Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α₂ + γ(Ti₃Al + TiAl) and orthorhombic Ti₂NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti₂NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.     

Consensus supertrees: The synthesis of rooted trees containing overlapping sets of labeled leaves

Given two dendrograms (rooted tree diagrams) which have some but not all of their base points in common, a supertree is a dendrogram from which each of the original trees can be regarded as samples The distinction is made between inconsistent and consistent sample trees, defined by whether or not the samples provide contradictory information about the supertree An algorithm for obtaining the strict consensus supertree of two consistent sample trees is presented, as are procedures for merging two inconsistent sample trees Some suggestions for future work are made     

Sterol carrier protein-2: structure reveals function

The multiple actions of sterol carrier protein-2 (SCP-2) in intracellular lipid circulation and metabolism originate from its gene and protein structure. The SCP-x/pro-SCP-2 gene is a fusion gene with separate initiation sites coding for 15-kDa pro-SCP-2 (no enzyme activity) and 58-kDa SCP-x (a 3-ketoacyl CoA thiolase). Both proteins share identical cDNA and amino acid sequences for 13-kDa SCP-2 at their C-termini. Cellular 13-kDa SCP-2 derives from complete, posttranslational cleavage of the 15-kDa pro-SCP-2 and from partial posttranslational cleavage of 58-kDa SCP-x. Putative physiological functions of SCP-2 have been proposed on the basis of enhancement of intermembrane lipid transfer (e.g., cholesterol, phospholipid) and activation of enzymes involved in fatty acyl CoA transacylation (cholesterol esters, phosphatidic acid) in vitro, in transfected cells, and in genetically manipulated animals. At least four important SCP-2 structural domains have been identified and related to specific functions. First, the 46-kDa N-terminal presequence present in 58-kDa SCP-x is a 3-ketoacyl-CoA thiolase specific for branched-chain acyl CoAs. Second, the N-terminal 20 amino acid presequence in 15-kDa pro-SCP-2 dramatically modulates the secondary and tertiary structure of SCP-2 as well as potentiating its intracellular targeting coded by the C-terminal peroxisomal targeting sequence. Third, the N-terminal 32 amino acids form an amphipathic a-helical region, one face of which represents a membrane-binding domain. Positively charged amino acid residues in one face of the amphipathic helices allow SCP-2 to bind to membrane surfaces containing anionic phospholipids. Fourth, the hydrophobic faces of the N-terminal amphipathic a helices along with beta strands 4, 5, and helix D form a ligand-binding cavity able to accommodate multiple types of lipids (e. g., fatty acids, fatty acyl CoAs, cholesterol, phospholipids, isoprenoids). Two-dimensional 1H-15N heteronuclear single quantum coherence spectra of both apo-SCP-2 and of the 1:1 oleate-SCP-2 complex, obtained at pH 6.7, demonstrated the homogenous formation of holo-SCP-2. While comparison of the apo- and holoprotein amide fingerprints revealed about 60% of the resonances remaining essentially unchanged, 12 assigned amide residues underwent significant chemical-shift changes upon oleic acid binding. These residues were localized in three regions: the juncture of helices A and B, the mid-section of the beta sheet, and the interface formed by the region of beta strands 4, 5, and helix D. Circular dichroism also showed that these chemical-shift changes, upon oleic acid binding, did not alter the secondary structure of SCP-2. The nuclear magnetic resonance chemical shift difference data, along with mapping of the nearby hydrophobic residues, showed the oleic acid-binding site to be comprised of a pocket created by the face of the beta sheet, helices A and B on one end, and residues associated with beta strands 4, 5, and helix D at the other end of the binding cavity. Furthermore, the hydrophobic nature of the previously ill-defined C-terminus suggested that these 20 amino acids may form a 'hydrophobic cap' which closes around the oleic acid upon binding. Thus, understanding the structural domains of the SCP-x/pro-SCP-2 gene and its respective posttranslationally processed proteins has provided new insights into their functions in intracellular targeting and metabolism of lipids.