Search PubMed⌕ Search

Biomedical subjects

D Peak

Publications and source records attributed to D Peak.

9 recordsLinked to original sources
PubMed2005

The influence of forestry activity on the structure of dissolved organic matter in lakes: implications for mercury photoreactions.

It is well known that dissolved organic matter (DOM) increases in lakes associated with forestry activity but characterization of the DOM structure is incomplete. Twenty-three lakes with a wide range of forestry activities located in central Quebec, Canada were sampled and analyzed for dissolved organic carbon (DOC) concentration, DOC fluorescence, and ultra violet-visible (UV-VIS) absorption spectra. The results show that DOC increases (as does the associated DOC fluorescence) with increased logging (slope=0.122, r2=0.581, p<0.001; and slope=0.283, r2=0.308, p<0.01, respectively) in the 23 lakes sampled however, the aromaticity of the DOM does not change with changes in logging (as found by UV-VIS ratios, absorbance slope in the UV region, and DOC normalized fluorescence (slope=1.42x10(-2), r2=0.331, p<0.01). The DOM from four of these lakes was concentrated using reverse osmosis (RO) followed by freeze-drying. The structures of the concentrated dissolved organic matter (DOM) samples were analyzed using X-ray analysis of near edge structures (XANES), X-ray diffraction (XRD), and 13C solid-state nuclear magnetic resonance (13C NMR) analysis. XANES analysis of functional groups in the four concentrated samples shows that there are significant differences in reduced sulphur between the samples, however there was no clear relationship with forestry activity in the associated catchment. XRD data showed the presence of amorphous sulphide minerals associated with the DOM concentrate that may be important sites for mercury binding. The 13C NMR spectra of these samples show that the percentage of carbon present in carboxylic functional groups increases with increasing logging. Such structures are important for binding photo-reducible mercury and their presence may limit mercury photo-reduction and volatilization. We propose a mechanism by which increased logging leads to increased carboxylic groups in DOM and thereby increased weak binding of photo-reducible mercury. These results, in part, explain the decrease in dissolved gaseous mercury (DGM) production rates with increased logging found in our previous work.

Air Pollutants↗
PubMed2002

Solid-state speciation of natural and alum-amended poultry litter using XANES spectroscopy.

While alum amendments have shown to be effective in lowering water-soluble phosphate levels in poultry litter, the mechanism by which this occurs is not fully known. To determine the solid-state speciation of phosphate in litter samples, experiments were conducted with X-ray absorption near edge structure (XANES) spectroscopy. XANES analysis reveals that, in unamended samples, phosphate is present as weakly bound inorganic as well as some organic phosphate, with some dicalcium phosphate-type calcium phosphates also present. When alum is applied in the houses, XANES results suggest that it precipitates out as amorphous Al(OH)3 and then reacts with phosphate via an adsorption mechanism. No evidence was found of aluminum phosphate precipitation in any samples.

Alum Compounds↗
PubMed2002

Mechanisms of selenate adsorption on iron oxides and hydroxides.

Selenate (SeO4(2-)) is an oxyanion of environmental importance because of its toxicity to animals and its mobility in the soil environment. It is known that iron(III) oxides and hydroxides are important sorbents for SeO4(2-) in soils and sediments, but the mechanism of selenate adsorption on iron oxides has been the subject of intense debate. Our research employed Extended X-ray absorption fine structure and attenuated total reflectance-Fourier transform infrared spectroscopies to determine SeO4(2-) bonding mechanisms on hematite, goethite, and hydrous ferric oxide (HFO). It was learned that selenate forms only inner-sphere surface complexes on hematite but forms a mixture of outer- and inner-sphere surface complexes on goethite and HFO. This continuum of adsorption mechanisms is strongly affected by both pH and ionic strength. These results suggest that adsorption experiments should be conducted on several different iron oxides and over a wide range of reaction conditions to accurately assess the reactivity of oxyanions on iron oxides.

Absorptiometry, Photon↗
PubMed1999

An in Situ ATR-FTIR Investigation of Sulfate Bonding Mechanisms on Goethite.

The mechanism of sulfate adsorption on goethite was investigated in situ using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Sulfate adsorption was investigated at ionic strengths between 0.005 and 0.1 M, reactant concentrations between 5 and 500 µM, and pH values between 3.5 and 9.0. It was determined that sulfate forms both outer-sphere and inner-sphere surface complexes on goethite at pH less than 6. At pH values greater than 6, sulfate adsorbs on goethite only as an outer-sphere complex. The relative amount of outer-sphere sulfate surface complexation increased with decreasing ionic strength. The spectrum of sulfate adsorbed on goethite was also compared to the infrared spectrum of synthetic schwertmannite, an iron(III) oxy-hydroxy-sulfate. It was determined that in situ spectra of both schwertmannite and adsorbed sulfate are quite similar, suggesting that a continuum of outer- and inner-sphere sulfate occurs in both cases. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Is blastocyst transfer useful as an alternative treatment for patients with multiple in vitro fertilization failures?

OBJECTIVE: To determine whether blastocyst transfer is of benefit to patients with multiple IVF failures. DESIGN: Retrospective cohort study. SETTING: The George Washington University Medical Center. PATIENT(S): Patients undergoing IVF between October 1, 1997, and November 30, 1998, who had previously undergone three or more unsuccessful IVF cycles. Patients who had at least three embryos at the 8- to 12-cell stage available on day 3 were eligible for the study. INTERVENTION(S): Patients were given the option of day 3 ET (group A) or blastocyst transfer (group B). MAIN OUTCOME MEASURE(S): Blastocyst-formation rate, clinical pregnancy rate (PR) per transfer, and implantation rate per transfer. RESULT(S): Groups A and B were similar in terms of age, the number of previous failed IVF cycles, fertilization rate, and the number of fertilized oocytes per cycle. The blastocyst-formation rate was 51.0%. Clinical pregnancy and implantation rates per transfer were statistically significantly higher in the blastocyst-transfer group. There were no multiple pregnancies after blastocyst transfer. CONCLUSION: Blastocyst transfer increases implantation rates and PRs in patients with multiple failed IVF cycles, without increasing the risk of multiple pregnancy.

Adult↗
PubMed1993

Inhibition of a reductive function of the plasma membrane by bacitracin and antibodies against protein disulfide-isomerase.

Evidence had been provided that a disulfide-linked [125I]iodotyramine/poly(D-lysine) conjugate was reductively cleaved when bound nonspecifically to the surface of Chinese hamster ovary (CHO) cells and that this cleavage was abolished by membrane-impermeant sulfhydryl blockers. The same blockers were subsequently found to inhibit the cytotoxicity of diphtheria toxin, a disulfide-linked heterodimer that binds to a specific surface receptor and must undergo chain separation to exert its cytotoxicity. This suggested that the disulfides of both macromolecules might be cleaved by a thiol-disulfide interchange reaction, possibly mediated by protein disulfide-isomerase (PDI, EC 5.3.4.1). We tested whether inhibitors of PDI--in particular, bacitracin and anti-PDI antibodies--might mimic the two effects of sulfhydryl blockers. Both bacitracin and anti-PDI antibodies were effective in inhibiting both reductive processes. This strongly suggests that the disulfide cleavage in the two membrane-bound macromolecules is mediated by PDI and that this enzyme, besides its known retention in the endoplasmic reticulum, must also be exposed at the plasma membrane. This paper points to other potentially important disulfide reductions that might be catalyzed by surface-associated PDI. It thereby broadens the known functions of an enzyme already known for its multifunctional properties.

4-Chloromercuribenzenesulfonate↗