Drift mobility of holes in single-crystal beta-AgI

The drift mobility of photoexcited holes in single-crystal beta-AgI has been measured between 260 and 312 °K. In this range the drift mobility µd increased with temperature due to trap-limited behavior. At 300 °K µd=12 cm2/V sec, the concentration and energy of the dominant traps being given by Nt=3×109 to 5×109/cm3 and Et=0.52 to 0.50 eV, respectively. Electron drift mobilities could not be determined due to low electron lifetimes. Journal of Applied Physics is copyrighted by The American Institute of Physics.

Functional interaction of diphenols with polyphenol oxidase - Molecular determinants of substrate/inhibitor specificity SO FEBS JOURNAL

Polyphenol oxidase (PPO) catalyzes the oxidation of o-diphenols to their respective quinones. The quinones autopolymerize to form dark pigments, an undesired effect. PPO is therefore the target for the development of antibrowning and antimelanization agents. A series of phenolic compounds experimentally evaluated for their binding affinity and inhibition constants were computationally docked to the active site of catechol oxidase. Docking studies suggested two distinct modes of binding, dividing the docked ligands into two groups. Remarkably, the first group corresponds to ligands determined to be substrates and the second group corresponds to reversible inhibitors. Analyses of the complexes provide structural explanations for correlating subtle changes in the position and nature of the substitutions on o-diphenols to their functional properties as substrates and inhibitors. Higher reaction rates and binding are reckoned by additional interactions of the substrates with key residues that line the hydrophobic cavity. The docking results suggest that inhibition of oxidation stems from an interaction between the aromatic carboxylic acid group and the apical His 109 of the four coordinates of the trigonal pyramidal coordination polyhedron of CuA. The spatial orientation of the hydroxyl in relation to the carboxylic group either allows a perfect fit in the substrate cavity, leading to inhibition, or because of a steric clash flips the molecule vertically, facilitating oxidation. This is the first study to explain, at the molecular level, the determinants Of substrate and inhibitor specificity of a catechol oxidase, thereby providing a platform for the design of selective inhibitors useful to both the food and pharmaceutical industries.

Importance of tyrosine residues of Bacillus stearothermophilus serine hydroxymethyltransferase in cofactor binding and l-allo-Thr cleavage

Serine hydroxymethyltransferase (SHMT) from Bacillus stearothermophilus (bsSHMT) is a pyridoxal 5'-phosphate-dependent enzyme that catalyses the conversion of l-serine and tetrahydrofolate to glycine and 5,10-methylene tetrahydrofolate. In addition, the enzyme catalyses the tetrahydrofolate-independent cleavage of 3-hydroxy amino acids and transamination. In this article, we have examined the mechanism of the tetrahydrofolate-independent cleavage of 3-hydroxy amino acids by SHMT. The three-dimensional structure and biochemical properties of Y51F and Y61A bsSHMTs and their complexes with substrates, especially l-allo-Thr, show that the cleavage of 3-hydroxy amino acids could proceed via Cα proton abstraction rather than hydroxyl proton removal. Both mutations result in a complete loss of tetrahydrofolate-dependent and tetrahydrofolate-independent activities. The mutation of Y51 to F strongly affects the binding of pyridoxal 5'-phosphate, possibly as a consequence of a change in the orientation of the phenyl ring in Y51F bsSHMT. The mutant enzyme could be completely reconstituted with pyridoxal 5'-phosphate. However, there was an alteration in the λmax value of the internal aldimine (396 nm), a decrease in the rate of reduction with NaCNBH3 and a loss of the intermediate in the interaction with methoxyamine (MA). The mutation of Y61 to A results in the loss of interaction with Cα and Cβ of the substrates. X-Ray structure and visible CD studies show that the mutant is capable of forming an external aldimine. However, the formation of the quinonoid intermediate is hindered. It is suggested that Y61 is involved in the abstraction of the Cα proton from 3-hydroxy amino acids. A new mechanism for the cleavage of 3-hydroxy amino acids via Cα proton abstraction by SHMT is proposed.

Perpetuation of immunological memory through common MHC-I binding modes of peptidomimic and antigenic peptides

Understanding the molecular mechanisms of immunological memory assumes importance in vaccine design. We had earlier hypothesized a mechanism for the maintenance of immunological memory through the operation of a network of idiotypic and anti-idiotypic antibodies (Ab2). Peptides derived from an internal image carrying anti-idiotypic antibody are hypothesized to facilitate the perpetuation of antigen specific T cell memory through similarity in peptide-MHC binding as that of the antigenic peptide. In the present work, the existence of such peptidomimics of the antigen in the Ab2 variable region and their similarity of MHC-I binding was examined by bioinformatics approaches. The analysis employing three known viral antigens and one tumor-associated antigen shows that peptidomimics from Ab2 variable regions have structurally similar MHC-I binding patterns as compared to antigenic peptides, indicating a structural basis for memory perpetuation. (C)) 2007 Elsevier Inc. All rights reserved.

The biology and pathogenesis of hepatitis viruses

Viral hepatitis is caused mainly by infection with one of the five hepatitis viruses, which use the liver as their primary site of replication. Each of these, known as hepatitis A through E viruses (HAV to HEV), belong to different virus families, have unique morphology, genomic organization and replication strategy. These viruses cause similar clinical manifestations during the acute phase of infection but vary in their ability to cause chronic infection. While HAV and HEV cause only acute disease with no chronic sequelae, HBV, HCV and HDV cause varying degrees of chronicity and liver injury, which can progress to cirrhosis and liver cancers. Though specific serological tests are available for the known hepatitis viruses, nearly 20% of all hepatitis cases show no markers. Antiviral therapy is also recommended for some hepatitis viruses and a preventive vaccine is available only for hepatitis B. More research and public awareness programmes are needed to control the disease. This review will provide an overview of the hepatitis viruses and the disease they cause.

Reservoir performance under uncertainty in hydrologic impacts of climate change

Relatively few studies have addressed water management and adaptation measures in the face of changing water balances due to climate change. The current work studies climate change impact on a multipurpose reservoir performance and derives adaptive policies for possible futurescenarios. The method developed in this work is illustrated with a case study of Hirakud reservoir on the Mahanadi river in Orissa, India,which is a multipurpose reservoir serving flood control, irrigation and power generation. Climate change effects on annual hydropower generation and four performance indices (reliability with respect to three reservoir functions, viz. hydropower, irrigation and flood control, resiliency, vulnerability and deficit ratio with respect to hydropower) are studied. Outputs from three general circulation models (GCMs) for three scenarios each are downscaled to monsoon streamflow in the Mahanadi river for two future time slices, 2045-65 and 2075-95. Increased irrigation demands, rule curves dictated by increased need for flood storage and downscaled projections of streamflow from the ensemble of GCMs and scenarios are used for projecting future hydrologic scenarios. It is seen that hydropower generation and reliability with respect to hydropower and irrigation are likely to show a decrease in future in most scenarios, whereas the deficit ratio and vulnerability are likely to increase as a result of climate change if the standard operating policy (SOP) using current rule curves for flood protection is employed. An optimal monthly operating policy is then derived using stochastic dynamic programming (SDP) as an adaptive policy for mitigating impacts of climate change on reservoir operation. The objective of this policy is to maximize reliabilities with respect to multiple reservoir functions of hydropower, irrigation and flood control. In variations to this adaptive policy, increasingly more weightage is given to the purpose of maximizing reliability with respect to hydropower for two extreme scenarios. It is seen that by marginally sacrificing reliability with respect to irrigation and flood control, hydropower reliability and generation can be increased for future scenarios. This suggests that reservoir rules for flood control may have to be revised in basins where climate change projects an increasing probability of droughts. However, it is also seen that power generation is unable to be restored to current levels, due in part to the large projected increases in irrigation demand. This suggests that future water balance deficits may limit the success of adaptive policy options. (C) 2010 Elsevier Ltd. All rights reserved.

Structural and photoelectrical characterization of hot wall deposited CuInSe2 thin films and the fabrication of CuInSe2 based solar cells

Films of CuInSe2 were deposited onto glass substrates by a hot wall deposition method using bulk CuInSe2 as a source material. All the deposited CuInSe2 films were found to be polycrystalline in nature exhibiting the chalcopyrite structure with the crystallite orientation along (101),(112),(103),(211),(220),(312) and (400) directions. The photocurrent was found to increase with increase in film thickness and also with increase of light intensity. Photocurrent spectra show a peak related to the band-to-band transition. The spectral response of CuInSe2 thin films was studied by allowing the radiation to pass through a series of interference filters in the wavelength range 700-1200 rim. Films of higher thickness exhibited higher photosensitivity while low thickness films exhibited moderate photosensitivity. CuInSe2-based Solar cells with different types of buffer layers such as US, Cdse, CuInSe2 and CdSe0.7Te0.3 were fabricated. The current and voltage were measured using an optical power meter and an electrometer respectively. The fabricated solar cells were illuminated using 100 mW/cm(2) white light under AM1 conditions. (C) 2006 Elsevier Inc. All rights reserved.

Structure determination and biochemical studies on Bacillus stearothermophilus E53Q serine hydroxymethyltransferase and its complexes provide insights on function and enzyme memory

Serine hydroxymethyltransferase (SHMT) belongs to the alpha-family of pyridoxal 5'-phosphate-dependent enzymes and catalyzes the reversible conversion of L-Ser and etrahydrofolate to Gly and 5,10-methylene tetrahydrofolate. 5,10-Methylene tetrahydrofolate serves as a source of one-carbon fragment in many biological processes. SHMT also catalyzes the tetrahydrofolate-independent conversion of L-allo-Thr to Gly and acetaldehyde. The crystal structure of Bacillus stearothermophilus SHMT (bsSHMT) suggested that E53 interacts with the substrate, L-Ser and etrahydrofolate. To elucidate the role of E53, it was mutated to Q and structural and biochemical studies were carried out with the mutant enzyme. The internal aldimine structure of E53QbsSHMT was similar to that of the except for significant changes at Q53, Y60 and Y61. The wild-type enzyme, carboxyl of Gly and side chain of L-Ser were in two conformations in the respective external aldimine structures. The mutant enzyme was completely inactive for tetrahydrofolate-depen dent cleavage of L-Ser, whereas there was a 1.5-fold increase in the rate of tetrahydrofolate-independent reaction with L-allo-Thr. The results obtained from these studies suggest that E53 plays an essential role in tetrahydrofolate/5-formyl tetrahydrofolate binding and in the proper positioning of C beta of L-Ser for direct attack by N5 of tetrahydrofolate. Most interestingly, the structure of the complex obtained by cocrystallization of E53QbsSHMT with Gly and 5-formyl tetrahydrofolate revealed the gem-diamine form of pyridoxal 5'-phosphate bound to Gly and active site Lys. However, density for 5-formyl tetrahydrofolate was not observed. Gly carboxylate was in a single conformation, whereas pyridoxal 5'-phosphate had two distinct conformations. The differences between the structures of this complex and Gly external aldimine suggest that the changes induced by initial binding of 5-formyl tetrahydrofolate are retained even though 5-formyl tetrahydrofolate is absent in the final structure. Spectral studies carried out with this mutant enzyme also suggest that 5-formyl tetrahydrofolate binds to the E53QbsSHMT-Gly complex forming a quinonoid intermediate and falls off within 4 h of dialysis, leaving behind the mutant enzyme in the gemdiamine form. This is the first report to provide direct evidence for enzyme memory based on the crystal structure of enzyme complexes.

Molten globule-like state of peanut lectin monomer retains its carbohydrate specificity - Implications in protein folding and legume lectin oligomerization

A central question in biological chemistry is the minimal structural requirement of a protein that would determine its specificity and activity, the underlying basis being the importance of the entire structural element of a protein with regards to its activity vis a vis the overall integrity and stability of the protein. Although there are many reports on the characterization of protein folding/ unfolding intermediates, with considerable secondary structural elements but substantial loss of tertiary structure, none of them have been reported to show any activity toward their respective ligands. This may be a result of the conditions under which such intermediates have been isolated or due to the importance of specific structural elements for the activity. In this paper we report such an intermediate in the unfolding of peanut agglutinin that seems to retain, to a considerable degree, its carbohydrate binding specificity and activity. This result has significant implications on the molten globule state during the folding pathway(s) of proteins in general and the quaternary association in legume lectins in particular, where precise subunit topology is required for their biologic activities.

Spectrophotometric and potentiometric investigations of copper (II)-ethanolamine complexes - Part II. Monoethanolamine (Analysis of mixed hydroxy complexes with polarographic evidence)

Potentiometric, spectrophotometric and polarographic evidence has been presented for the formation of mixed hydroxy complexes in coppermonoethanolamine system. A method has been developed for the analysis of Bjerrum formation curves taken in presence of 0·1, 0·2, 0·5 and 1·0 M monoethanolammonium ion with respect to hydroxy complexes. The formation of CuAOH+, CuA2OH+ and CuA3OH+ is shown and the corresponding stability constants are calculated at different concentrations of MEA ion. Curves showing the distribution of pure and hydroxy complexes at various pA values in solutions containing different concentrations of MEA ion have also been given.

Analysis of gradually and spatially varied flow in sand-bed channels

Seepage through a sand bed affects the channel hydrodynamics, which in turn alters channel stability. Thus, the effect of seepage on its hydrodynamic parameters needs to be ascertained. The present work analyses spatially varied flow of a sand-bed channel subjected to seepage in the downward direction through a sand bed. Numerically calculated flow profiles affected by seepage have been verified using experimental observations. The present work also analyses the friction slope, velocity and bed shear stress variations along the channel for both seepage and no-seepage conditions. It was found that the downward seepage-induced channel flow has larger friction slope and bed shear stress than that of no-seepage.

The emission spectrum of bismuth bromide (BiBr)-"A" system

The emission spectrum of bismuth monobromide has been investigated and a vibrational analysis of the A→X system has been made. About 286 bands were recorded in the region λλ 4595–6063 and the isotope effect due to Br79 and Br81 was observed in about 87 bands. A value of 2·74 ev. for the dissociation energy of the excited state has been obtained and arguments have been given to show that the dissociation products in the excited state are Bi(4S3/2) and Br(2P3/2) and that those of the ground state are most probably Bi (4S3/2) and Br (2P1/2) atoms.

Characteristics of yeasts isolated from phenol- and catechol-adapted activated sludges

Phenol- and catechol-adapted sludges contained large numbers of the yeasts, Candida tropicalis and Trichosporon cutaneum. Both were able to grow on a variety of aromatic compounds and utilized phenol and catechol at a high rate. This property was inducible. The feasibility of using these yeasts for removing phenols from waste waters is suggested.

Reversible polyelectrolyte capsules as carriers for protein delivery

A reversible drug delivery system based on spontaneous deposition of a model protein into preformed microcapsules has been demonstrated for protein delivery applications. Layer-by-Layer assembly of poly(allylamine hydrochloride) (PAH) and poly(methacrylic acid) (PMA) onto polystyrene sulfonate (PSS) doped CaCO3 particles, followed by core removal yielded intact hollow microcapsules having a unique property to induce spontaneous deposition of bovine serum albumin (BSA) at pH below its isoelectric point of 4.8, where it was positively charged. These capsules showed reversible pH dependent open and closed states to fluorescence labeled dextran (FITC-Dextran) and BSA (FITC-BSA). The loading capacity of BSA increased from 9.1 x 10(7) to 2.03 x 10(8) molecules per capsule with decrease in pH from 4.5 to 3.The loading of BSA-FITC was observed by confocal laser scanning microscopy (CLSM), which showed homogeneous distribution of protein inside the capsule. Efficient loading of BSA was further confirmed by atomic force microscopy (AFM) and scanning electron microscopy (SEM).The interior capsule concentration was as high as 209 times the feeding concentration when the feeding concentration was increased from 1 to 10 mg/ml. The deposition was initially controlled by spontaneous loading mechanism at lower BSA concentration followed by diffusion controlled loading at higher concentration; which decreased the loading efficiency from 35% to 7%. Circular dichroism (CD) measurements and Fourier transform infrared spectroscopy (FTIR) confirmed that there was no significant change in conformation of released BSA in comparison with native BSA. The release was initially burst in the first 0.5 h and sustained up to 5 h. The hollow capsules were found to be biocompatible with mouse embryonic fibroblast (MEF) cells during in vitro cell culture studies. Thus these pH sensitive polyelectrolyte microcapsules may offer a promising delivery system for water soluble proteins and peptides. (C) 2010 Elsevier B.V. All rights reserved.