Structures of (5′S)-8,5′-Cyclo-2′-deoxyguanosine Mismatched with dA or dT

pubs.acs.org/cr

Precious-metal-base advanced materials

The precious metals are basically known for their applications in jewelry, coins, bullions and catalysis. The reality is thas precious metals constitute also the base of several advanced materials used in the industry in hundreds of metric tons. Since forties, platinum alloys have been used as structural materials for equipments in the glass industry. The essential reason for this is the excellent resistance of platinum alloys to oxidation and eletrolytical corrosion in molten glasses at temperatures as high as 1200-1500°C. The major drawback is a weak creep resistance. In the case of all conventional platinum alloys, the 100 h creep resistance at 1100°C is smaller than 10 MPa while the stress to cause rupture in 1000 h at 1100°C for superalloys is about 100 MPa. The unique way for significant improvement of platinum base materials acep resistance is a strengthening by an oxide dispersion (ODs). In the cas of CLAL's patented "Plativer" materials, 0.05 wt% of Y2O3 is incorporated within the alloy matrix by the name spraying process. The creep behaviour of such ODs materials shows a 100 times creep rate reduction comparing to conventional platinum alloys. Thanks to the fact that the mean size of oxide particles is included between 2000 and 4000 A, the obtained materials don't exihibit any loss of plasticity in comparison of conventional Pt alloys. Further improvement of platinum base materials is related, in the authors opinion, to the development of precious metals base intermetallics. Anothe interesting applications of precious metals are silver base electrical contacts. They are in fad silver matrix composites containing varying amounts of well-dispersed particles of constituents such as CdO, SnO2, Ni, WC or C. In the case of such materials, particular properties are required and tested : resistance to arc erosion, resistance to welding and contact resistance. Choice of the second phase depends on the specfic service conditions. During last 10 years, an interesting and successful battlle took place in order to replace, without alteration of the properties of conventional materials, toxic CdO by inert SnO2. Electrical contacts are manufactured by various powder metallurgy techniques, including coprecipitation from solution, internal oxidation, internal oxidation of atomized powders, unitary pressing and hot extrusion. A supplementary difficulty comes from the fact that in order to enable brazing or welding of contacts to supports, the parts had to be manufactured as sandwiches : silver composite/pure silver. Many other technically fascinating precious metals base materials exist : brazing alloys for assembling metals, superconductors and ceramics ; dental materials including magnetic biocompatible alloys ; silver composites for superconductor wire jackets. One can ask the fundamental question if the development of materials science will induce the replacement of precious metals by cheaper materials. The observation of current evolution indicate very clearly the opposed tendency : precious metals cannot be replaced by common metals because of their unique characteristics due to their atomic level properties. The relatively high cost of raw materials authorizes the use of advanced metallurgical techniques, the conversion cost being only a small portion of the total material value

An NMR structural study of deaminated base pairs in DNA.

The structurally aberrant base pairs TG, UG and TI may occur in DNA as a consequence of deamination of 5-methylcytosine, cytosine and adenine respectively. Results of NMR spectroscopic studies are reported here for these deaminated base pairs in a model seven base pair long oligonucleotide duplex. We find that in all three cases, the DNA helix is a normal B form and both mispaired bases are intrahelical and hydrogen bonded with one another in a wobble geometry. Similarly, in all three cases, all sugars are found to be normal C2' endo in conformation. Symmetric structural perturbations are observed in the helix twist on the 3' side of the mispaired pyrimidine and on the 5' side of the mispaired purine. In all three cases, the amino group of the G residue on the 3' side of the mispaired pyrimidine shows hindered rotation. Although less thermodynamically stable than helices containing only Watson-Crick base pairs, these helices melt normally from the ends and not from the mispair outwards

AUGER ELECTRON SPECTROSCOPY STUDY OF GRAIN BOUNDARY AND SURFACE SULFUR SEGREGATION ON SINTERED STEELS

Nous avons étudié la fragilisation par du soufre ségrégé d'aciers frittés à 1120°C pendant 30 minutes (Fe-4% Ni - 2% Cu - 0,5 % Mo - 0,3 % S - 0,5 % C). Dans ces aciers, le soufre améliore l'usinabilité et accélère les processus de frittage par formation d'une phase liquide. La fragilité intergranulaire a été étudiée "in situ" par spectroscopie d'électrons Auger (A.E.S.). La ségrégation du soufre fut révélée par bombardement d'ions argon sur la surface de rupture. En utilisant le modèle de Mc LEAN, nous avons déterminé l'enthalpie libre de ségrégation : ƊGG.B.S = - 88400 J.mol-1. Cette valeur est en accord avec celle obtenue par Seah et Hondros (en 1973) pour le Feα: ƊGG.B.S = - 88400 J.mol-1. Les échantillons pour l'étude de la ségrégation du soufre en surface ont été laminés et recuits à 925°C durant 15 minutes avant de procéder aux analyses Auger. Les mesures de cinétique de ségrégation du soufre en surface nous ont permis de déterminer le coefficient de diffusion "apparent" de S comme étant égal à 3.10-13et 3, 8.10-12cm2.s-1 à 400 et 500°C respectivement. Ces valeurs sont plus grandes que celles trouvées pour DV mais sont du même ordre de grandeur que DJ. La taille de grain moyenne étant égale à 10µm, la ségrégation du soufre en surface pourrait donc avoir comme principale source la diffusion intergranulaire.The purpose of this paper is to study the embrittlement of sintered steels (Fe - 4% Ni - 2% Cu - 0,5% Mo - 0,3% S - 0,5% C) at 1120°C during 30 minutes. In these steels, the sulfur improves usually the machinability and decreases the temperature when the shrinkage occurs. The intergranular brittleness was studied "in situ" by A.E.S. The S segregation was revealed by ion sputtering of the intergranular surface. Mc LFANS's approach was used to determine the free energy of segregation, ƊGG.B.S = - 88400 J.mol-1. This value is close to the value obtained by SEAH and HONDROS (1973) in Fe(α) : ƊGG.B.S = - 88400 J.mol-1. The samples for the free surface study were cold rolled and annealed at 925°C (15 mn) before analyses by A.E.S. The kinetic measurements allowed us to determine the "apparent" S diffusion coefficient as being equal to 3x10-l3 and 3,8 x 10-12cm2s-1 at 400°C and 500°C respectively. These values are greater than the values found for S bulk diffusion coefficient but they are close to intergranular diffusion coefficient. The average grain size is equal to 10µm ; this could explain that the surface sulfur segregation comes rather from the grain boundaries than from the bulk

Phase knowledge enables rational screens for protein crystallization

We show that knowledge of the phase behavior of a protein allows one to create a rational screen that increases the success rate of crystallizing challenging proteins. The strategy is based on using microfluidics to perform large numbers of protein solubility experiments across many different chemical conditions to identify reagents for crystallization experiments. Phase diagrams were generated for the identified reagents and used to design customized crystallization screens for every protein. This strategy was applied with a 75% success rate to the crystallization of 12 diverse proteins, most of which failed to crystallize when using traditional techniques. The overall diffraction success rate was 33%, about double what was achieved with conventional automation in large-scale protein structure consortia. The higher diffraction success rates are achieved by designing customized crystallization screens using the phase behavior information for each target. The identification of reagents based on an understanding of protein solubility and the use of phase diagrams in the design of individualized crystallization screens therefore promotes high crystallization rates and the production of diffraction-quality crystals

Systematic investigation of protein phase behavior with a microfluidic formulator

We demonstrated a microfluidic device for rapidly generating complex mixtures of 32 stock reagents in a 5-nl reactor. This “formulation chip” is fully automated and allows thousands of experiments to be performed in a single day with minimal reagent consumption. It was applied to systematically study the phase behavior of the protein xylanase over a large and complex chemical space. For each chemical formulation that demonstrated a pronounced effect on solubility, the protein phase behavior was completely mapped in the chip, generating a set of empirical phase diagrams. This ab initio phase information was used to devise a rational crystallization screen that resulted in 72-fold improvement in successful crystallization hits compared with conventional sparse matrix screens. This formulations tool allows a physics-based approach to protein crystallization that may prove useful in structural genomics efforts