Diffusion of localized optical excitations and trap emission in ruby

Using the critical percolation conductance method the energy-dependent diffusion coefficient associated with thermally assisted transfer of the R1 line excitation between single Cr3+ ions with strain-induced randomness has been calculated in the 4A2 to E(2E) transition energies. For localized states sufficiently far away from the mobility edge the energy transfer is dominated by dipolar interactions, while very close to the mobility edge it is determined by short-range exchange interactions. Using the above energy-dependent diffusion coefficient a macroscopic diffusion equation is solved for the rate of light emission by Cr3+ ion-pair traps to which single-ion excitations are transferred. The dipolar mechanism leads to good agreement with recent measurements of the pair emission rate by Koo et al. (Phys. Rev. Lett., vol.35, p.1669 (1975)) right up to the mobility edge.

Studies on the reduction of hematite by carbon

Single pellet experiments have been carried out in a nitrogen atmosphere to study the reduction of hematite by graphite in the temperature range 925 to 1060°C. The effect of variables such as c/Fe2O3 molar ratio, pellet size, and so forth, has been investigated. Gas analysis data show a continuous decrease in CO2/CO ratio during reduction, the values being far away from Fe/FeO equilibrium for wustite reduction by CO. The activation energies associated with different degrees of reduction appear to be widely different suggesting a possible changeover in reaction mechanism during the progress of reduction. X-ray diffraction studies confirm the stepwise nature of hematite reduction.

Computational analyses of the catalytic and heparin-binding sites and their interactions with glycosaminoglycans in glycoside hydrolase family 79 endo-d-glucuronidase (heparanase)

Mammalian heparanase is an endo-β-glucuronidase associated with cell invasion in cancer metastasis, angiogenesis and inflammation. Heparanase cleaves heparan sulfate proteoglycans in the extracellular matrix and basement membrane, releasing heparin/heparan sulfate oligosaccharides of appreciable size. This in turn causes the release of growth factors, which accelerate tumor growth and metastasis. Heparanase has two glycosaminoglycan-binding domains; however, no three-dimensional structure information is available for human heparanase that can provide insights into how the two domains interact to degrade heparin fragments. We have constructed a new homology model of heparanase that takes into account the most recent structural and bioinformatics data available. Heparin analogs and glycosaminoglycan mimetics were computationally docked into the active site with energetically stable ring conformations and their interaction energies were compared. The resulting docked structures were used to propose a model for substrates and conformer selectivity based on the dimensions of the active site. The docking of substrates and inhibitors indicates the existence of a large binding site extending at least two saccharide units beyond the cleavage site (toward the nonreducing end) and at least three saccharides toward the reducing end (toward heparin-binding site 2). The docking of substrates suggests that heparanase recognizes the N-sulfated and O-sulfated glucosamines at subsite +1 and glucuronic acid at the cleavage site, whereas in the absence of 6-O-sulfation in glucosamine, glucuronic acid is docked at subsite +2. These findings will help us to focus on the rational design of heparanase-inhibiting molecules for anticancer drug development by targeting the two heparin/heparan sulfate recognition domains.

Can current force fields reproduce ring puckering in 2-O-sulfo-α-l-iduronic acid? A molecular dynamics simulation study

The monosaccharide 2-O-sulfo-α-l-iduronic acid (IdoA2S) is one of the major components of glycosaminoglycans. The ability of molecular mechanics force fields to reproduce ring-puckering conformational equilibrium is important for the successful prediction of the free energies of interaction of these carbohydrates with proteins. Here we report unconstrained molecular dynamics simulations of IdoA2S monosaccharide that were carried out to investigate the ability of commonly used force fields to reproduce its ring conformational flexibility in aqueous solution. In particular, the distribution of ring conformer populations of IdoA2S was determined. The GROMOS96 force field with the SPC/E water potential can predict successfully the dominant skew-boat to chair conformational transition of the IdoA2S monosaccharide in aqueous solution. On the other hand, the GLYCAM06 force field with the TIP3P water potential sampled transitional conformations between the boat and chair forms. Simulations using the GROMOS96 force field showed no pseudorotational equilibrium fluctuations and hence no inter-conversion between the boat and twist boat ring conformers. Calculations of theoretical proton NMR coupling constants showed that the GROMOS96 force field can predict the skew-boat to chair conformational ratio in good agreement with the experiment, whereas GLYCAM06 shows worse agreement. The omega rotamer distribution about the C5–C6 bond was predicted by both force fields to have torsions around 10°, 190°, and 360°.

Specific polyadenylation and purification of total messenger RNA from Escherichia coli

Obtaining pure mRNA preparations from prokaryotes has been difficult, if not impossible, for want of a poly(A) tail on these messages, We have used poly(A) polymerase from yeast to effect specific polyadenylation of Escherichia coli polysomal mRNA in the presence of magnesium and manganese, The polyadenylated total mRNA, which could be subsequently purified by binding to and elution from oligo(dT) beads, had a size range of 0.4-4.0 kb. We have used hybridization to a specific plasmid-encoded gene to further confirm that the polyadenylated species represented mRNA, Withdrawal of Mg2+ from the polyadenylation reaction rRNA despite the presence of Mn2+, indicating the vital role of Mg2+ in maintaining the native structure of polysomes, Complete dissociation of polysomes into ribosomal subunits resulted in quantitative polyadenylation of both 16S and 23S rRNA species, Chromosomal lacZ gene-derived messages were quantitatively recovered in the oligo(dT)-bound fraction, as demonstrated by RT-PCR analysis, Potential advantages that accrue from the availability of pure total mRNA from prokaryotes is discussed.

Free energy calculations of the interactions of c-Jun-based synthetic peptides with the c-Fos protein

The c-Fos–c-Jun complex forms the activator protein 1 transcription factor, a therapeutic target in the treatment of cancer. Various synthetic peptides have been designed to try to selectively disrupt the interaction between c-Fos and c-Jun at its leucine zipper domain. To evaluate the binding affinity between these synthetic peptides and c-Fos, polarizable and nonpolarizable molecular dynamics (MD) simulations were conducted, and the resulting conformations were analyzed using the molecular mechanics generalized Born surface area (MM/GBSA) method to compute free energies of binding. In contrast to empirical and semiempirical approaches, the estimation of free energies of binding using a combination of MD simulations and the MM/GBSA approach takes into account dynamical properties such as conformational changes, as well as solvation effects and hydrophobic and hydrophilic interactions. The predicted binding affinities of the series of c-Jun-based peptides targeting the c-Fos peptide show good correlation with experimental melting temperatures. This provides the basis for the rational design of peptides based on internal, van der Waals, and electrostatic interactions.

Calculations of the free energy of interaction of the c-Fos−c-Jun coiled coil: Effects of the solvation model and the inclusion of polarization effects

The leucine zipper region of activator protein-1 (AP-1) comprises the c-Jun and c-Fos proteins and constitutes a well-known coiled coil protein−protein interaction motif. We have used molecular dynamics (MD) simulations in conjunction with the molecular mechanics/Poisson−Boltzmann generalized-Born surface area [MM/PB(GB)SA] methods to predict the free energy of interaction of these proteins. In particular, the influence of the choice of solvation model, protein force field, and water potential on the stability and dynamic properties of the c-Fos−c-Jun complex were investigated. Use of the AMBER polarizable force field ff02 in combination with the polarizable POL3 water potential was found to result in increased stability of the c-Fos−c-Jun complex. MM/PB(GB)SA calculations revealed that MD simulations using the POL3 water potential give the lowest predicted free energies of interaction compared to other nonpolarizable water potentials. In addition, the calculated absolute free energy of binding was predicted to be closest to the experimental value using the MM/GBSA method with independent MD simulation trajectories using the POL3 water potential and the polarizable ff02 force field, while all other binding affinities were overestimated.

Role of Arg-401 of cytosolic serine hydroxymethyltransferase in subunit assembly and interaction with the substrate carboxy group.

In an attempt to identify the arginine residue involved in binding of the carboxylate group of serine to mammalian serine hydroxymethyltransferase, a highly conserved Arg-401 was mutated to Ala by site-directed mutagenesis. The mutant enzyme had a characteristic visible absorbance at 425 nm indicative of the presence of bound pyridoxal 5'-phosphate as an internal aldimine with a lysine residue. However, it had only 0.003% of the catalytic activity of the wild-type enzyme. It was also unable to perform reactions with glycine, beta-phenylserine or d-alanine, suggesting that the binding of these substrates to the mutant enzyme was affected. This was also evident from the interaction of amino-oxyacetic acid, which was very slow (8.4x10(-4) s-1 at 50 microM) for the R401A mutant enzyme compared with the wild-type enzyme (44.6 s-1 at 50 microM). In contrast, methoxyamine (which lacks the carboxy group) reacted with the mutant enzyme (1.72 s-1 at 250 microM) more rapidly than the wild-type enzyme (0.2 s-1 at 250 microM). Further, both wild-type and the mutant enzymes were capable of forming unique quinonoid intermediates absorbing at 440 and 464 nm on interaction with thiosemicarbazide, which also does not have a carboxy group. These results implicate Arg-401 in the binding of the substrate carboxy group. In addition, gel-filtration profiles of the apoenzyme and the reconstituted holoenzyme of R401A and the wild-type enzyme showed that the mutant enzyme remained in a tetrameric form even when the cofactor had been removed. However, the wild-type enzyme underwent partial dissociation to a dimer, suggesting that the oligomeric structure was rendered more stable by the mutation of Arg-401. The increased stability of the mutant enzyme was also reflected in the higher apparent melting temperature (Tm) (61 degrees C) than that of the wild-type enzyme (56 degrees C). The addition of serine or serinamide did not change the apparent Tm of R401A mutant enzyme. These results suggest that the mutant enzyme might be in a permanently 'open' form and the increased apparent Tm could be due to enhanced subunit interactions.

Analysis of human chorionic gonadotropin-monoclonal antibody interaction in BIAcore

Kinetic studies of macromolecular ligand-ligate interaction have generated ample interest since the advent of plasmon resonance based instruments like BIAcore. Most of the studies reported in literature assume a simple 1 : 1 Langmuir binding and complete reversibility of the system. However we observed that in a high affinity antigen-antibody system [human chorionic gonadotropin-monoclonal antibody (hCG-mAb)] dissociation is insignificant and the sensogram data cannot be used to measure the equilibrium and kinetic parameters. At low concentrations of mAb the complete sensogram could be fitted to a single exponential. Interestingly we found that at higher mAb concentrations, the binding data did not conform to a simple bimolecular model. Instead, the data fitted a two-step model, which may be because of surface heterogeneity of affinity sites. In this paper, we report on the global fit of the sensograms. We have developed a method by which a single two-minute sensogram can be used in high affinity systems to measure the association rate constant of the reaction and the functional capacity of the ligand (hCG) immobilized on the chip. We provide a rational explanation for the discrepancies generally observed in most of the BIAcore sensograms

Thermodynamics of Monosaccharide and Disaccharide Binding to Erythrina corallodendron Lectin

Isothermal titration calorimetry measurements of the binding of 2′-fucosyllactose, lactose, N-acetyllactosamine, galactopyranose, 2-acetamido-2-deoxygalactopyranoside, methyl α-N-dansylgalactosaminide (Me-α-DNS-GalN), methyl α-D-galactopyranoside, methyl β-D-galactopyranoside, and fucose to Erythrina corallodendron lectin (ECorL), a dimer with one binding site per subunit, were performed at 283-286 and 297-299 K. The site binding enthalpies, ΔHb, with the exception of Me-α-DNS-GalN, are the same at both temperatures and range from −47.1 ± 1.0 kJ mol−1 for N-acetyllactosamine to −4.4 ± 0.3 kJ mol−1 for fucose, and the site binding constants range from 3.82 ± 0.9 × 105 M−1 for Me-α-DNS-GalN at 283.2 K to 0.46 ± 0.05 × 103 M−1 for fucose at 297.2 K. The binding reactions are mainly enthalpically driven except for fucose and exhibit enthalpy-entropy compensation. The binding enthalpies of the disaccharides are about twice the binding enthalpies of the monosaccharides in contrast to concanavalin A where the binding enthalpies do not double for the disaccharides. Differential scanning calorimetry measurements show that denaturation of the ECorL dimer results in dissociation into its monomer subunits. The binding constants from the increase in denaturation temperature of ECorL in the presence of saccharides are in agreement with values from isothermal titration calorimetry results. The thermal denaturation of ECorL occurs around 333 K, well below the 344-360 K denaturation temperature of other legume lectins of similar size and tertiary structure, undoubtedly due to the difference in its quaternary structure relative to other legume lectins. This is also apparent from the independent unfolding of its two domains.

The hydrogen bond: a molecular beam microwave spectroscopist's view with a universal appeal

In this manuscript, we propose a criterion for a weakly bound complex formed in a supersonic beam to be characterized as a `hydrogen bonded complex'. For a `hydrogen bonded complex', the zero point energy along any large amplitude vibrational coordinate that destroys the orientational preference for the hydrogen bond should be significantly below the barrier along that coordinate so that there is at least one bound level. These are vibrational modes that do not lead to the breakdown of the complex as a whole. If the zero point level is higher than the barrier, the `hydrogen bond' would not be able to stabilize the orientation which favors it and it is no longer sensible to characterize a complex as hydrogen bonded. Four complexes, Ar-2-H2O, Ar-2-H2S, C2H4-H2O and C2H4-H2S, were chosen for investigations. Zero point energies and barriers for large amplitude motions were calculated at a reasonable level of calculation, MP2(full)/aug-cc-pVTZ, for all these complexes. Atoms in molecules (AIM) theoretical analyses of these complexes were carried out as well. All these complexes would be considered hydrogen bonded according to the AIM theoretical criteria suggested by Koch and Popelier for C-H center dot center dot center dot O hydrogen bonds (U. Koch and P. L. A. Popelier, J. Phys. Chem., 1995, 99, 9747), which has been widely and, at times, incorrectly used for all types of contacts involving H. It is shown that, according to the criterion proposed here, the Ar-2-H2O/H2S complexes are not hydrogen bonded even at zero kelvin and C2H4-H2O/H2S complexes are. This analysis can naturally be extended to all temperatures. It can explain the recent experimental observations on crystal structures of H2S at various conditions and the crossed beam scattering studies on rare gases with H2O and H2S.

Effect of mechanical cycling on the stress-strain response of a martensitic Nitinol shape memory alloy

An experimental investigation into the ambient temperature, load-controlled tension�tension fatigue behavior of a martensitic Nitinol shape memory alloy (SMA) was conducted. Fatigue life for several stress levels spanning the critical stress for detwinning was determined and compared with that obtained on an alloy similar in composition but in the austenitic state at room temperature. Results show that the fatigue life of the pseudo-plastic alloy is superior to superelastic shape memory alloy. The stress�strain hysteretic response, monitored throughout the fatigue loading, reveals progressive strain accumulation with the cyclic loading. In addition, the area of hysteresis and recoverable and frictional energies were found to decrease with increasing number of fatigue cycles. Post-mortem characterization of the fatigued specimens through calorimetry and fractography was conducted in order to get further insight into the fatigue micromechanisms. These results are discussed in terms of reversible and irreversible microstructural changes that take place during cyclic loading. Aspects associated with self-heating of martensitic alloy undergoing high frequency stress cycling are discussed.

Theoretical calculations of base-base interactions in nucleic acids: II Stacking interactions in polynucleotides

Base-base interactions were computed for single- and double- stranded polynucleotides, for all possible base sequences. In each case, both right and left stacking arrangements are energetically possible. The preference of one over the other depends upon the base-sequence and the orientation of the bases with respect to helix-axis. Inverted stacking arrangement is also energetically possible for both single- and double-stranded polynucleotides. Finally, interaction energies of a regular duplex and the alternative structures3 were compared. It was found that the type II model3 is energetically more favourable than the rest.

Solution NMR studies of acetohydroxy acid synthase I: Identification of the sites of inter-subunit interactions using multidimensional NMR methods

The novel multidomain organization in the multimeric Escherichia coli AHAS I (ilvBN) enzyme has been dissected to generate polypeptide fragments. These fragments when cloned, expressed and purified reassemble in the presence of cofactors to yield a catalytically competent enzyme. Structural characterization of AHAS has been impeded due to the fact that the holoenzyme is prone to dissociation leading to heterogeneity in samples. Our approach has enabled the structural characterization using high-resolution nuclear magnetic resonance methods. Near complete sequence specific NMR assignments for backbone H-N, N-15, C-13 alpha and C-13(beta) atoms of the FAD binding domain of ilvB have been obtained on samples isotopically enriched in H-2, C-13 and N-15. The secondary structure determined on the basis of observed C-13(alpha) secondary chemical shifts and sequential NOEs indicates that the secondary structure of the FAD binding domain of E. coli AHAS large Subunit (ilvB) is similar to the structure of this domain in the catalytic subunit of yeast AHAS. Protein-protein interactions involving the regulatory subunit (ilvN) and the domains of the catalytic subunit (ilvB) were studied using circular dichroic and isotope edited solution nuclear magnetic resonance spectroscopic methods. Observed changes in circular dichroic spectra indicate that the regulatory subunit (ilvN) interacts with ilvB alpha and ilvB beta domains of the catalytic subunit and not with the ilvB gamma domain. NMR chemical shift mapping methods show that ilvN binds close to the FAD binding site in ilvB beta and proximal to the intrasubunit ilvB alpha/ilvB beta domain interface. The implication of this interaction on the role of the regulatory subunit oil the activity of the holoenzyme is discussed. NMR studies of the regulatory domains show that these domains are structured in solution. Preliminary evidence for the interaction of ilvN with the metabolic end product of the pathway, viz., valine is also presented.

Which One Is Preferred: Myers-Saito Cyclization of Ene-Yne-Allene or Garratt-Braverman Cyclization of Conjugated Bisallenic Sulfone? A Theoretical and Experimental Study

A competitive scenario between Myers-Saito (MS) and Garraff-Braverman (GB) cyclization has been created in a molecule. High-level computations indicate a preference for GB over MS cyclization. The activation energies for the rate-determining steps of the GB and MS cyclizations were found to be the same (24.4 kcal/mol) at the B3LYP/6-31G* level of theory; thus, from the kinetic point of view, both reactions are feasible. However, the main biradical intermediate GB2 of the GB reaction is 6.2 kcal/mol lower in energy than the biradical MS2, which is the main intermediate of MS reaction, so GB cyclization is thermodynamically favored over MS cyclization. To verify the prediction by computational techniques, bisenediynyl sulfones 1-4 and bisenediynyl sulfoxide 17 were synthesized. Under basic conditions, these molecules isomerized to a system possessing both the ene-yne-allene and the bisallenic sulfone. The isolation of only one product, identified as the corresponding naphthalene- or benzene-fused sulfone 8-11, indicated the occurrence of GB cyclization as the sole reaction pathway. No product corresponding to the MS cyclization pathway could be isolated. Though the theoretical prediction showed a preference for the GB pathway over the MS pathway, the exclusive preference for GB over MS cyclization is very striking. Further analysis showed that the intramolecular self-quenching nature of the GB pathway may play an important role in the complete preference for this reaction. Apart from the mechanistic studies, these sulfones showed DNA cleavage activity that had an inverse relation with the reactivity order. Our findings are important for the design of artificial DNA-cleaving agents.