55,644 research outputs found
Time course of Ca2+ concentration triggering exocytosis in neuroendocrine cells.
No full textWe have used the secretory response of chromaffin cells to estimate the submembrane intracellular Ca2+ concentration ([Ca2+]i) "seen" by secretory granules during short depolarizations. The rate of secretion during a depolarization was assessed by combining the electrochemical method of amperometry and electrical capacitance measurements. The rate was then related to [Ca2+]i based on a previous characterization of how Ca2+ affects the dynamics of vesicle priming and fusion in chromaffin cells [Heinemann, C., Chow, R. H., Neher, E. & Zucker, R. S. (1994) Biophys. J. 67, in press]. Calculated [Ca2+]i rose during the depolarization to a peak of < 10 microM, then decayed over tens of milliseconds. In synapses, vesicles are presumed to be located within nanometers of Ca2+ channels where [Ca2+]i is believed to rise in only microseconds to near steady-state levels of hundreds of micromolar. Channel closure should lead to a decrease in [Ca2+]i also in microseconds. Our findings of the slower time course and the lower peak [Ca2+]i suggest that in chromaffin cells, unlike synapses, Ca2+ channels and vesicles are not strictly colocalized. This idea is consistent with previously published data on dense-core vesicle secretion from diverse cell types
Chow forms of congruences
No full textFor X PN an n-dimensional variety the set of linear spaces of dimension N − n − 1 meeting X defines a hypersurface, H, in the Grassmann variety G(N − n,N + 1).
The homogeneous form in the Pl¨ucker coordinates defining H or H itself is called the Chow form of X. This notion was defined by Cayley [A. Cayley, “On a new analytical representation of curves in space”, Q. J. Pure Appl. Math. 3, 225-236 (1860), and 5, 81-86 (1862); for a modern treatment see M. Green and I. Morrison, Duke Math. J.
53, 733-747 (1986; Zbl 0621.14028)].
In the present paper the authors study Chow forms of integral surfaces in G(2, 4) following the approach of M. Green and I. Morrison. Let V be a fixed 4-dimensional space and F P3 סP3, the flag variety parametrizing all chains V1 V3, where Vi is a subspace of V with dim Vi = i. F parametrizes the lines of G and to each integral surface Y in G there corresponds, in a natural way, an integral hypersurface X in F. The main result in this paper is a characterization of integral hypersurfaces X in F that are Chow forms of integral surfaces in G, in terms of some differential equations.Depto. de Álgebra, Geometría y TopologíaFac. de Ciencias MatemáticasTRUEpu
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Activity of circular muscle of rat uterus at different times in pregnancy
No full text Page C216: B. Bengtsson, E. M. H. Chow, and J. M. Marshall. “Activity of circular muscle of rat uterus at different times in pregnancy.” Page C216: the citation of the journal should read: Am. J. Physiol. 246 (Cell Physiol. 15): C216– C223, 1984. </jats:p
A backward selection procedure for approximating a discrete probability distribution by decomposable models
No full textsummary:Decomposable (probabilistic) models are log-linear models generated by acyclic hypergraphs, and a number of nice properties enjoyed by them are known. In many applications the following selection problem naturally arises: given a probability distribution over a finite set of discrete variables and a positive integer , find a decomposable model with tree-width that best fits . If is the generating hypergraph of a decomposable model and is the estimate of under the model, we can measure the closeness of to by the information divergence , so that the problem above reads: given and , find an acyclic, connected hypergraph of tree-width such that is minimum. It is well-known that this problem is -hard. However, for it was solved by Chow and Liu in a very efficient way; thus, starting from an optimal Chow-Liu solution, a few forward-selection procedures have been proposed with the aim at finding a `good' solution for an arbitrary . We propose a backward-selection procedure which starts from the (trivial) optimal solution for , and we show that, in a study case taken from literature, our procedure succeeds in finding an optimal solution for every
A comparative field study of free-fall cone and sphere penetrometers in soft sediment
No full textThis paper describes the field performance of two instrumented free-fall penetrometers (FFPs), which offer the possibility of low cost rapid strength profiling without requiring the equipment associated with conventional CPTs or drilling and sampling. The devices considered are a free-fall cone penetrometer (FFcP) and an instrumented free-fall sphere (IFFS). The performance of the FFPs in assessing soil undrained shear strength was investigated and compared through field trials in the soft sediments of the Swan River, Perth, Western Australia. The penetrometers achieved impact velocities of 1.4–5.8 m/s from drop heights of 0.7–3.0 m above the riverbed, and embedded to depths of up to 1.6 times the FFP length (1.45 m) and 4.5 times the IFFS diameter (0.89 m). The FFP data provided the shear strength indirectly using the acceleration measured from an onboard inertial measurement unit. The accuracy depends on the assumptions made regarding the magnitude of dynamic bearing and shaft frictional resistance. Uncertainty associated with dynamic bearing resistance causes a ±10% and ±13% variation in the calculated strength for the IFFS and the FFcP, respectively. However, the equivalent uncertainty in dynamic shaft resistance can cause a variation in the calculated strength of up to ±76% for the FFcP. The FFcP has higher embedment potential than the IFFS, but the IFFS has the advantage of a simpler interpretation without needing to account for dynamic shaft resistance
Rearrangement of alkylchlorocarbenes: 1,2-H shift in free carbene, carbene-olefin complex, and excited states of carbene precursors
No full textPhotolysis of alkylchlorodiazirines (1) in the presence of olefins gives a cyclopropane (3) by addition of the generated carbene to the olefin and a vinyl chloride derivative (2) resulting from a 1,2-H shift rearrangement. This rearrangement may occur either in the carbene or in some excited state, precursor of the carbene (RIES mechanism), or in a ''carbene + olefin complex'' on the way to the formation of 3 (COC mechanism). Results obtained by time-resolved photoacoustic calorimetry as well as by thermolysis and photolysis of ClCH2C(N-2)Cl and CH3(CH2)(2)C(N-2)Cl in the presence of tetramethylethylene clearly indicate that both the RIES and COC mechanisms play a role but with efficiencies which greatly depend on the nature of the diazirine. Reexamination of the results previously obtained with benzylchlorodiazirines indicates that, for this class of diazirines, the RIES mechanism is temperature dependent and has a very low efficiency at room temperature and below, whereas the nonlinearity of the plots [3]/[2] vs [olefin] is mainly due to the COC mechanism.PT: J; CR: BIGOT B, 1978, J AM CHEM SOC, V100, P8575 CHANG KT, 1979, J AM CHEM SOC, V101, P5082 FREY HM, 1966, ADV PHOTOCHEM, V4, P225 GANZER GA, 1986, J AM CHEM SOC, V108, P1517 GRAHAM WH, 1965, J AM CHEM SOC, V87, P4396 HEIHOFF K, 1987, BIOCHEMISTRY-US, V22, P1422 HOUK KN, 1984, J AM CHEM SOC, V106, P4291 HOUK KN, 1984, J AM CHEM SOC, V106, P4293 JACKSON JE, 1994, ADV CARBENE CHEM, V1 KANABUSKAMINSKA JM, 1987, J AM CHEM SOC, V109, P5267 LAVILLA JA, 1989, J AM CHEM SOC, V111, P6877 LAVILLA JA, 1989, J AM CHEM SOC, V111, P712 LAVILLA JA, 1990, TETRAHEDRON LETT, V31, P5109 LIU MTH, 1987, J ORG CHEM, V52, P4223 LIU MTH, 1989, J CHEM SOC CHEM COMM, P12 LIU MTH, 1990, J AM CHEM SOC, V112, P3915 MODARELLI DA, 1991, J AM CHEM SOC, V113, P8985 MODARELLI DA, 1992, J AM CHEM SOC, V114, P7034 MOSS RA, 1993, J CHEM SOC CHEM COMM, P1597 MOSS RA, 1994, ADV CARBENE CHEM, V1 MULDER P, 1988, J AM CHEM SOC, V110, P4090 MULLERREMMERS PL, 1985, J AM CHEM SOC, V107, P7275 NI T, 1989, J AM CHEM SOC, V111, P457 NICKON A, 1993, ACCOUNTS CHEM RES, V26, P84 RUDZKI JE, 1985, J AM CHEM SOC, V107, P7849 SKELL PS, 1969, J AM CHEM SOC, V91, P7131 TOMIOKA H, 1984, J AM CHEM SOC, V106, P454 TURRO NJ, 1982, J AM CHEM SOC, V104, P1754 WARNER PM, 1984, TETRAHEDRON LETT, V25, P4211 WESTRICK JA, 1987, BIOCHEMISTRY-US, V26, P8313 WHITE WR, 1992, J ORG CHEM, V57, P2841 WIERLACHER S, 1993, J AM CHEM SOC, V115, P8943 YAMAMOTO N, 1994, J AM CHEM SOC, V116, P2064; NR: 33; TC: 30; J9: J AMER CHEM SOC; PG: 9; GA: UG695Source type: Electronic(1
Catalytic P-H activation by Ti and Zr catalysts
No full textCatalytic dehydrocoupling of phosphines was investigated using the anionic zirconocene trihydride salts [Cp*Zr-2(mu-H)(3)Li](3) (1a) or [Cp*Zr-2(mu-H)(3)K(thf)(4)] (1b), and the metallocycles [CpTi(NPtBu3)(CH2)(4)] (6) and [Cp*M(NPtBu3)(CH2)(4)] (M = Ti 20, Zr 21) as catalyst precursors. Dehydrocoupling of primary phosphines RPH2 (R = Ph, C6H2Me3, Cy, C10H7) gave both dehydrocoupled dimers RP(H)P(H)R or cyclic oligophosphines (RP)(n) (n = 4, 5) while reaction of tBu(3)C(6)H(2)PH(2) gave the phosphaindoline tBu(2)(Me2CCH2)C6H2PH (9). Stoichiometric reactions of these catalyst precursors with primary phosphines afforded [Cp*Zr-2((PR)(2))H][K(thf)(4)] (R = Ph 2, Cy 3, C6H2Me3 4), [Cp*Zr-2((PPh)(3))H] [K(thf)(4)] (5), [CpTi(NPtBu3)(PPh)(3)] (7) and [CpTi(NPtBu3)(mu-PHPh)](2) (8), while reaction of 6 with (C(6)H(2)tBu3)PH2 in the presence of PMe3 afforded [CpTi(NPtBu3)(PMe3)(p(C(6)H(2)tBu(3))] (10). The secondary phosphines Ph2PH and (PhHPCH2)(2)CH2 also undergo dehydrocoupling affording (Ph2P)(2) and (PhPCH2)(2)CH2. The bisphosphines (CH2PH2)(2) and C6H4(PH2)(2) are dehydrocoupled to give (PCH2CH2PH)(2) (12) and (C6H4P(PH))(2) (13) while prolonged reaction of 13 gave (C6H4P2)(8) (14). The analogous bisphosphine Me2C6H4(PH)(2) (17) was prepared and dehydrocoupling catalysis afforded (Me2C6H2P(PH))(2) (18) and subsequently [(Me2C6H2P2)(2)(mu-Me2C6H2P2)](2) (19). Stoichiometric reactions with these bisphosphines gave [Cp*Zr-2(H)(PH)(2)C6H4] [Li(thf)(4)] (22), [Cp*Ti(NPtBu3)(PH)(2)C6H4](2) (23) and [Cp*Ti(NPtBu3)(PH)(2)C6H4] (24). Mechanistic implications are discussed.PT: J; CR: ALBRAND JP, 1976, J CHEM SOC CHEM COMM, P876 ANSELME JP, 1969, TETRAHEDRON, V25, P855 BASULI F, 2003, J AM CHEM SOC, V125, P10170 BAUDLER M, 1976, Z NATURFORSCH B, V31, P558 BAUDLER M, 1978, CHEM BER, V111, P1210 BAUDLER M, 1978, CHEM BER, V111, P1217 BAUDLER M, 1983, CHEM BER, V116, P2711 BAUDLER M, 1984, Z NATURFORSCH B, V39, P438 BAZAN GC, 1991, J AM CHEM SOC, V113, P6899 BOHM VPW, 2001, ANGEW CHEM, V113, P4832 CHAUVIN Y, 1971, MAKROMOL CHEM, V141, P161 COREY JY, 2004, ADV ORGANOMET CHEM, V51, P1 COURET C, 1986, ORGANOMETALLICS, V5, P113 COWLEY AH, 1984, TETRAHEDRON LETT, V25, P2125 COWLEY AH, 1990, INORG SYNTH, V27, P235 CROMER DT, 1974, INT TABLES CRYSTALLO, V4, P71 ETKIN N, 1997, J AM CHEM SOC, V119, P11420 ETKIN N, 1997, J AM CHEM SOC, V119, P2954 ETKIN N, 1997, ORGANOMETALLICS, V16, P3504 FEHLNER TP, 1992, INORGANOMETALLLICS FERMIN MC, 1995, J AM CHEM SOC, V117, P12645 FERMIN MC, 1995, ORGANOMETALLICS, V14, P4247 FU GC, 1993, J AM CHEM SOC, V115, P9856 GAUVIN F, 1998, ADV ORGANOMET CHEM, V42, P363 GRAHAM TW, 2004, ORGANOMETALLICS, V23, P3309 GRUBBS RH, 1972, J AM CHEM SOC, V94, P2538 GRUBBS RH, 2003, HDB METATHESIS HEY E, 1988, CHEM BER, V121, P561 HEY E, 1989, J ORGANOMET CHEM, V378, P375 HO JW, 1991, ORGANOMETALLICS, V10, P3001 HO JW, 1994, INORG CHEM, V33, P865 HOFFMAN PR, 1975, INORG CHEM, V14, P1997 HOSKIN AJ, 2001, ANGEW CHEM, V113, P1917 HOU ZM, 1993, ORGANOMETALLICS, V12, P3158 INAGAKI Y, 1980, B CHEM SOC JPN, V53, P205 ISSLEIB K, 1972, ANGEW CHEM, V84, P582 ISSLEIB K, 1987, J ORGANOMET CHEM, V330, P17 JACOBSEN EN, 1988, J AM CHEM SOC, V110, P1968 KATSUKI T, 1980, J AM CHEM SOC, V102, P5974 KAUFFMANN T, 1984, TETRAHEDRON LETT, V25, P1963 KAUFFMANN T, 1985, CHEM BER, V118, P1022 KITAMURA M, 1988, J AM CHEM SOC, V110, P629 KNOWLES WS, 1983, ACCOUNTS CHEM RES, V16, P106 KOEPF H, 1981, CHEM BER, V114, P2731 KOHLER EP, 1935, J AM CHEM SOC, V57, P367 KYBA EP, 1983, ORGANOMETALLICS, V2, P1877 MILLER AR, 1976, J AM CHEM SOC, V98, P1860 MILLER SJ, 1996, J AM CHEM SOC, V118, P9606 MIYASHITA A, 1980, J AM CHEM SOC, V102, P7932 MURDZEK JS, 1987, ORGANOMETALLICS, V6, P1373 NGUYEN ST, 1992, J AM CHEM SOC, V114, P3974 NGUYEN ST, 1993, J AM CHEM SOC, V115, P9858 NOVAK BM, 1988, J AM CHEM SOC, V110, P960 OHKUMA T, 1995, J AM CHEM SOC, V117, P2675 OHTA T, 1988, INORG CHEM, V27, P566 OSHIKAWA T, 1985, CHEM IND-LONDON, P126 ROCKLAGE SM, 1981, J AM CHEM SOC, V103, P1440 SCHOLL M, 1999, TETRAHEDRON LETT, V40, P2247 SCHROCK RR, 1974, J AM CHEM SOC, V96, P6796 SCHROCK RR, 1980, J MOL CATAL, V8, P73 SCHROCK RR, 1988, J MOL CATAL, V46, P243 SCHROCK RR, 1990, J AM CHEM SOC, V112, P3875 SCHWAB P, 1995, ANGEW CHEM INT EDIT, V34, P2039 SCHWAB P, 1995, ANGEW CHEM, V107, P2179 SCHWAB P, 1996, J AM CHEM SOC, V118, P100 SENDERIKHIN AI, 1988, ZH OBSHCH KHIM+, V58, P1662 SENDERIKHIN AI, 1989, ZH OBSHCH KHIM+, V59, P2141 SEYFERTH D, 1969, J ORG CHEM, V34, P1483 SHELDRICK GM, 2000, SHELXTL SHU RH, 1998, J AM CHEM SOC, V120, P12988 SMIT CN, 1983, TETRAHEDRON LETT, V24, P2031 SOUFFLET JP, 1973, CR ACAD SCI C CHIM, V276, P169 STEPHAN DW, 2000, ANGEW CHEM, V112, P322 STEPHAN DW, 2005, ORGANOMETALLICS, V24, P2548 STRADIOTTO M, 2001, HELV CHIM ACTA, V84, P2958 TILLEY TD, 1990, COMMENTS INORG CHEM, V10, P37 TILLEY TD, 1993, ACCOUNTS CHEM RES, V26, P22 TVERDOMED SN, 2003, RUSS J GEN CHEM+, V73, P319 VANDENWINKEL Y, 1991, J ORGANOMET CHEM, V405, P183 WATERMAN R, 2006, ANGEW CHEM INT EDIT, V45, P2926 WATERMAN R, 2006, ANGEW CHEM, V118, P2992 WEAST RC, 1974, HDB CHEM PHYS, P2436 WOOD CD, 1979, J AM CHEM SOC, V101, P3210 WU Z, 1995, J AM CHEM SOC, V117, P5503 XIN SX, 1997, J AM CHEM SOC, V119, P5307; NR: 85; TC: 0; J9: CHEM-EUR J; PG: 12; GA: 113PJSource type: Electronic(1
Evidence for the decay B0→J/ψω and measurement of the relative branching fractions of meson decays to J/ψη and J/ψη′
No full textFirst evidence of the B 0 → J / ψ ω decay is found and the B s 0 → J / ψ η and B s 0 → J / ψ η ′ decays are studied using a dataset corresponding to an integrated luminosity of 1.0 fb -1 collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV. The branching fractions of these decays are measured relative to that of the B 0 → J / ψ ρ 0 decay:frac(B (B 0 → J / ψ ω), B (B 0 → J / ψ ρ 0)) = 0.89 ± 0.19 (stat) - 0.13 + 0.07 (syst),frac(B (B s 0 → J / ψ η), B (B 0 → J / ψ ρ 0)) = 14.0 ± 1.2 (stat) - 1.5 + 1.1 (syst) - 1.0 + 1.1 (frac(f d, f s)),frac(B (B s 0 → J / ψ η ′), B (B 0 → J / ψ ρ 0)) = 12.7 ± 1.1 (stat) - 1.3 + 0.5 (syst) - 0.9 + 1.0 (frac(f d, f s)), where the last uncertainty is due to the knowledge of f d / f s, the ratio of b-quark hadronization factors that accounts for the different production rate of B 0 and B s 0 mesons. The ratio of the branching fractions of B s 0 → J / ψ η ′ and B s 0 → J / ψ η decays is measured to befrac(B (B s 0 → J / ψ η ′), B (B s 0 → J / ψ η)) = 0.90 ± 0.09 (stat) - 0.02 + 0.06 (syst)
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