C2 Fraction Research Articles

Scanning tunneling microscopy studies on the reversible phase transition between metastable structures of Si(111)-c2×8 and “1×1”

The reversible structural phase transition at 230°C ( T c) between the metastable structures of Si(111)-c2×8 and “1×1” studied by high-temperature scanning tunneling microscopy (STM), reported briefly in a previous letter [Surf. Sci. 411(1998) L822], is given in detail. An ordered fraction of c2×8 regions was measured as a function of temperature, ordering and disordering dynamics (relaxation time measurements after an abrupt temperature change near T c) and short-range ordering characteristics (Fourier transform of STM images) were studied. There are similarities between the present phase transition and that on Ge(111), where the transition is between the thermally stable c2×8 and 1×1 structures.

Mechanical restitution and recirculation fraction in cardiac myocytes and left ventricular muscle of adult rats.

Unloaded cell shortening was measured in electrically stimulated myocytes from adult rat hearts to compare the contractile response to stimulation with that in isometrically contracting left ventricular papillary muscles under similar experimental conditions, but preloaded to produce maximum twitch tension. Mechanical restitution in cells followed a biexponential function with time constants of 0.19 +/- 0.03 s and 36.4 +/- 10.2 s (7 cells from 5 hearts, n = 7/5). The time constants for papillary muscles were 0.58 +/- 0.05 s and 14.6 +/- 1.0 s (n = 6/6). In myocytes, maximum post-rest potentiation occurred after 30 to 60 s of rest. The potentiation after 60 s of rest was 2.48 +/- 0.31 times the steadystate in cells and 2.63 +/- 0.16 in papillary muscles. Recirculation fraction of C2+ as calculated from the decay of post-rest potentiation was 0.84 +/- 0.04 in single cells and 0.59 +/- 0.02 in papillary muscles (p < 0.005). Caffeine (3mM) abolished post-rest potentiation in both types of preparations. The numerical values for the time constants of mechanical restitution, potentiation factor and recirculation fraction in papillary muscles did not depend on preload. It is concluded that interval-dependent changes of contractility are preserved in single cardiac cells but the kinetics of decay of potentiation appear to have changed quantitatively.

Nitroethane as a probe molecule for CO hydrogenation over Ru/SiO 2

Studies of C{sub 2}H{sub 5}NO{sub 2} hydrogenation and addition to CO hydrogenation over a Ru/SiO{sub 2} catalyst have been carried out as part of a current effort by this group to investigate the usefulness of nitroparaffins as probe molecules capable of generating C{sub y}H{sub x} groups and to better understand the role of the latter in hydrocarbon formation during CO hydrogenation. Among other observations, it was noted that during CH{sub 3}NO{sub 2} addition the weight percent of the C2 fraction remained almost constant while there were major changes in the weight percent of the C1 and C3 through C6 hydrocarbon fractions. The results of this study were viewed as an indication that C{sub 2} surface species may play a special role in chain growth initiation during hydrocarbon formation on Ru catalysts. In an attempt to probe the possible role of C{sub 2} groups in chain growth initiation 1.65 vol% C{sub 2}H{sub 5}NO{sub 2} was added to He:CO:H{sub 2} mixtures at two molar ratios (Conditions A, 1:2:2; B, 1:1:3) flowing at 3 liters/h over 0.25 g of Ru/SiO{sub 2} at 533 K and 1 atm. C{sub f2}H{sub 5}NO{sub 2} (1.7 vol%) hydrogenation in He:H{sub 2} (1:4 molar ratio) was also studiedmore » under the same conditions. The rates of product formation and product distributions for CO hydrogenation in the absence and presence of C{sub 2}H{sub 5}NO{sub 2} after 5 min reaction are given. The values of the chain growth probabilities, {alpha}, and the olefin-to-paraffin molar ratios for the C2 and C3 fractions in the products are also listed. A table also gives the rates of formation of CO{sub 2} and of CH{sub 3}CN, the only nitrogen-containing hydrocarbon product detected.« less

Serial permeability barriers to water transport in human placental vesicles

Microvillous vesicles were prepared from term human placenta by shearing, differential centrifugation and Mg2+ precipitation. Vesicles were purified further on a sucrose density gradient producing two bands with densities of 1.16 to 1.18 g/ml (C1) and 1.13 to 1.15 g/ml (C2). The C2 fraction, which had a 24-fold enrichment of alkaline phosphatase and a three-fold reduction in Na+, K+-ATPase activity compared to homogenates, was used to measure osmotic water (Pf) permeability. Pf was measured from the time course of scattered light intensity following exposure of vesicles to specified gradients of impermeant solutes. Pf decreased from 3.0 X 10(-3) to 0.6 X 10(-3) cm/sec with increasing gradient size (65 to 730 mM; 23 degrees C). Four possible causes of this behavior were examined theoretically and experimentally: an unstirred layer, saturation of water transport, large changes in the vesicle surface area with changes in volume and a structural restriction to vesicle volume change. The measured dependence of Pf on gradient size and the effect of the channel-forming ionophore gramicidin on Pf fit best to the theoretical dependences predicted by a structural restriction mechanism. This finding was supported by experiments involving the effects on Pf of increased solution viscosity, initial vesicle volume, the magnitude of transmembrane volume flow, and the effects of gradient size on activation energy (Ea) for Pf. The decreased Pf resulting from a structural restriction limiting vesicle volume change was modeled mathematically as a second barrier in series with the vesicle membrane. Ea measured using a 250-mM inwardly directed sucrose gradient was 5.4 +/- 0.6 kcal/mol (T greater than 27 degrees C) and 10.0 +/- 0.6 kcal/mol (T less than 27 degrees C). Ea above 27 degrees C is in the range normally associated with transmembrane passage of water via aqueous channels. Water transport was not inhibited by p-chloromercuribenzenesulfonate.

Hydrolysis of manganese carbides Mn5C2 and Mn23C6

Diffractographically pure manganese carbides Mn5C2 and Mn23C6 were hydrolyzed with water at 25 and 60 °C. Based on the model of contracting spherical particles in the carbide powder particulate system, the corrosion rates were calculated from the course of gas evolution during the hydrolysis; the values obtained were as follows in μm/h: Mn5C2: 1.2 ± 0.4 at 25 °C and 10 ± 3 at 60 °C, and Mn23C6: 0.15 ± 0.01 at 25 °C and 2.2 ± 0.2 at 60 °C. The hydrolysis gives rise to hydrogen and a concentration-decreasing sequence of predominantly saturated hydrocarbons, the fraction of C2 and higher hydrocarbons being about 6 times lower for Mn23C6 than for Mn5C2. The hydrogen and hydrocarbon contents of the mixture after hydrolysis are consistent with the assumed reaction course, where the adsorbed active hydrogen, formed during the hydrolysis reaction due to the metallic nature of the carbide, partly reacts with the methane arising from the hydrolysis of the C1 groups in the carbide and partly combines to molecular hydrogen.

The amino acid sequence of soybean leghaemoglobin c2

1. Introduction The leghaemoglobin found in the root nodules of the soybean plant (G&&e max cultivar Lincoln) after rhizobial infection was originally fractionated into two major and two minor components [ 1,2]. Recently, the second of the major components, leghaemoglobin c, has been found to consist of two distinct molecular species, leghaemoglobin cl and c2, separable by elution from a DEAE cellulose column using a continuous acetate gradient at pH 5.2 [3]. Since the amino acid sequence of the first major component, leghaemoglobin a, is known [4] and it has been well characterized both structurally, and functionally [S-8] we felt it would be of value to elucidate the primary structure of a second major component, leghaemoglobin c2, as a step towards establishing a function (if any) for the multiplicity of leghaemoglobin species found in the root nodules of most legumes. 2. Materials and methods Soybean plants were grown as previously described [9] and isolated by gradient elution (10 to 50 mM acetate) at pH 5.2 from a DEAE cellulose column [3]. The leghaemoglobin c2 fraction was pooled and rechro- matographed on a shallow gradient (25-35 mM acetate) to ensure removal of the cl component. The protein (65 mg) was dehaemed by the HCl-acetone method [lo] and succinylated in an auto-pH titrator at pH 9.2 by the addition of eight lots of