PubMed Health⌕ Search

Biomedical subjects

Nan Xu

Publications and source records attributed to Nan Xu.

13 recordsLinked to original sources
PubMed2006

Dynamic alignment of C2H4 investigated by using two linearly polarized femtosecond laser pulses.

We have studied multielectron ionization and Coulomb explosion of C2H4 irradiated by 110 fs, 800 nm laser pulses at an intensity of approximately 10(15) W/cm2. Strong anisotropic angular distributions were observed for the atomic ions Cn+(n = 1-3). Based on the results of two crossed linearly polarized laser pulses, we conclude that such anisotropic angular distributions result from dynamic alignment, in which the rising edge of the laser pulses aligns the neutral C2H4 molecules along the laser polarization direction. The angular distribution of the exploding fragments, therefore, reflects the degree of the alignment of molecules before ionization. Using the same femtosecond laser with intensity below the ionization threshold, the alignment of C2H4 molecules was also observed.

Journal Article↗
PubMed2006

Field-induced alignment of oxygen and nitrogen by intense femtosecond laser pulses.

Field-induced alignment of O2 and N2 was experimentally studied with laser intensities varying from 10(13) to 10(15) W/cm2. When the laser intensity was below the ionization threshold for these molecules, the interaction between the induced dipole moment of molecules and the laser electric field aligned the molecules along the laser polarization direction. After extinction of the exciting laser, the transient alignment revived periodically. Thus macroscopic ensembles of highly aligned O2 and N2 molecules were obtained under field-free conditions. When the laser intensity exceeded the ionization threshold for these molecules, multielectron ionization and Coulomb explosion occurred. Using two linearly polarized laser pulses with crossed polarization, we demonstrated that the rising edge of the laser pulse aligned the molecules along the laser polarization direction prior to ionization, which resulted in strong anisotropic angular distributions of exploding fragments. These results suggest that the degree of alignment should be taken into account when qualitatively comparing the ion yield of these molecules with their companion atoms.

Journal Article↗
PubMed2006

Modeling the competitive effect of phosphate, sulfate, silicate, and tungstate anions on the adsorption of molybdate onto goethite.

The mobility of Mo in soils and sediments depends on several factors including soil mineralogy and the presence of other oxyanions that compete with Mo for the adsorbent's retention sites. Batch experiments addressing Mo adsorption onto goethite were conducted with phosphate, sulfate, silicate, and tungstate as competing anions in order to produce competitive two anions adsorption envelopes, as well as competitive two anions adsorption isotherms. Tungstate and phosphate appear to be the strongest competitors of Mo for the adsorption sites of goethite, whereas little competitive effects were observed in the case of silicate and sulfate. Mo adsorption isotherm from a phosphate solution was similar to the one from a tungstate solution. The charge distribution multi-site complexation (CD-MUSIC) model was used to predict competitive adsorption between MoO(4)(2-) and other anions (i.e., phosphate, sulfate, silicate and tungstate) using model parameters obtained from the fitting of single ion adsorption envelopes. CD-MUSIC results strongly agree with the experimental adsorption envelopes of molybdate over the pH range from 3.5 to 10. Furthermore, CD-MUSIC prediction of the molybdate adsorption isotherm show a satisfactory fit of the experimental results. Modeling results suggest that the diprotonated monodentate complexes, FeOW(OH)(5)(-0.5) and FeOMo(OH)(5)(-0.5), were respectively the dominant complexes of adsorbed W and Mo on goethite 110 faces at low pH. The model suggests that Mo and W are retained mainly by the formation of monodentate complexes on the goethite surface. Our results indicate that surface complexation modeling may have applications in predicting competitive adsorption in more complex systems containing multiple competing ions.

Adsorption↗
PubMed2005

Nuclear magnetic resonance structural studies of a potassium channel-charybdotoxin complex.

Ion channels play critical roles in signaling processes and are attractive targets for treating various diseases. Here we describe an NMR-based strategy for structural analyses of potassium channel-ligand complexes using KcsA (residues 1-132, with six mutations to impart toxin binding and to mimic the eukaryotic hERG channel). Using this approach, we determined the solution structure of KcsA in complex with the high-affinity peptide antagonist charybdotoxin. The structural data reveal how charybdotoxin binds to the closed form of KcsA and makes specific contacts with the extracellular surface of the ion channel, resulting in pore blockage. This represents the first direct structural information about an ion channel complexed to a peptide antagonist and provides an experimental framework for understanding and interpreting earlier mutational analyses. The strategy presented here overcomes many of the limitations of conventional NMR approaches to helical membrane protein structure determination and can be applied in the study of the binding of druglike molecules to this important class of proteins.

Amino Acid Sequence↗
PubMed2005

Adsorption of molybdate and tetrathiomolybdate onto pyrite and goethite: effect of pH and competitive anions.

The adsorption of two major molybdenum (Mo) species, molybdate (MoO4(2-)) and tetrathiomolybdate (MoS4(2-)) onto two main iron minerals pyrite (FeS2) and goethite (FeOOH) is addressed to elucidate the possible mechanisms of molybdenum immobilization in anoxic sediments. Suspensions of MoS4(2-) (or MoO4(2-)) and goethite (or pyrite) in 0.1M NaCl solution were equilibrated under anoxic conditions at 25 degrees C in the pH range from 3 to 10. The competitive effects of sulfate, phosphate, and silicate on the adsorption of MoO4(2-) and MoS4(2-) by pyrite and goethite are also addressed. Adsorption of MoO4(2-) and MoS4(2-) on pyrite and goethite is in general well described by a Langmuir model at low pH; the extent of sorption is a function of pH and the surface loading. Maximum sorption is observed in the acidic pH range (pH<5) at low surface loading. The adsorption of molybdenum (micromol g(-1)) depends upon Mo species and on the type of iron mineral following the order: MoS4(2-)-goethite > MoO4(2-)-goethite > MoS4(2-)-pyrite > MoO4(2-)-pyrite. Phosphate appears to compete strongly with MoO4(2-) and MoS4(2-) for the sorption sites of pyrite and goethite. The strength of the phosphate competitive effect follows the sequence of MoO4(2-)-goethite approximately = MoO4(2-)-pyrite > MoS4(2-)-pyrite > MoS4(2-)-goethite. Silicate and sulfate have a negligible effect on the sorption of MoO4(2-) and MoS4(2-). The preferred adsorption by iron mineral of MoS4(2-), as well as its behavior in the presence of competitive anions suggests that tetrathiomolybdate species may be an ultimate reservoir and may control Mo enrichment in the sediments.

Adsorption↗
PubMed2005

[Effects of rhubarb on the immune substances secreted from intestine in mice].

OBJECTIVE: To investigate the therapeutic mechanism of rhubarb in protecting the intestinal muco-membranous barrier in the mice. METHOD: Bal b/c mice were divided into 2 groups, gavaged with normal saline and 10% rhubarb decoction, respectively. The animals were killed after 24 hours after the treatments. The intestinal juice was collected after intestinal lavage and centrifuged for determination of IgA, total protein, C3, high density lipoprotein, type II PLA2 activity, and content of lysozyme. At the same time, 40 mg of small intestine were incised in each mouse. Reverse transcription polymerase chain reaction (RT-PCR) and gel image analysis were performed to detect the content of the cryptdin gene expression. RESULT: The content of IgA, total protein, the C3, lysozyme, and the type II PLA2 activity in intestinal lavaged juice exhibited the statistical differences between the two groups (P < 0.05). There were no significant difference in the ontents of HDL, cryptdin-1 and cryptdin-4 gene expression between the two groups (P > 0.05). CONCLUSION: Rhubarb could increase secretion of several immune associated substances of the mucous membrane in normal intestine, indicating a possibility to abate the injury of intestine mucus resulted from severe stress induced by trauma, burn and shock. Through above mechanisms Rhubarb may also reduce the incidence of bacterial translocation and systemic inflammatory reaction syndrome (SIRS).

Animals↗
PubMed2004

Fluorescence assay of SIRT protein deacetylases using an acetylated peptide substrate and a secondary trypsin reaction.

A novel fluorescent substrate was devised for the sirtuin (SIRT) class of human protein deacetylases comprised of a peptide sequence containing a single acetyl-lysine residue, with a fluorescent group (tetramethylrhodamine-6-carboxylic acid, 6-TAMRA) near the carboxyl terminus and a nonfluorescent quenching group (QSY-7) near the amino terminus. The peptide sequence is modeled after the p53 acetylation site but is unreactive toward trypsin because all other lysine and arginine residues have been replaced by serine. However, the SIRT-deacetylated peptide is readily cleaved by trypsin, resulting in a maximal 30-fold enhancement of the 6-TAMRA fluorescence. Nicotinamide at millimolar concentrations stops the deacetylation but does not inhibit trypsin, and a microtiter plate assay of the SIRTs has been devised using the fluorescent substrate and these reagents. Using this method, the kinetics of the reaction of the cosubstrate nicotinamide adenine dinucleotide and the competitive inhibitor nicotinamide with SIRT1 and SIRT2 has been analyzed. Several nicotinamide analogs have also been tested as inhibitors and found to have much lower affinity for these enzymes than does the parent compound.

Chromatography, High Pressure Liquid↗
PubMed2004

Structure of the N-terminal RNA-binding domain of the SARS CoV nucleocapsid protein.

The severe acute respiratory syndrome (SARS) virus belongs to the Coronaviridea family of viruses. Its virion encodes several proteins including a replicase and four structural proteins. Here we describe the three-dimensional structure of the N-terminal domain of the SARS coronavirus (CoV) nucleocapsid protein. The protein consists of a five-stranded beta sheet with a folding topology distinct from other RNA-binding proteins. Single-stranded RNAs bind to the protein surface at the junction between a flexible, positively charged beta hairpin and the core structure. NMR-based screening was used to identify low molecular weight compounds that bind to this site.

Animals↗
PubMed2004

Defining the p53 DNA-binding domain/Bcl-x(L)-binding interface using NMR.

p53 exerts its tumor suppressor activity through both transcription-dependent and transcription-independent processes. Although the transcription-dependent activity of p53 has been extensively studied, the mechanism for transcription-independent p53-mediated tumor suppression is less well known. Recently, it was reported that p53 can directly induce mitochondrial permeabilization and promote apoptosis. This occurs through complexation of the DNA-binding region of p53 with the anti-apoptotic proteins Bcl-x(L) and Bcl-2 (Mihara, M. et al. (2003) Mol. Cell 11, 577-590). Using nuclear magnetic resonance (NMR) spectroscopy we show that the interaction surface on p53 involves the same region that is used by the protein to contact DNA. The p53-binding site on Bcl-x(L) consists of the carboxy-terminus of the first alpha-helix, the loop between alpha3 and alpha4, and the loop between alpha5 and alpha6 of Bcl-x(L). Furthermore, the interaction of p53 with Bcl-x(L) is blocked by the binding of a 25-residue peptide derived from the BH3 region of the pro-apoptotic protein referred to as Bad.

Binding Sites↗
PubMed2003

[Toxic effects of Cd2+ pollution on the biochemical and physiological characters and ultrastructure of Alternanthera philoxeroides].

By imitating different concentration Cd2+ polluted water environment, this paper dealt with the changes of some biochemical and physiological characters and the damage of ultrastructure in leaves of Alternanthera philoxeroides. The result showed that SOD and POD activity first increased in the low concentration pollution and then decreased with the concentration of pollution raised. The content of chlorophyll, chlorophyll a/b value, CAT activity and the content of soluble protein all declined continually. With the concentration of pollution increased, the ultrastructure of cell nuclei, chloroplast and mitochondria became deformed, the permutation of structure inside disordered, swelled, membrane disrupted and vesiculated. The changes of some biochemical and physiological characters accorded with the damage of cell ultrastructure.

Amaranthaceae↗
PubMed2002

Tamoxifen-induced nonalcoholic steatohepatitis in breast cancer patients treated with adjuvant tamoxifen.

OBJECTIVE: The benefits of 5-year tamoxifen administration for patients with estrogen receptor-positive breast cancer undoubtedly outweigh the risk of any adverse effects. Massive hepatic steatosis is an example of its side effects. Here we show risk factors for the development of massive hepatic steatosis and describe a representative clinical course of these patients treated with fibrates for the first time. METHODS: Computed tomography (CT) numbers of the liver and spleen were measured and correlated to body mass index (BMI) and pregnanetriol/pregnanediol ratio in urine (P3/P2 ratio). PATIENTS: We enrolled 56 premenopausal women treated with breast conservation treatment. They received oral tamoxifen (40 mg/day for 2 to 3 years) as adjuvant endocrine therapy with systemic chemotherapy. RESULTS: Serum estradiol level in 48 of 56 patients treated with tamoxifen was less than 10 pg/ml. The ratio of hepatic CT number to splenic CT number<0.9 was related to increased BMI (>23.6 kg/sqm) and reduced P/P2 ratio (<1). Fibrates are potent enough to improve hepatic steatosis in tamoxifen-induced hepatic steatosis. CONCLUSION: P3/P2 ratio<1 was related to sufficient blockade of estrogen receptor proven by the development of massive hepatic steatosis. Therefore, we propose that tamoxifen is not a mere antagonist of estrogen, but it may also suppress estrogen synthesis when estrogen receptor is blocked sufficiently. The undetectable level of serum estrogen in tamoxifen-treated premenopausal women may partially explain why tamoxifen was so effective in estrogen receptor-positive early breast cancer.

Adult↗
PubMed2002

[Multicenter evaluation on the efficacy and safety of sparfloxacin in the treatment of acute bacterial infections].

OBJECTIVE: To evaluate the efficacy and safety of domestic sparfloxacin. METHODS: A multicentre randomized controlled clinical trial was conducted to compare sparfloxacin versus ofloxacin in the treatment of acute bacterial infections. 58 patients received 200 mg of sparfloxacin once daily intravenous drip and 61 patients received 200 mg of ofloxacin twice daily intravenous drip. Both drugs were given for 7-14 days. RESULTS: The clinical cure rates of sparfloxacin group and ofloxacin group were 44.83% and 42.62%, and the efficacy rates were 87.93% and 81.97%, respectively; no significant differences were noted between the two groups (P > 0.05). The bacterial eradication rates were 86.00% and 84.62% respectively (P > 0.05). The adverse reactions in the two groups were 13.79% and 22.58% (P > 0.05). Photosensitivity reaction was not observed in this study. CONCLUSION: Domestic sparfloxacin is effective and safe in treating acute bacterial infections.

Acute Disease↗