of the incorporation in vitro of C14-labeled glycine, L-histidine, L-leucine, and L-lysine into the proteins of rabbit reticulocytes. In preliminary experiments the incorporation into the hemoglobin isolated from the reticulocytes was determined. But, after it was found that plasma contains factors accelerating amino acid incorporation, it was decided to proceed as rapidly as possible toward the identification of these factors; we have, therefore, measured incorporation into the total proteins of the reticulocytes, since isolation of the hemoglobin was time-consuming. The results obtained with hemoglobin and with the total proteins are essentially the same, indicating that the other proteins of the reticulocytes incorporate amino acids at approximately the same rate as hemoglobin
Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles
We have studied the magnetic behavior of dextran-coated magnetite
(FeO) nanoparticles with median particle size \left=8 .
Magnetization curves and in-field M\"ossbauer spectroscopy measurements showed
that the magnetic moment of the particles was much smaller than the bulk
material. However, we found no evidence of magnetic irreversibility or
non-saturating behavior at high fields, usually associated to spin canting. The
values of magnetic anisotropy from different techniques indicate that
surface or shape contributions are negligible. It is proposed that these
particles have bulk-like ferrimagnetic structure with ordered A and B
sublattices, but nearly compensated magnetic moments. The dependence of the
blocking temperature with frequency and applied fields, ,
suggests that the observed non-monotonic behavior is governed by the strength
of interparticle interactions.Comment: 11 pages, 7 figures, 3 Table
A peptide fraction in liver
We reported in a preliminary communication (1) the isolation of a peptide fraction from guinea pig liver. The following points of interest appeared at once: many different amino acids were obtained on hydrolysis; the peptide fraction contained most of the indispensable amino acids, which indicated that it probably is important in protein metabolism; when guinea pig liver homogenate was incubated with C14-labeled glycine, leucine, or lysine, these were rapidly incorporated into this peptide fraction, which is further evidence that it is metabolically active; the peptide fraction had not been described hitherto; a fraction containing one or more large peptides can be separated from so complex a mixture as liver homogenate by starch chromatography
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