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Hai Xu

Publications and source records attributed to Hai Xu.

24 records · Page 2Linked to original sources
PubMed2004

Nanoparticle dispersion on reconstructed carbon nanomeshes.

A nanoporous template which can be used for the preparation of monodispersed metal nanoparticles can have wide-ranging applications in the catalyzed growth of single-walled nanotubes, as well as the preparation of energetic, nanostructured ferromagnetic particle arrays. Here, we found that a honeycomb-like carbon nanomesh with periodically arranged pores of approximately 2-nm dimension could be fabricated on the reconstructed 6H-SiC(0001) surface. The carbon nanomesh arises from the periodic arrangement of segregated carbon clusters on the 6H-SiC surface to form a highly regular, nanoporous film. The carbon nanomesh can be dynamically structured to control the periodicity and depth of the pores by annealing in a vacuum. We evaporated cobalt on the surface of the nanomesh and investigated the diffusion and agglomeration behavior of cobalt clusters using in situ scanning tunneling microscopy. It is found that monodispersed Co nanoclusters that resist aggregation up to a temperature of 500 degrees C can be fabricated on this template.

Catalysis↗
PubMed2004

Probe-induced native oxide decomposition and localized oxidation on 6H-SiC (0001) surface: an atomic force microscopy investigation.

We report, for the first time, the native oxide decomposition/etching and direct local oxide growth on 6H-SiC (0001) surface induced by atomic force microscopy (AFM). Surface native oxide was decomposed and assembled into protruded lines when the negatively biased AFM tip was scanned over surface areas. The mechanism of decomposition was found to be governed by the Fowler-Nordheim emission current enhanced by the negatively biased AFM tip. Direct oxide growth on the SiC surface was achieved when the AFM tip was immobilized and longer bias duration applied. In particular, the aspect ratio of oxide grown on SiC was found to be several times higher than that on the Si surface. The improved aspect ratio on SiC was attributed to the anisotropic OH(-) diffusion involved in vertical and lateral oxidation along the polar and nonpolar directions such as [0001] and [110] axis in SiC crystal. The electron transport in the above AFM grown oxide on SiC was further investigated by I-V characteristics. The dielectrical strength of AFM oxide against degradation and breakdown under electrical stressing was evaluated.

Journal Article↗
PubMed2004

Isoleucine biosynthesis in Leptospira interrogans serotype lai strain 56601 proceeds via a threonine-independent pathway.

Three leuA-like protein-coding sequences were identified in Leptospira interrogans. One of these, the cimA gene, was shown to encode citramalate synthase (EC 4.1.3.-). The other two encoded alpha-isopropylmalate synthase (EC 4.1.3.12). Expressed in Escherichia coli, the citramalate synthase was purified and characterized. Although its activity was relatively low, it was strictly specific for pyruvate as the keto acid substrate. Unlike the citramalate synthase of the thermophile Methanococcus jannaschii, the L. interrogans enzyme is temperature sensitive but exhibits a much lower K(m) (0.04 mM) for pyruvate. The reaction product was characterized as (R)-citramalate, and the proposed beta-methyl-d-malate pathway was further confirmed by demonstrating that citraconate was the substrate for the following reaction. This alternative pathway for isoleucine biosynthesis from pyruvate was analyzed both in vitro by assays of leptospiral isopropylmalate isomerase (EC 4.2.1.33) and beta-isopropylmalate dehydrogenase (EC 1.1.1.85) in E. coli extracts bearing the corresponding clones and in vivo by complementation of E. coli ilvA, leuC/D, and leuB mutants. Thus, the existence of a leucine-like pathway for isoleucine biosynthesis in L. interrogans under physiological conditions was unequivocally proven. Significant variations in either the enzymatic activities or mRNA levels of the cimA and leuA genes were detected in L. interrogans grown on minimal medium supplemented with different levels of the corresponding amino acids or in cells grown on serum-containing rich medium. The similarity of this metabolic pathway in leptospires and archaea is consistent with the evolutionarily primitive status of the eubacterial spirochetes.

2-Isopropylmalate Synthase↗
PubMed2003

Structure of DNA helicase RepA in complex with sulfate at 1.95 A resolution implicates structural changes to an "open" form.

The structure of a new crystal form (space group C2), grown at pH 8.0 and diffracting to 1.95 A resolution, of the replicative homo-hexameric DNA helicase RepA encoded by plasmid RSF1010 is reported. In contrast to previous crystals grown at pH 6.0 in space group P2(1) (Niedenzu et al., 2001), only one half (a trimer) of the RepA hexamer occupies the asymmetric unit of the space-group C2 crystals. The new crystal packing explains the pH-dependent hexamer-hexamer association mechanism of RepA. The C-terminus (264)VLERQRKSKGVPRGEA(279), which could not be modelled in the previous structure, is clearly defined in the present electron density except for the last four amino acids. Sulfate anions occupy the six ATPase active sites of RepA at positions where the product phosphates are supposed to bind. Binding of sulfate anions induces conformational changes both at the ATPase active sites and throughout the whole molecular structure. In agreement with electron microscopy, the above studies implicate structural changes to an "open" form that may occur upon binding and hydrolysis of nucleotide 5'-triphosphates and could be essential for DNA duplex-unwinding activity.

Adenosine Triphosphatases↗
PubMed2003

Unique physiological and pathogenic features of Leptospira interrogans revealed by whole-genome sequencing.

Leptospirosis is a widely spread disease of global concern. Infection causes flu-like episodes with frequent severe renal and hepatic damage, such as haemorrhage and jaundice. In more severe cases, massive pulmonary haemorrhages, including fatal sudden haemoptysis, can occur. Here we report the complete genomic sequence of a representative virulent serovar type strain (Lai) of Leptospira interrogans serogroup Icterohaemorrhagiae consisting of a 4.33-megabase large chromosome and a 359-kilobase small chromosome, with a total of 4,768 predicted genes. In terms of the genetic determinants of physiological characteristics, the facultatively parasitic L. interrogans differs extensively from two other strictly parasitic pathogenic spirochaetes, Treponema pallidum and Borrelia burgdorferi, although similarities exist in the genes that govern their unique morphological features. A comprehensive analysis of the L. interrogans genes for chemotaxis/motility and lipopolysaccharide synthesis provides a basis for in-depth studies of virulence and pathogenesis. The discovery of a series of genes possibly related to adhesion, invasion and the haematological changes that characterize leptospirosis has provided clues about how an environmental organism might evolve into an important human pathogen.

Bacterial Adhesion↗