PubMed Health⌕ Search

Biomedical subjects

K T Leung

Publications and source records attributed to K T Leung.

At least 19 recordsLinked to original sources
PubMed2006

Surface chemistry of monochlorinated and dichlorinated benzenes on Si(100)2x1: comparison study of chlorine content and isomeric effects.

Using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), the room temperature (RT) adsorption and thermal evolution of monochlorobenzene (MCB) and 1,3-dichlorobenzene (1,3-DCB) on Si(100)2x1 have been investigated and compared with that of 1,2-dichlorobenzene (1,2-DCB) reported previously. Like 1,2-DCB, the C 1s features observed at 284.6 (C(1)) and 286.0 eV (C(2)) for both MCB and 1,3-DCB could be attributed to the C-H and C-Cl bonds, respectively. The C(1)/C(2) intensity ratios for MCB (5.0) and 1,3-DCB (2.0) are found to follow the stoichiometric ratios of the C-H to C-Cl bonds for MCB and 1,3-DCB, respectively, indicating that both MCB and 1,3-DCB adsorb on Si(100)2x1 molecularly with negligible C-Cl dissociation at RT, in marked contrast to the partial C-Cl dissociation found for 1,2-DCB. Unlike 1,2-DCB with two discernible Cl 2s features at 270.3 and 271.2 eV, a single Cl 2s feature at 271.2 eV is observed for MCB and 1,3-DCB, in accord with the single local chemical environment for Cl. The TPD results show that MCB undergoes molecular desorption exclusively, similar to that found for benzene. Both molecular desorption and recombinative HCl desorption are found for 1,3-DCB, similar to that for 1,2-DCB. Despite the different Cl contents and relative Cl locations on the benzene ring, both MCB and 1,3-DCB exhibit RT adsorption behavior remarkably similar to that of benzene. To explain the C-Cl dissociation observed for 1,2-DCB, we propose a possible transition state involving the Cl atoms located at more physically compatible positions with the surface Si dimers in order to facilitate the conversion of 1,2-DCB (preferentially over 1,3-DCB) to dissociated products at RT. However, the thermal evolution of 1,3-DCB is closer to that of 1,2-DCB than that of MCB and benzene. The breakage of C-Cl bonds is found to occur at a relatively low temperature of 425 K, which suggests a relatively low activation barrier for the dechlorination of 1,3-DCB adspecies. Calculated energetics for 1,4-DCB on Si(100)2x1 shows that double dechlorination is not as favorable a process as those for 1,2-DCB and 1,3-DCB.

Journal Article↗
PubMed2006

Formation of C=C and Si-Cl adstructures by insertion reactions of cis-dichloroethylene and perchloroethylene on Si(100)2 x 1.

The room-temperature adsorption and thermal evolution of cis-dichloroethylene (DCE) and perchloroethylene (PCE) on Si(100)2 x 1 have been studied by X-ray photoelectron spectroscopy and temperature programmed desorption (TPD) mass spectrometry. Unlike ethylene that is found to adsorb on Si(100)2 x 1 through a [2+2] cycloaddition reaction, cis-DCE and PCE appear to dechlorinate upon adsorption on the 2 x 1 surface through an insertion reaction preserving the C=C bond. Our C 1s XPS spectra are consistent with the existence of mono-sigma-bonded and di-sigma bonded dechlorinated adstructures for both cis-DCE and PCE. The presence of the XPS C 1s feature at 283.9 eV, characteristic of the (=C<(Si)(Si)) component, supports the formation of a unique tetra-sigma-bonded C(2) dimer (i.e., by full dechlorination) for PCE, which is found to be stable to 800 K. In marked contrast to PCE for which no organic desorption fragments are observed, m/z 26 TPD features at 590 and 750 K have been observed for cis-DCE. These features could be attributed to the formation of acetylene resulting from Cl beta-elimination of 2-chlorovinyl adspecies and to direct desorption of vinylene, respectively. Further annealing the cis-DCE and PCE samples to above 800 K produces SiC and/or carbon clusters. The TPD data also show HCl evolution over 810-850 K for both cis-DCE and PCE, the latter of which also exhibits an additional SiCl(2) evolution above 850 K. The present work illustrates that the insertion mechanism could be quite common in the surface chemistry of chlorinated ethylenes on the 2 x 1 surface.

Journal Article↗
PubMed2006

In vitro antiviral activities of Chinese medicinal herbs against duck hepatitis B virus.

Fifty-six different Chinese medicinal herbs from 29 families were evaluated for their antiviral activities against duck hepatitis B virus (DHBV) in vitro. The DHBV DNA level in primary duck hepatocyte cultures was monitored by dot blot hybridization and the cytotoxicity was evaluated by MTT assay. Anti-DHBV activities were found more strongly in the aqueous extracts of Ardisia chinensis and Pithecellobium clypearia with selective indices of 2.6 and >2.7, respectively, which were comparable to that of 2',3'-dideoxycytidine. Further research on the isolation of the active antiviral phytochemicals from these herbs may provide alternative options for the treatment of chronic hepatitis B.

Animals↗
PubMed2006

Acute renal failure associated with prolonged intake of slimming pills containing anthraquinones.

Chinese herbal medicine preparations are widely available and often regarded by the public as natural and safe remedies for a variety of medical conditions. Nephropathy caused by Chinese herbs has previously been reported, usually involving the use of aristolochic acids. We report a 23-year-old woman who developed acute renal failure following prolonged use of a proprietary Chinese herbal slimming pill that contained anthraquinone derivatives, extracted from Rhizoma Rhei (rhubarb). The renal injury was probably aggravated by the concomitant intake of a non-steroidal anti-inflammatory drug, diclofenac. Renal pathology was that of hypocellular interstitial fibrosis. Spontaneous renal recovery occurred upon cessation of the slimming pills, but mild interstitial fibrosis and tubular atrophy was still evident histologically 4 months later. Although a causal relationship between the use of an anthraquinone-containing herbal agent and renal injury remains to be proven, phytotherapy-associated interstitial nephropathy should be considered in patients who present with unexplained renal failure.

Acute Kidney Injury↗
PubMed2005

Hydrogen-bonding interactions in acetic acid monohydrates and dihydrates by density-functional theory calculations.

Equilibrium structures and the respective binding energies of acetic acid monohydrates and dihydrates have been determined by density-functional theory calculations with different basis sets, including 6-31+G(3d,p), 6-311++G(d,p), and 6-311++G(3df,3pd). Given that the C=O and OH groups in acetic acid provide the predominant hydrogen-bonding interactions with water, six stable conformer structures have been found each for the monohydrate and syn-dihydrate. Of the three syn- and three anti-conformers of acetic acid with water, the most stable monohydrate structure is found to be that of the syn-conformer bonding with water in a cyclic double H-bonded geometry. Similarly, the syn-conformer bonding with two water molecules in a cyclic double H-bonded geometry has also been determined to be the most stable among the six plausible structures for the syn-dihydrate. Frequency analysis of the stable conformers has been performed and the vibrational spectra of the most stable monohydrate and dihydrate structures are compared with the experimental gas-phase and matrix data. Furthermore, the calculated binding energies between an acetic acid and a water molecule for both monohydrate and dihydrate are larger than that between two water molecules, which supports our recent experimental observation of coevaporation of acetic acid with water upon annealing acetic acid on ice.

Journal Article↗
PubMed2005

Room-temperature chemisorption and thermal evolution of 1,1-dichloroethylene and monochloroethylene on Si(111)7 x 7: formation of vinylidene and vinylene adspecies.

The room-temperature (RT) adsorption and thermal evolution of 1,1-dichloroethylene (1,1-C2H2Cl2 or iso-DCE) and monochloroethylene (C2H3Cl or MCE) on Si(111)7 x 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm(-1) suggests that upon adsorption iso-DCE dissociates via C-Cl bond breakage on the 7x7 surface to form mono-sigma-bonded 1-chlorovinyl (ClC=CH2) and/or di-sigma-bonded vinylidene (: C=CH(2)) adspecies. Upon annealing to 450 K, the 1-chlorovinyl adspecies undergoes further dechlorination to vinylidene adspecies, which may be converted to di-sigma-bonded vinylene (HC=CH) before dehydrogenating to hydrocarbon fragments above 580 K. TDS studies reveal both molecular desorption of iso-DCE near 350 K and C2H2 fragments near 700 K, and the presence of the latter confirms the existence of the di-sigma-bonded vinylene adspecies. Like the other chlorinated ethylene homologues, iso-DCE also exhibits TDS features of an etching product SiCl2 at 800-950 K and a dehydrochlorination product HCl at 700-900 K. Unlike iso-DCE, MCE is found to adsorb on the 7 x 7 surface predominantly through a [2 + 2] cycloaddition mechanism at RT, with similar di-sigma bonding structure as ethylene. The thermal evolution of MCE however follows that of iso-DCE, with the formation of vinylene above 580 K. Despite the lack of TDS feature attributable to HCl, weaker SiCl2 TDS feature could be observed at 800-950 K. For both iso-DCE and MCE, strong recombinative desorption of H2 is observed near 780 K. The differences in the Cl content among iso-DCE, MCE, and ethylene therefore play a key role in the RT chemisorption and thermally driven chemical processes on Si(111)7 x 7.

Journal Article↗
PubMed2005

Thermal and ion-induced surface reactions of 1,1-difluoroethylene on Si(111)7 x 7 and vitreous SiO2.

Thermal and ion-induced reactions of 1,1-difluoroethylene (1,1-C2H2F2 or iso-DFE) on Si(111)7 x 7 and vitreous SiO2 surfaces have been investigated by vibrational electron energy loss spectroscopy and thermal desorption spectrometry. Like ethylene, iso-DFE predominantly chemisorbs via a [2 + 2] cycloaddition mechanism onto the 7 x 7 surface as a di-sigma-bonded difluoroethane-1,2-diyl adstructure, which undergoes H abstraction and defluorination, producing hydrocarbon fragments and SiF(x) (x = 1-3) upon annealing to >700 K. Ion irradiation of Si(111)7 x 7 in iso-DFE at 50 eV impact energy appears to substantially enhance the production of hydrocarbon fragments and SiF(x)(), leading to stronger SiF4 desorption products over an extended temperature range (400-900 K). The observed SiC and SiF(x) produced on the 7 x 7 surface by ion irradiation in iso-DFE are found to be similar to those obtained by ion irradiation in the fluoromethane homologues, CF4 and CH2F2. The production of higher relative concentrations for the larger SiF(x) and C2-containing fragments is evidently favored on the 7 x 7 surface. On a vitreous SiO2 surface, ion irradiation in iso-DFE, unlike that in CF4 and CH2F2, appears to produce less SiF(x) than that on the 7 x 7 surface, which indicates that surface O does not interact strongly with the C2-containing fragments. The presence or absence of a C=C bond and the relative F-to-C ratio of the sputtering gas could therefore produce important effects on the resulting surface products obtained by low-energy ion irradiation.

Journal Article↗
PubMed2005

Thermal evolution of acetic acid nanodeposits over 123-180 K on noncrystalline ice and polycrystalline ice studied by FTIR reflection-absorption spectroscopy: hydrogen-bonding interactions in acetic acid and between acetic acid and ice.

Acetic acid vapor-deposited on ultrathin noncrystalline ice (NCI) and polycrystalline ice (PCI) films (less than 6 nm thick) under ultrahigh vacuum conditions has been investigated by using Fourier Transform Infrared Reflection-Absorption Spectroscopy. Pristine acetic acid deposited at 123 K (on a copper support) appears as an amorphous solid, which undergoes an irreversible phase transformation to a more structurally ordered (polycrystalline) form upon annealing to 153 K. Acetic acid is found to adsorb on NCI and PCI films initially through hydrogen bonding between C=O and dangling OH (of ice), followed by the formation of multilayers at 123 K. Thermal evolution studies of a low exposure of acetic acid on the ultrathin NCI and PCI films show that acetic acid undergoes coevaporation with water likely as an acetic acid hydrate at 155 K, which continues until the entire ice film has been exhausted at 165 K. Above 165 K, the remaining acetic acid solid appears to evaporate without undergoing the phase transformation, in contrast to the case of a high acetic acid exposure. Coevaporation of acetic acid with water is also found to proceed at a faster rate than the subsequent evaporation of acetic acid, which is consistent with the weaker interactions observed in the H-bonded acetic acid hydrate than that in acetic acid solid.

Journal Article↗
PubMed2005

Thermal chemistry of styrene on Si(100)2x1 and modified surfaces: electron-mediated condensation oligomerization and posthydrogenation reactions.

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2x1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2x1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2x1 reconstructed surface. The chemisorption of styrene on the 2x1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2x1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2x1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2x1 upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.

Adsorption↗
PubMed2005

Mycobacterium chelonae exit site infection in a patient on peritoneal dialysis.

Atypical mycobacterial infe tion of the Tenckhoff catheter exit site is rare. Eradication of the infection is often difficult without the removal of the Tenckhoff cath ter. We report here a case of Mycobacterium chelonae exit site infection in a peritoneal dialysis patient. He was treated with a combination regimen of prolonged antibiotics, local heat therapy, deroofing and shaving of the Tenckhoff catheter outer cuff. This resulted in the successful treatment of the infection without the need for removal of the Tenckhoff catheter. We recommend that this therapeutic approach could be considered in similar cases and that removal of Tenckhoff catheter is not mandatory.

Anti-Bacterial Agents↗
PubMed2005

Acute renal failure related to intravenous immunoglobulin infusion in an elderly woman.

Intravenous immunoglobulin infusion induces acute renal failure via a mechanism of osmotic nephrosis. Most reported cases are related to the use of sucrose-based intravenous immunoglobulin. Maltose-based intravenous immunoglobulin is thought to be a safer alternative and have a lower risk of renal toxicity than sucrose-based preparations. Maltase, but not sucrase, is present in the brush border of proximal convoluted renal tubules, where the maltose is metabolised. We report a case of maltose-based intravenous immunoglobulin-induced acute renal failure in an elderly diabetic woman. In this case, the risk factors included advanced age, hypovolaemia, sepsis, diabetes mellitus, and the high infusion rate of the intravenous immunoglobulin. Maltase is readily inhibited by hyperglycaemia; therefore, poor glycaemic control may predispose patients to develop acute renal failure even with the better-tolerated maltose-based intravenous immunoglobulin.

Acute Kidney Injury↗
PubMed2004

Parametric study on electrochemical deposition of copper nanoparticles on an ultrathin polypyrrole film deposited on a gold film electrode.

Monoshaped and monosized copper nanostructured particles have been prepared by potentiostatic electrochemical deposition on an ultrathin polypyrrole (PPY) film, electrochemically grown on a Si(100) substrate sputter-coated with a thin gold film or gold-film electrode (GFE). The crystal size and the number density of the copper nanocrystals have been examined by varying several deposition parameters, including the thickness of the gold film, the PPY film thickness, the applied potential, and the Cu2+ and the electrolyte concentrations for copper deposition. Optimal conditions for uniform growth ofnanocrystals well-dispersed on the GFE have been determined, along with insight into the mechanism of crystal growth. A minimum gold film thickness of 80 nm is required to eliminate the effects of the gold-silicon interface. The PPY film thickness and homogeneity principally affect the shape uniformity of the nanocrystals, while the copper deposition potential could be used to regulate the size and number density of the nanocrystals. Both the Cu2+ and electrolyte concentrations are also found to play important roles in controlling the electrodeposition of nanocrystal growth.

Journal Article↗
PubMed2004

Treatment of vancomycin-intermediate Staphylcoccus aureus endocarditis with linezolid.

We report a case of infective endocarditis due to vancomycin-intermediate Staphylococcus aureus that developed after repeated courses of vancomycin. The patient had underlying end stage renal disease and dissecting aortic aneurysm with aortic graft and prosthetic aortic valve replacement. He responded to prolonged combination therapy with linezolid and amikacin without undergoing surgical intervention.

Acetamides↗
PubMed2004

Successful kidney re-transplantation in a patient with previous allograft kidney tuberculosis.

Opportunistic infections, and in particular tuberculosis (TB), carry substantial morbidity and mortality in solid organ transplant recipients. We report a 39-year-old man who underwent a cadaveric renal transplant. Three months postoperatively, he was diagnosed to have tuberculous infection of his graft kidney manifested as fever and renal impairment. The diagnosis was confirmed by renal biopsy, which showed granuloma formation and positive stain for acid-fast bacilli (AFB). His systemic symptoms responded well to a complete course of anti-tuberculous therapy, but his renal function continued to deteriorate. Graft nephrectomy was performed and the patient underwent a second kidney transplant 1 year later. He remained well and asymptomatic thereafter. No signs of recurrence of tuberculous infection were noted up until the present time. This case illustrates that TB remains an important threat to transplant recipients. Although reactivation of dormant TB is the usual mode of infection, acquisition from the donor graft is also possible. The latter may account for the infection in our case, as our patient had a negative tuberculin skin test and normal chest radiograph prior to transplant. The identification of AFB in the kidney graft less than 3 months postoperatively also suggested that causal relationship. While diagnosing TB in post-transplant recipients is difficult and may require renal biopsy, as in our case, treatment on the other hand is no different from the standard protocols. However, no consensus has been reached on the safety of re-transplantation. Also, the need for graft nephrectomy and chemoprophylaxis remains unclear.

Adult↗
PubMed2004

Corticosteroid therapy in a Chinese patient with nephropathy associated with human immunodeficiency virus infection.

A 52-year-old man with 6 years' history of human immunodeficiency virus infection who was receiving highly active antiretroviral therapy presented with acute renal failure and nephrotic syndrome. Renal biopsy revealed features consistent with nephropathy associated with human immunodeficiency virus infection. Treatment consisted of intravenous methylprednisolone followed by oral prednisolone. The patient's renal function improved, although proteinuria persisted. Human immunodeficiency virus-associated nephropathy is very rare in Asian populations and is more common among blacks. To the best of our knowledge, this is the first documented case of nephropathy associated with human immunodeficiency virus infection occurring in Hong Kong.

AIDS-Associated Nephropathy↗
PubMed2002

Characterization of tetrachlorohydroquinone reductive dehalogenase from Sphingomonas sp. UG30.

Tetrachlorohydroquinone reductive dehalogenase (PcpC) is the second of three enzymes that catalyze the initial degradation of pentachlorophenol in Sphingomonas sp. UG30 and several other bacterial strains. The UG30 PcpC shares a high degree (94%) of primary sequence identity with the well-studied PcpC from Sphingobium chlorophenolicum ATCC 39723. Significant differences, however, were observed between the two PcpC enzymes in some of their functional and kinetic properties. The temperature optimum of the UG30 PcpC is 10 degrees C higher and the pH optimum is approximately 2 units higher than the S. chlorophenolicum PcpC. In addition, the S. chlorophenolicum PcpC is subject to inhibition by the substrate tetrachlorohydroquinone (TCHQ), and this has necessitated the use of a mutant enzyme, which was not inhibited by TCHQ, for kinetic studies. In contrast, the UG30 PcpC was not inhibited by TCHQ and this may allow detailed kinetic and mechanistic studies using the wild-type enzyme.

Amino Acid Sequence↗
PubMed2001

Hybridization of single-stranded DNA targets to immobilized complementary DNA probes: comparison of hairpin versus linear capture probes.

A microtiter-based assay system is described in which DNA hairpin probes with dangling ends and single-stranded, linear DNA probes were immobilized and compared based on their ability to capture single-strand target DNA. Hairpin probes consisted of a 16 bp duplex stem, linked by a T(2)-biotin.dT-T(2) loop. The third base was a biotinylated uracil (U(B)) necessary for coupling to avidin coated microtiter wells. The capture region of the hairpin was a 3' dangling end composed of either 16 or 32 bases. Fundamental parameters of the system, such as probe density and avidin adsorption capacity of the plates were characterized. The target DNA consisted of 65 bases whose 3' end was complementary to the dangling end of the hairpin or to the linear probe sequence. The assay system was employed to measure the time dependence and thermodynamic stability of target hybridization with hairpin and linear probes. Target molecules were labeled with either a 5'-FITC, or radiolabeled with [gamma-(33)P]ATP and captured by either linear or hairpin probes affixed to the solid support. Over the range of target concentrations from 10 to 640 pmol hybridization rates increased with increasing target concentration, but varied for the different probes examined. Hairpin probes displayed higher rates of hybridization and larger equilibrium amounts of captured targets than linear probes. At 25 and 45 degrees C, rates of hybridization were better than twice as great for the hairpin compared with the linear capture probes. Hairpin-target complexes were also more thermodynamically stable. Binding free energies were evaluated from the observed equilibrium constants for complex formation. Results showed the order of stability of the probes to be: hairpins with 32 base dangling ends > hairpin probes with l6 base dangling ends > 16 base linear probes > 32 base linear probes. The physical characteristics of hairpins could offer substantial advantages as nucleic acid capture moieties in solid support based hybridization systems.

Avidin↗