Structure and magnetism in Cr-embedded Co nanoparticles.

We present the results of an investigation into the atomic structure and magnetism of 2 nm diameter Co nanoparticles embedded in an antiferromagnetic Cr matrix. The nanocomposite films used in this study were prepared by co-deposition directly from the gas phase, using a gas aggregation source for the Co nanoparticles and a molecular beam epitaxy (MBE) source for the Cr matrix material. Co K and Cr K edge extended x-ray absorption fine structure (EXAFS) experiments were performed in order to investigate atomic structure in the embedded nanoparticles and matrix respectively, while magnetism was investigated by means of a vibrating sample magnetometer. The atomic structure type of the Co nanoparticles is the same as that of the Cr matrix (bcc) although with a degree of disorder. The net Co moment per atom in the Co/Cr nanocomposite films is significantly reduced from the value for bulk Co, and decreases as the proportion of Co nanoparticles in the film is decreased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.25 µ B/atom. After field cooling to below 30 K all samples showed an exchange bias, which was largest for the most dilute sample. Both the structural and magnetic results point towards a degree of alloying at the nanoparticle/matrix interface, leading to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core.

Structure and magnetism in Fe/FexPd1-x core/shell nanoparticles formed by alloying in Pd-embedded Fe nanoparticles.

We have investigated atomic structure and magnetism in Fe nanoparticles with a diameter of 2 nm embedded in a Pd matrix. The samples for these studies were prepared directly from the gas phase by co-deposition, using a gas aggregation source and an MBE-type source for the Fe nanoparticles and Pd matrix respectively. Extended absorption fine structure (EXAFS) measurements indicate that there is an appreciable degree of alloying at the nanoparticle/matrix interface; at dilute nanoparticle concentrations, more than half of the Fe atoms are alloyed with Pd. This leads to a core/shell structure in the embedded nanoparticles, with an FexPd1-x shell surrounding a reduced pure Fe core. Magnetism in the nanocomposite samples was probed by means of magnetometry measurements, which were interpreted in the light of their atomic structure. These point to a magnetized cloud of Pd atoms surrounding the embedded nanoparticles which is significantly larger than around single Fe atoms in Pd. The coercivities in the Fe/Pd nanocomposite samples are larger than in FexPd1-x atomic alloys of corresponding composition, which is consistent with exchange coupling between the magnetically harder and softer regions in the nanocomposite samples.

Realizing high magnetic moments in fcc Fe nanoparticles through atomic structure stretch.

We describe the realization of a high moment state in fcc Fe nanoparticles through a controlled change in their atomic structure. Embedding Fe nanoparticles in a Cu(1-x)Au(x) matrix causes their atomic structure to switch from bcc to fcc. Extended x-ray absorption fine structure (EXAFS) measurements show that the structure in both the matrix and the Fe nanoparticles expands as the amount of Au in the matrix is increased, with the data indicating a tetragonal stretch in the Fe nanoparticles. The samples were prepared directly from the gas phase by co-deposition, using a gas aggregation source and MBE-type sources respectively for the nanoparticle and matrix materials. The structure change in the Fe nanoparticles is accompanied by a sharp increase in atomic magnetic moment, ultimately to values of ~2.5 ± 0.3 µ(B)/atom .

Probing atomic structure in magnetic core/shell nanoparticles using synchrotron radiation.

Core/shell Fe/Cu and Fe/Au nanoparticles were prepared directly by deposition from the gas phase. A detailed study of the atomic structure in both the cores and shells of the nanoparticles was undertaken by means of extended absorption fine structure (EXAFS) measurements. For Fe/Cu nanoparticles, a Cu shell ∼ 20 monolayers thick appears similar in structure to bulk Cu and is sufficient to cause the structure in the Fe core to switch from body centred cubic (bcc; as in bulk Fe) to face centred cubic. This is not the case for thinner Cu shells, 1-2 monolayers in thickness, in which there is a considerable contraction in nearest-neighbour interatomic distance as the shell structure changes to bcc. In Fe/Au nanoparticles, the crystal structure in the Fe core remains bcc for all Au thicknesses although there is some stretching of the lattice. In thin Au shells ∼ 2 monolayers thick, there is strong contraction in interatomic distances. There does not appear to be significant alloying at the Fe/Au interface.

Extended x-ray absorption fine structure studies of the atomic structure of nanoparticles in different metallic matrices.

It has been appreciated for some time that the novel properties of particles in the size range 1-10 nm are potentially exploitable in a range of applications. In order to ultimately produce commercial devices containing nanosized particles, it is necessary to develop controllable means of incorporating them into macroscopic samples. One way of doing this is to embed the nanoparticles in a matrix of a different material, by co-deposition for example, to form a nanocomposite film. The atomic structure of the embedded particles can be strongly influenced by the matrix. Since some of the key properties of materials, including magnetism, strongly depend on atomic structure, the ability to determine atomic structure in embedded nanoparticles is very important. This review focuses on nanoparticles, in particular magnetic nanoparticles, embedded in different metal matrices. Extended x-ray absorption fine structure (EXAFS) provides an excellent means of probing atomic structure in nanocomposite materials, and an overview of this technique is given. Its application in probing catalytic metal clusters is described briefly, before giving an account of the use of EXAFS in determining atomic structure in magnetic nanocomposite films. In particular, we focus on cluster-assembled films comprised of Fe and Co nanosized particles embedded in various metal matrices, and show how the crystal structure of the particles can be changed by appropriate choice of the matrix material. The work discussed here demonstrates that combining the results of structural and magnetic measurements, as well as theoretical calculations, can play a significant part in tailoring the properties of new magnetic cluster-assembled materials.

Carbon cycling: the prokaryotic contribution.

Although the debate continues, the concept of global warming as a consequence of the increased production of 'greenhouse gases' via human activities is now widely accepted. The role of microbes, especially the prokaryotes, in the formation, trapping and retention of 'greenhouse gases' has, for the most part, been overlooked. The future requires that we pay close attention to these organisms for possible solutions to adverse global changes.

Influence of Cropping Systems on Stem Rot (Sclerotium rolfsii), Meloidogyne arenaria, and the Nematode Antagonist Pasteuria penetrans in Peanut.

The effect of crop rotation (main plots) and pesticide treatment (subplots) on stem rot (Sclerotium rolfsii), Meloidogyne arenaria, and the nematode antagonist Pasteuria penetrans was determined in a field experiment. The field site was naturally infested with all three organisms. Peanut (P) was rotated with 2 years of either cotton (Ct), corn (C), or bahiagrass (B). The pesticide treatments for the peanut crop were aldicarb (31 g a.i. per 100-m row), flutolanil (1.7 kg a.i./ha), aldicarb + flutolanil, and a control without either pesticide. Populations of M. arenaria were lower in peanut in the Ct-Ct-P than in P-P-P, C-C-P, or B-B-P plots and tended to be lower in plots treated with aldicarb. Abundance of P. penetrans endospores was highest in the P-P-P plots, intermediate in the B-B-P rotations, lowest in all other rotations, and was unaffected by aldicarb. The high endospore densities in the P-P-P plots may have contributed to the uncharacteristically low nematode populations in the monoculture. Incidence of stem rot in peanut was lowest in treatments with flutolanil, intermediate in the control, and highest in treatments with aldicarb alone. The greater canopy cover in aldicarb-treated plots may have created a conducive environment for S. rolfsii infection.

Identification and localization of the carboxysome peptide Csos3 and its corresponding gene in Thiobacillus neapolitanus.

Four genes encoding carboxysome shell peptides (csoS1A, csoS1B, csoS1C, csoS2), the genes encoding the large and small subunits of RuBisCO (cbbL, cbbS), and three unidentified ORFs constitute an operon in Thiobacillus neapolitanus. An unidentified ORF 1.54 kb in size is predicted from sequence analysis to encode a protein with a molecular mass of approximately 57 kDa. When this ORF was expressed in Escherichia coli under the control of its endogenous ribosome-binding site, no peptide product was observed. In order to correlate this ORF with a carboxysome peptide, the ORF was overexpressed in E. coli by cloning it into pProExHTb, a prokaryotic expression vector containing an E. coli ribosome binding site. When antibodies raised against the recombinant protein were used to probe an immunoblot containing carboxysome peptides, a 60-kDa peptide was recognized. The peptide was subsequently named CsoS3. CsoS3 is a minor component of the carboxysome; a peptide of this size is commonly not observed or is very faint on Coomassie blue-stained SDS-polyacrylamide gels of purified carboxysomes. Immunogold labeling established CsoS3 to be a component of the carboxysome shell.

Bahiagrass, corn, cotton rotations, and pesticides for managing nematodes, diseases, and insects on peanut.

Florunner peanut was grown after 1 and 2 years of Tifton 9 bahiagrass, corn, cotton, and continuous peanut as whole-plots. Pesticide treatments aldicarb (3.4 kg a.i./ha), flutolanil (1.7 kg a.i./ha), aldicarb + flutolanil, and untreated (control) were sub-plots. Numbers of Meloidogyne arenaria second-stage juveniles in the soil and root-gall indices of peanut at harvest were consistently lower in plots treated with aldicarb and aldicarb + flutolanil than in flutolanil-treated and untreated plots. Percentages of peanut leaflets damaged by thrips and leafhoppers were consistently greater in flutolaniltreated and untreated plots than in plots treated with aldicarb or aldicarb + flutolanil but not affected by cropping sequences. Incidence of southern stem rot was moderate to high for all chemical treatments except those that included flutolanil. Stem rot loci were low in peanut following 2 years of bahiagrass, intermediate following 2 years of corn or cotton, and highest in continuous peanut. Rhizoctonia limb rot was more severe in the peanut monoculture than in peanut following 2 years of bahiagrass, corn, or cotton. Flutolanil alone or combined with aldicarb suppressed limb rot compared with aldicarb-treated and untreated plots. Peanut pod yields were 4,186 kg/ha from aldicarb + flutolanil-treated plots, 3,627 kg/ha from aldicarb-treated plots, 3,426 kg/ha from flutolanil-treated plots, and 3,056 kg/ha from untreated plots. Yields of peanut following 2 years of bahiagrass, corn, and cotton were 29% to 33% higher than yield of monocultured peanut.

Insertion mutation of the form I cbbL gene encoding ribulose bisphosphate carboxylase/oxygenase (RuBisCO) in Thiobacillus neapolitanus results in expression of form II RuBisCO, loss of carboxysomes, and an increased CO2 requirement for growth.

It has been previously established that Thiobacillus neapolitanus fixes CO2 by using a form I ribulose bisphosphate carboxylase/oxygenase (RuBisCO), that much of the enzyme is sequestered into carboxysomes, and that the genes for the enzyme, cbbL and cbbS, are part of a putative carboxysome operon. In the present study, cbbL and cbbS were cloned and sequenced. Analysis of RNA showed that cbbL and cbbS are cotranscribed on a message approximately 2,000 nucleotides in size. The insertion of a kanamycin resistance cartridge into cbbL resulted in a premature termination of transcription; a polar mutant was generated. The mutant is able to fix CO2, but requires a CO2 supplement for growth. Separation of cellular proteins from both the wild type and the mutant on sucrose gradients and subsequent analysis of the RuBisCO activity in the collected fractions showed that the mutant assimilates CO2 by using a form II RuBisCO. This was confirmed by immunoblot analysis using antibodies raised against form I and form II RuBisCOs. The mutant does not possess carboxysomes. Smaller, empty inclusions are present, but biochemical analysis indicates that if they are carboxysome related, they are not functional, i.e., do not contain RuBisCO. Northern analysis showed that some of the shell components of the carboxysome are produced, which may explain the presence of these inclusions in the mutant.

Peanut-cotton-rye rotations and soil chemical treatment for managing nematodes and thrips.

In the southeastern United States, a cotton-peanut rotation is attractive because of the high value and extensive planting of both crops in the region. The objective of this experiment was to determine the effects of cotton-peanut rotations, rye, and soil chemical treatments on management of plant-parasitic nematodes, thrips, and soilborne fungal diseases and on crop yield. Peanut-cotton-rye rotations were conducted from 1988 to 1994 on Tifton loamy sand (Plinthic Kandiudult) infested primarily with Meloidogyne incognita race 3, Belonolaimus longicaudatus, Sclerotium rolfsii, Rhizoctonia solani, and Fusarium oxysporum. Continuous peanut, continuous cotton, cotton-peanut rotation, or peanut-cotton rotation were used as main plots; winter rye or fallow as sub-plots; and cotton with and without aldicarb (3.36 kg a.i./ha), or peanut with and without aldicarb (3.36 kg a.i./ha) plus flutolanil (1.12 kg a.i./ha), as sub-sub-plots. Population densities of M. incognita and B. longicaudatus declined rapidly after the first crop in continuous peanut and remained low thereafter. Neither rye nor soil chemical treatment affected M. incognita or B. longicaudatus population density on peanut or cotton. Cotton and peanut yields from the cotton-peanut rotation were 26% and 10% greater, respectively, than those from monoculmre over the 7-year study. Cotton and peanut yields were improved 9% and 4%, respectively, following rye vs. fallow. Soil chemical treatments increased yields of cotton 23% and peanut 32% over those of untreated plots. Our data demonstrate the sustainable benefits of using cotton-peanut rotations, winter rye, and soil chemical treatments to manage plant-parasitic nematodes and other pests and pathogens and improve yield of both cotton and peanut.

Crop yields and nematode population densities in triticale-cotton and triticale-soybean rotations.

Triticale cv. Beagle 82, cotton cv. McNair 235, and soybean cv. Twiggs were arranged in three cropping sequences to determine the effects of fenamiphos and cropping sequence on nematode population densities and crop yields under conservation tillage for 4 years. The cropping sequences were triticale (T)-cotton (C)-T-C, T-soybean (S)-T-S, and T-C-T-S. Numbers of Meloidogyne incognita second-stage juveniles declined on trificale but increased on cotton and soybean each year. Root-gall indices of cotton and soybean ranged from 1.00 to 1.08 (1 to 5 scale: 1 = 0%, 2 = 1% to 25%, 3 = 26% to 50%, 4 = 51% to 75%, and 5 = 76% to 100% of roots galled) each year and were not affected by fenamiphos treatment or cropping sequence. Numbers of Pratylenchus brachyurus were maintained on trificale and generally increased more on soybean than on cotton. Population densities of Helicotylenchus dihystera were near or below detection levels in all plots during the first year and increased thereafter in untreated plots in the T-C-T-C and T-S-T-S sequences. Generally, yields of triticale in all cropping sequences declined over the years. Yields of cotton and soybean were not affected by fenamiphos at 6.7 kg a.i./ha. Cotton and soybean were grown successfully with little or no suppression in yields caused by nematodes in conservation tillage following triticale harvested for grain.

Alanine-scanning mutagenesis of protein phosphatase type 1 in the yeast Saccharomyces cerevisiae.

Protein phosphatase type 1, encoded by GLC7 in Saccharomyces cerevisiae, is an essential serine/threonine phosphatase implicated in the regulation of a diverse array of physiological functions. We constructed and examined 20 mutant alleles of GLC7 in which codons encoding clusters of charged residues were changed to alanine codons. Three of 20 mutant alleles alter residues in the active site of the phosphatase and are unable to rescue the lethality of a glc7::LEU2 disruption. The 17 alleles that support growth confer a range of mutant traits including cell cycle arrest, 2-deoxyglucose resistance, altered levels of glycogen, sensitivity to high salt, and sporulation defects. For some traits, such as 2-deoxyglucose resistance and cell cycle arrest, the mutated residues map to specific regions of the protein whereas the mutated residues in glycogen-deficient mutants and sporulation-defective mutants are more widely distributed over the protein surface. Many mutants have complex phenotypes, each displaying a diverse range of defects. The wide range of phenotypes identified from the collection of mutant alleles is consistent with the hypothesis that Glc7p-binding proteins, which are thought to regulate the specificity of Glc7p, have overlapping binding sites on the surface of Glc7p. This could account for the high level of sequence conservation found among type 1 protein phosphatases from different species.

Deduced amino acid sequence, functional expression, and unique enzymatic properties of the form I and form II ribulose bisphosphate carboxylase/oxygenase from the chemoautotrophic bacterium Thiobacillus denitrificans.

The cbbL cbbS and cbbM genes of Thiobacillus denitrificans, encoding form I and form II ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), respectively, were found to complement a RubisCO-negative mutant of Rhodobacter sphaeroides to autotrophic growth. Endogenous T. denitrificans promoters were shown to function in R. sphaeroides, resulting in high levels of cbbL cbbS and cbbM expression in the R. sphaeroides host. This expression system provided high levels of both T. denitrificans enzymes, each of which was highly purified. The deduced amino acid sequence of the form I enzyme indicated that the large subunit was closely homologous to previously sequenced form I RubisCO enzymes from sulfur-oxidizing bacteria. The form I T. denitrificans enzyme possessed a very low substrate specificity factor and did not exhibit fallover, and yet this enzyme showed a poor ability to recover from incubation with ribulose 1,5-bisphosphate. The deduced amino acid sequence of the form II T. denitrificans enzyme resembled those of other form II RubisCO enzymes. The substrate specificity factor was characteristically low, and the lack of fallover and the inhibition by ribulose 1,5-bisphosphate were similar to those of form II RubisCO obtained from nonsulfur purple bacteria. Both form I and form II RubisCO from T. denitrificans possessed high KCO2 values, suggesting that this organism might suffer in environments containing low levels of dissolved CO2. These studies present the initial description of the kinetic properties of form I and form II RubisCO from a chemoautotrophic bacterium that synthesizes both types of enzyme.

The 20,000 dalton structural variant of recombinant DNA-derived methionyl human growth hormone has early insulin-like effects in hypophysectomized rats.

The 20,000 dalton variant of recombinant DNA-derived methionyl human growth hormone (20K-Met-hGH) induced decreases in blood glucose and free fatty acid concentrations one hour after intraperitoneal injection into fasted, hypophysectomized rats. Similar results were obtained using the 22,000 dalton form of recombinant DNA-derived methionyl human growth hormone (22K-Met-hGH). The data reported show that 20K-Met-hGH induces early insulin-like effects similar to the responses produced by 22K-Met-hGH in fasted hypophysectomized rats.