PubMed Health⌕ Search

Biomedical subjects

K K Cheung

Publications and source records attributed to K K Cheung.

At least 19 recordsLinked to original sources
PubMed2005

Expression of P2X purinoceptors during rat brain development and their inhibitory role on motor axon outgrowth in neural tube explant cultures.

Extracellular ATP is well known as a neurotransmitter and neuromodulator in the CNS of adults. However, little is known about the involvement of ATP during the development of mammalian brain. In the present study, we have examined the expression pattern of P2X receptor subtype mRNA and protein during perinatal rat brain development (from embryonic day (E) 10 to postnatal day (P) 16 brain). While P2X3 receptors appeared early at E11, they declined in the stages that follow. P2X2 and P2X7 receptors were expressed from E14 onwards, while P2X4, P2X5 and P2X6 receptors were expressed from P1 onwards. P2X1 receptor expression was not observed in any of the developmental ages examined. We investigated the effect of 100 microM ATP and alpha,beta-methylene ATP (alpha,beta-meATP; selective agonist for P2X1, P2X2/3 and P2X3 receptors) on motor axon outgrowth in collagen-embedded neural tube explant cultures. Both ATP- and alpha,beta-meATP-treated neural tubes showed a significant reduction in neurite outgrowth compared with the control explants. This inhibitory effect could not be reproduced by uridine triphosphate. In conclusion, all P2X receptor subtypes, except for P2X1, were strongly represented in the developing rat brain. ATP was shown to inhibit motor axon outgrowth during early embryonic neurogenesis, most likely via the P2X3 receptor. It is speculated that P2X7 receptors may be involved in programmed cell death during embryogenesis and that P2X4, P2X(5) and P2X6 receptors might be involved in postnatal neurogenesis.

Adenosine Triphosphate↗
PubMed2002

Acute vertebral body compression fractures: discrimination between benign and malignant causes using apparent diffusion coefficients.

Diffusion weighted MRI was performed on patients with acute vertebral body compression. The usefulness of the apparent diffusion coefficient (ADC) in differentiating between benign and malignant fractures was evaluated. A total of 49 acute vertebral body compression fractures were found in 32 patients. 25 fractures in 18 patients were due to osteoporosis, 18 fractures in 12 patients were histologically proven to be due to malignancy, and 6 fractures in 2 patients were due to tuberculosis. Signal intensities on T(1) weighted, short tau inversion recovery (STIR) and diffusion weighted images were compared. ADC values of normal and abnormal vertebral bodies were calculated. Except for two patients with sclerotic metastases, benign acute vertebral fractures were hypointense and malignant acute vertebral fractures were hyperintense with respect to normal bone marrow on diffusion weighted images. Mean combined ADCs (ADC(cmb); average of the combined ADCs in the x, y and z diffusion directions) were 0.23 x 10(-3) mm(2) s(-1) in normal vertebrae, 0.82 x 10(-3) mm(2) s(-1) in malignant acute vertebral fractures and 1.94 x 10(-3) mm(2) s(-1) in benign acute vertebral fractures. The differences between ADC(cmb) values were statistically significant (p<0.001). The ADC is useful in differentiating benign from malignant acute vertebral body compression fractures, but there may be overlapping ADC values between malignant fractures and tuberculous spondylitis.

Acute Disease↗
PubMed2001

Synthesis, structural characterization, and photophysics of dinuclear gold(II) complexes [(Au(dppn)Br)(2)](PF(6))(2) and [(Au(dppn)I)(2)](PF(6))(2) with an unsupported Au(II)-Au(II) Bond.

A series of novel dinuclear gold(II) complexes having an unsupported Au(II)-Au(II) bond with well-defined oxidation state at the gold center were synthesized. Red shifts in both the low-energy absorption and emission bands were observed upon increasing the donor ability of the coordinated halogen atoms.

Journal Article↗
PubMed2001

Palladium(II) and platinum(II) analogues of luminescent diimine Triangulo complexes supported by triply bridging sulfide ligands: structural and spectroscopic comparisons.

Reaction of [M(L-L)Cl(2)] [M = Pd, Pt; L-L = 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bu(2)bpy), 4,4'-dimethylcarboxylate-2,2'-bipyridine ((CO(2)Me)(2)bpy), bis(diphenylphosphino)methane (dppm)] with Na(2)S in refluxing methanol afforded [M(3)((t)Bu(2)bpy)(3)(mu(3)-S)(2)](2+) [M = Pd (1a), Pt (2a)], [M(3)((CO(2)Me)(2)bpy)(3)(mu(3)-S)(2)](2+) [M = Pd (1b), Pt (2b)], and [Pt(3)(dppm)(3)(mu(3)-S)(2)](2+) (3) as perchlorate salts. X-ray crystal analysis revealed that 1a, 1b, 2a, and 3 have triangular M(3)S(2) core structures. The three metal atoms in 1a, 2a, and 3 form virtual equilateral triangles with intramolecular Pd-Pd and Pt-Pt separations of 3.027(1)-3.065(1) and 3.104(1)-3.154(1) A, respectively. An isosceles triangle of Pd(3) atoms is observed in the molecular structure of 1b. The (1)MLCT absorption of 2a and 2b appears at 415 and 448 nm, respectively, in dichloromethane and is significantly red-shifted from the lowest energy absorption band of the Pd(3) analogues. Complex 1a exhibits weak photoluminescence in the solid state at 77 and 298 K (uncorrected lambda(max) 760 and 730 nm, respectively) while the 77 K solid-state emission of 1b (uncorrected lambda(max) 760 nm) is also weak. At 77 K, complexes 2a, 2b, and 3 display broad unstructured emissions at lambda(max) 616-630 nm in the solid state. Ligand-field excited states are tentatively assigned for these emissions.

Journal Article↗
PubMed2001

Substrate-Binding reactions of the 3[dsigma*psigma] excited state of binuclear gold(I) complexes with bridging bis(dicyclohexylphosphino)methane ligands: emission and time-resolved absorption spectroscopic studies.

The complexes [Au2(dcpm)2]-Y2 (dcpm = bis(dicyclohexylphosphino)methane; Y=ClO4 (1), PF6- (2), CF3SO3- (3), Au(CN)2- (4), Cl- (5), SCN (6) and I- (7)) were prepared, and the structures of 1 and 4-7 were determined by X-ray crystallography. Complexes 1-4 display intense phosphorescence with lambdamax at 360-368 nm in the solid state at room temperature as well as in glassy solutions at 77 K. The solid-state emission quantum yields of the powdered samples are 0.37 (1), 0.74 (2), 0.53 (3) and 0.12 (4). Crystalline solid 5 displays both high-energy UV (lambdamax = 366 nm) and low-energy visible emissions (lambdamax = 505 nm) at room temperature, whereas either 6 or 7 shows only an intense emission with lambdamax at 465 or 473 nm, respectively. All the complexes in degassed acetonitrile solutions exhibit an intense phosphorescence with lambdamax ranging from 490 to 530 nm. The high-energy UV emission is assigned to the intrinsic emission of the 3[dsigma*psigma] excited state of [Au2(dcpm)2]2+, whereas the visible emission is attributed to the adduct formation of the triplet excited state with the solvent/counterion. The quenching rate constants of the visible emission of [Au2(dcpm)2]2+ in acetonitrile by various anions are 6.08 x 10(5) (ClO4-), 9.18 x l0(5) (PF6 ), 1.55 x 10(7) (Cl-) and 4.06 x 10(9) (I-) mol(-1) dm3s(-1). The triplet-state difference absorption spectra of 1-4 in acetonitrile show an absorption band with lambdamax at 350 nm and a shoulder/absorption maxima at 395-420 nm; their relative intensities are dependent upon the halide ion present in solution. Substrate binding reactions of the 3[dsigma*psigma] excited state with halide (X-) to give [Au2(dcpm)2X]+* would account for the lower energy absorption maxima in the triplet-state difference absorption spectra. With iodide as the counterion, complex 7 undergoes a photoinduced electron-transfer reaction with I- to give the radical anion I2-.

Journal Article↗
PubMed2001

Organic light-emitting materials based on bis(arylacetylide)platinum(II) complexes bearing substituted bipyridine and phenanthroline ligands: photo- and electroluminescence from 3MLCT excited states.

We present the synthesis and photophysical and electroluminescent properties for a series of platinum(II) alpha-diimine bis(arylacetylide) complexes. The molecular structures of five derivatives have been elucidated by X-ray crystallography. Intermolecular pi-pi interactions (between aromatic diimine and phenylacetylide moieties) are apparent in the crystal lattices of two of these. All bis(phenylacetylide) derivatives exhibit intense triplet metal-to-ligand charge transfer (MLCT) photoluminescence in the solid state and in fluid solutions at room temperature. The impact of different solvents, substituents on the diimine ligands, and complex concentrations upon their emissive behavior have been examined and demonstrates that their emission energies can be systematically modified. Application of the 3MLCT excited state of the [Pt(alpha-diimine) (C(triple bond)CPh)2] materials in single- and double-layer organic light-emitting devices are described. The bis(butadiynyl) complex [Pt(4,4'-dtbpy)(C(triple bond)C-C(triple bond)CPh)2] (dtbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) displays strong solid-state and solution phosphorescence at 77 and 298 K; the associated excited state is proposed to arise from both acetylenic 3pi pi*(C(triple bond)C-C(triple bond)CPh) and 3MLCT [Pt --> pi*(diimine)] transitions.

Journal Article↗
PubMed2001

Photoluminescent metal-sulfur clusters derived from tetrathiometalates: metal-to-metal charge-transfer excited states of d0-d10 heterobimetallic sulfido clusters with bulky phosphine ligands.

Reactions of MS4(2-) (M = Mo, W) with M'(PCy3)X (M'=Ag/Au, X= ClO4/Cl) and [Cu2(dcpm)2(MeCN)2](ClO4)2 (dcpm = bis(dicyclohexylphosphino)methane) afforded heterometallic sulfido clusters [M'2(PCy3)2(MS4)] (M=Mo, M'=Au: 2; M=W, M'=Ag: 3, Au: 4) and [Cu4(dcpm)4(MS4)](ClO4)2 (M=Mo: 5 x (ClO4)2, W: 6 x (ClO4)2), all of which, except 4, have been characterized by X-ray structure determination. Clusters 5 x(ClO4)2 and 6 x (ClO4)2 feature unusual 16-membered [Cu4P5C4] metallamacrocycles formed on the respective tetrathiometalate anion templates and have unusually long Cu-S bonds and Cu...M distances for metal sulfur clusters that contain a saddle-shaped [Cu4MS4] core. Low-energy absorption bands are observed in their electronic spectra at approximately 562 and 467 nm, respectively, assignable to MMCT transitions; quasireversible reduction waves are observed with E(1/2) = -1.43 (52+) and -1.78 V (62+) versus FeCp2(0/+); and they are emissive either in the solid state or in solution. The emission of 6(2+) can be quenched by both electron acceptors, such as methylviologen, or electron donors, such as aromatic amines, with the excited state reduction potential E(62+*/6+) estimated to be approximately 1.13V versus a normal hydrogen electrode.

Journal Article↗
PubMed2001

Syntheses, photophysics, and photochemistry of trinuclear copper(I) thiolate and hexanuclear copper(I) selenolate complexes: X-ray crystal structures of [Cu(6)(mu-dppm)4(mu(3)-SePh)4](BF(4))2 and [Cu(6)(mu-(Ph(2)P)(2)NH(4))(mu(3)-SePh)4](BF(4)2.

A series of trinuclear copper(I) thiolate complexes, [Cu(3)(mu-dppm)(3)(mu(3)-SR)(2)]BF(4) (R = C(6)H(4)Cl-4, C(6)H(4)CH(3)-4, C(6)H(4)OCH(3)-4, C(6)H(4)(OCH(3))(2)-3,4, C(6)H(4)-benzo-15-crown-5, or (t)()Bu), [Cu(3)(mu-dppm)(3)(mu(3)-S(t)()Bu)](BF(4))(2), and [Cu(3)(mu-dppm)(3)(mu(3)-SR)(mu(3)-Cl)]BF(4) (R = C(6)H(4)CH(3)-4, C(6)H(4)(t)()Bu-4, or C(6)H(4)(CH(3))(3)-2,4,6) and two hexanuclear copper(I) selenolate complexes, [Cu(6)(mu-P(wedge)P)(4)(mu(3)-SePh)(4)](BF(4))(2) (P(wedge)P = dppm, (Ph(2)P)(2)NH), have been synthesized, and their photophysical properties have been studied. The X-ray crystal structures of both copper(I) selenolate complexes have been determined. These complexes have been shown to exhibit long-lived low-energy emission in solution, attributed to an excited state of predominantly ligand-to-metal charge-transfer [chalcogenolate to copper(I)] origin.

Journal Article↗
PubMed2001

Interaction between dioxoruthenium(VI) porphyrins and hydroxylamines: coordination of N-substituted hydroxylamine to ruthenium and X-ray crystal structures of ruthenium complexes with a unidentate nitrosoarene ligand.

The interactions between dioxoruthenium(VI) porphyrins 1 with N-phenylhydroxylamine or unsubstituted hydroxylamine are described. Reaction of complexes 1 with excess PhNHOH leads to isolation of bis(nitrosobenzene)ruthenium(II) porphyrins 3 and mono(nitrosobenzene)ruthenium(II) porphyrins 4. Both the types of ruthenium complexes are characterized by 1H NMR, IR, and UV/Vis spectroscopy, and mass spectrometry. The X-ray structure determinations on [Ru(II)(TPP)(PhNO)2] (3a), [Ru(II)(2,6-Cl-TPP)(PhNO)2] (3e), and [Ru(II)(4-MeO-TPP)(PhNO)(PhNH2)] (4d) (TPP tetraarylporphyrin) disclose a unidentate nitrosoarene coordination in all these complexes, with Ru-N(PhNO) bond lengths of 2.003(3) (3a, average), 1.991(3) (3e, average), and 2.042(2) A (4d). In the case of 4d, the Ru-N(PhNH2) bond length is found to be 2.075(3) A. Mechanistic investigations reveal the formation of intermediates [Ru(II)(Por)(PhNO)(PhNHOH)] (5; Por=porphyrin), a ruthenium complex with N-substituted hydroxylamine ligand, in the "1 + PhNHOH" system. The Ru-NH(OH)Ph moiety in 5 undergoes no rapid exchange with free PhNHOH in solution at room temperature, as revealed by 1H NMR spectroscopy. Unlike the interaction between complexes 1 and PhNHOH, reaction of such complexes with NH2OH affords nitrosylruthenium(II) porphyrins [Ru(II)(Por)(NO)(OH)] (6).

Catalysis↗
PubMed2001

Luminescent mu-ethynediyl and mu-butadiynediyl binuclear gold(I) complexes: observation of (3)(pi pi*) emissions from bridging C(n)(2-) units.

The synthesis and X-ray structural and spectroscopic characterization for LAuC triple bond CAuL x 4CHCl(3) and LAuC triple bond C--C triple bond CAuL x 2CH(2)Cl(2) (1 x 4CHCl(3) and 2 x 2CH(2)Cl(2), respectively; L = PCy(3), tricyclohexylphosphine) are reported. The bridging C(n)(2-) units are structurally characterized as acetylene or diacetylene units, with C triple bond C distances of 1.19(1) and 1.199(8) A for 1 x 4CHCl(3) and 2 x 2CH(2)Cl(2), respectively. An important consequence of bonding to Au(I) for the C(n)(2-) moieties is that the lowest-energy electronic excited states, which are essentially acetylenic (3)(pi pi*) in nature, acquire sufficient allowedness via Au spin-orbit coupling to appear prominently in both electronic absorption and emission spectra. The origin lines for both complexes are well-defined and are observed at 331 and 413 nm for 1 and 2, respectively. Sharp vibronic progressions corresponding to v(C triple bond C) are observed in both emission and absorption spectra. The acetylenic (3)(pi pi) excited state of 2 has a long lifetime (tau(0) = 10.8 mus) in dichloromethane at room temperature and is a powerful reductant (E degrees [Au(2)(+)/Au(2)] < or = -1.85 V vs SSCE).

Journal Article↗
PubMed2001

SPARC (secreted protein acidic and rich in cysteine) induces apoptosis in ovarian cancer cells.

Secreted protein acidic and rich in cysteine (SPARC) is an extracellular Ca(2+)-binding matricellular glycoprotein that associates with cell populations undergoing migration, morphogenesis, and differentiation. Studies on endothelial cells have established that its principal functions in vitro are counteradhesion and antiproliferation. The mechanism(s) underlying these antitumor effects is unknown. In this study, we showed that SPARC expression in ovarian cancer cells is inversely correlated with the degree of malignancy. The immunohistochemical data presented here confirmed the importance of diminished SPARC expression in ovarian cancer development. Treating human ovarian surface epithelial cells and ovarian cancer cells with SPARC revealed that as SPARC inhibits the proliferation of both normal and cancer cells, it induces apoptosis only in cancer cells. This observation indicates that down-regulation of SPARC is essential for ovarian carcinogenesis as cancer cells become sensitized to the apoptotic activity of SPARC during malignant transformation. We also showed here the first direct evidence that putative SPARC receptors are present on ovarian epithelial cells. Their levels are higher in human ovarian surface epithelial cells than cancer cells. Binding of SPARC to its receptor is likely to trigger tissue-specific signaling pathways that mediate its tumor suppressing functions. Decrease in ligand-receptor interaction by the down-regulation of SPARC and/or its receptor is essential for ovarian carcinogenesis.

Apoptosis↗
PubMed2001

Compounds from Kadsura ananosma.

The 18-(13-->12 beta)-abeo-lanostene triterpenoid acid, ananosic acid A (1), and the dibenzocyclooctadiene lignan, ananosin A (2), were isolated from the stem bark of Kadsura ananosma. Their structures were elucidated by extensive spectral studies and the structure of 1 was confirmed by single crystal X-ray diffraction analysis.

Crystallography, X-Ray↗
PubMed2000

Synthesis, emission, and electrochemical properties of luminescent dinuclear zinc(II) chalcogenolate complexes. Dynamic 1H NMR studies and X-ray crystal structure of [(bpy)Zn2(SC6H4-Cl-p)(mu-SC6H4-Cl-p)(mu-OAc)2].

A series of novel luminescent dinuclear zinc(II) diimine complexes with bridging chalcogenolate ligands have been synthesized, in which the two zinc atoms were found to exist in different coordination environment. The luminescence and electrochemical behavior of these complexes have been studied. These complexes have also been shown to exhibit dynamic fluxional behavior in solution due to an exchange of the bridging and terminal thiolate ligands. The mechanism and kinetics of which have been investigated by variable-temperature 1H NMR studies. The X-ray crystal structure of [(bpy)Zn2(SC6H4-Cl-p)(mu-SC6H4-Cl-p)(mu-OAc)2] has also been determined.

Crystallography, X-Ray↗
PubMed2000

Ruthenium complexes with a terminal hydrazido ligand. Synthesis, spectroscopy, and X-ray crystal structure.

Bis(1,1-diphenylhydrazido(1-))ruthenium(IV) porphyrins, [Ru(IV)(Por)(NHNPh2)2] (Por = TPP, TTP, 4-Cl-TPP, 4-MeO-TPP), were prepared in approximately 60% yields through the reaction of dioxoruthenium(VI) porphyrins, [Ru(VI)(Por)O2], with 1,1-diphenylhydrazine in ethanol. This new type of ruthenium complex has been characterized by 1H NMR, IR, UV-vis, and FABMS with elemental analysis. The crystal structure of [Ru(IV)(TTP)(NHNPh2)2], which reveals an eta1-coordination mode for both hydrazido axial ligands, has been determined. The average Ru-NHNPh2 distance and Ru-N-N angle were found to be 1.911(3) A and 141.1(3) degrees, respectively. The porphyrin ring exhibits a ruffling distortion that is unprecedentedly large for ruthenium complexes with simple porphyrinato ligands (such as TTP). This is probably due to the steric effect of the axial hydrazido(1-) ligands.

Journal Article↗
PubMed2000

Luminescent mononuclear and binuclear cyclometalated palladium(II) complexes of 6-phenyl-2,2 bipyridines: spectroscopic and structural comparisons with platinum(II) analogues.

The mononuclear cyclometalated Pd(II) complexes [Pd(L1)X] (HL1 = 6-phenyl-2,2'-bipyridine; X = Cl, la; Br, 1b; I, 1c), [Pd(L1)PPh3]+ (1d), [Pd(L2-5)Cl] [2a-5a, HL2-5 = 4-(aryl)-6-phenyl-2,2'-bipyridine; aryl = phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5)] and the binuclear derivatives [Pd2(L1-5)2(mu-dppm)]2+ (1e-5e, dppm = bis(diphenylphosphino)methane) and [Pd2(L1)2(mu-dppCs)]2+, (1f, dppC5 = 1,5-bis(diphenylphosphino)pentane) were prepared. The crystal structures of 1d(ClO4), 1e(ClO4)2 x DMF, and 2e(ClO4)2 have been determined by X-ray crystallography. The magnitude of the Pd-Pd distances in le and 2e (3.230(1) and 3.320(2) A, respectively) suggest minimal metal-metal interaction, although pi-stacking of the aromatic ligands (interplanar separations 3.34 and 3.35 A, respectively) is evident. All complexes display low-energy UV absorptions at lambda approximately 390 nm, which are tentatively assigned to 1MLCT transitions; red shifts resulting from Pd-Pd interactions in the binuclear species are not apparent. The complexes in this work are non-emissive at 298 K, but the cationic derivatives exhibit intense luminescence at 77 K. The structured emissions of 1d and 1f in MeOH/EtOH glass (lambdamax 467-586 nm) and all cationic species in the solid state (lambdamax 493-578 nm) are assigned to intraligand excited states. Complexes le-5e display dual emissions in MeOH/EtOH glass at 77 K, and the broad structureless bands at lambdamax 626-658 nm are attributed to pi-pi excimeric IL transitions. A comparison between the photophysical properties of Pd(II) and Pt(II) congeners is presented.

Journal Article↗
PubMed2000

Reactivity of dioxoruthenium(VI) porphyrins toward amines. Synthesis and characterization of bis(arylamine)ruthenium(II), bis(arylamido)- and bis(diphenylamido)ruthenium(IV), and oxo(tert-butylimido)ruthenium(VI) porphyrins.

Reactions of dioxoruthenium(VI) porphyrins, [Ru(VI)O2(Por)], with p-chloroaniline, trimethylamine, tert-butylamine, p-nitroaniline, and diphenylamine afforded bis(amine)ruthenium(II) porphyrins, [Ru(II)(Por)(L)2] (L-p-ClC6H4NH2, Me3N, Por=TTP, 4-Cl-TPP; L=tBuNH2, Por = TPP, 3,4,5-MeO-TPP, TTP, 4-Cl-TPP, 3,5-Cl-TPP) and bis(amido)ruthenium(IV) porphyrins, [Ru(IV)(Por)(X)2] (X=p-NO2C6H4NH, Por=TTP, 4-Cl-TPP; X = Ph2N, Por = 3,4,5-MeO-TPP, 3,5-Cl-TPP), respectively. Oxidative deprotonation of [Ru(II)(Por)(NH2-p-C6H4Cl)2] in chloroform by air generated bis(arylamido)ruthenium(IV) porphyrins, [RuIV(Por)(NH-p-C6H4Cl)2] (Por=TTP. 4-Cl-TPP). Oxidation of [RuII(Por)-(NH2tBu)2] by bromine in dichloromethane in the presence of tert-butylamine and traces of water produced oxo(imido)ruthenium(VI) porphyrins, [RuVI-O(Por)(NtBu)] (Por=TPP, 3,4,5-MeO-TPP, TTP, 4-Cl-TPP, 3,5-Cl-TPP). These new classes of ruthenium complexes were characterized by 1H NMR, IR, and UV/visible spectroscopy, mass spectrometry, and elemental analysis. The structure of [Ru(IV)(TTP)(NH-p-C6H4Cl)2 . CH2Cl2 was determined by X-ray crystallography. The Ru-N bond length and the Ru-N-C angle of the Ru-NHAr moiety are 1.956(7) A and 135.8(6) degrees, respectively.

Amines↗