10,000 publications from this institution
Abstract The progenitors of stripped-envelope supernovae (SNe Ibc) remain to be conclsuively identified, but correlations between SN rates and host-galaxy properties can constrain progenitor models. Here, we present one result from a re-analysis of the rates from the Lick Observatory Supernova Search. Galaxies with stellar masses ≲10 10 M ⊙ are less efficient at producing SNe Ibc than more massive galaxies. Any progenitor scenario must seek to explain this new observation.
The Perspective “Structural nano-composites” (Y. Dzenis, 25 January, p. 419) describes a quest for improved structural materials and indicates that composites with nanoscale reinforcements would have “exceptional mechanical properties.” Is this true? Why would reinforcements that are small
We have previously presented a precursor-product stable isotopic technique for measuring in vivo the fraction of very low-density lipoprotein-fatty acids (VLDL-FA) derived from de novo lipogenesis (fractional DNL). Here, we propose a technique for converting fractional DNL into absolute rates of DNL and describe its explicit underlying assumptions. The technique combines the fractional DNL method with a modification of the method of S. Klein, V. R. Young, G. L. A. Blackburn, B. R. Bistrain, and R. R. Wolfe (J. Clin. Invest. 78: 928-933, 1986), for estimating hepatic reesterification of free fatty acids (FFA). Infusions of [1,2,3,4-13C]palmitate and [1-13C]acetate are performed concurrently with indirect calorimetry in human subjects. Fractional DNL (based on mass isotopomer distribution analysis of VLDL-FA), the rate of appearance of plasma FFA (Ra of FFA), and net fat oxidation in the whole body are measured. Equations from the hepatic reesterification model, modified to include the contribution from DNL, allow calculation of absolute DNL (= fractional DNL x [Ra of FFA - net whole body fat oxidation], when respiratory quotient < 1.0). Sample results from human subjects with different dietary energy intakes are presented, with calculations of absolute DNL, absolute reesterification, and absolute fat oxidation rates. The assumptions of this technique (in particular, that all fat oxidized is derived at steady state from circulating FFA and that DNL and reesterification of FFA both occur exclusively in liver) are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)
Smoking is known to affect appetite and weight in both animals and humans. We examined the relation between smoking deprivation and caloric intake in five women with bulimia nervosa in whom the weight regulation aspects of smoking may be particularly salient
2,3,4,6-Tetra-O-benzyl-α-D-glucopyranosyl bromide (1) and, in one instance, its α-D-galacto isomer (2) were condensed under conditions of the Heiferich modification of the Koenigs–Knorr reaction with the four stereoisomeric methyl 4,6-O-benzylidene-3-deoxy-3-nitrohexopyranosides having the α-D-galacto (3), β-D-galacto (4), α-D-gluco (5), and β-D-gluco (6) configurations. Six new, 1 → 2 linked, fully blocked nitro disaccharides (7–12) were isolated. The configurations of the disaccharidic linkages were established by n.m.r. spectroscopy to be α in four of the products (8, 9, 11, and 12) and β in two (7 and 10). The steric course of the disaccharide bond formation was found to depend on the anomeric as well as the over-all configuration of the alcoholic components 3–6.
A/Asovzc~--I ~ this paper we show that systems consisting of a memory-less nonlinearity sandwiched between two linear time-invariant (LTI) oper-ators are unique module scaling and delays. We mention a few corollaries and applications of general circuit and system theoretic interest. I I.
Complexes [PhBP3]RuH(η(3)-H2SiRR') (RR' = Me,Ph, 1a; RR' = Ph2, 1b; RR' = Et2, 1c) react with XylNC to form carbene complexes [PhBP3]Ru(H)═[C(H)(N(Xyl)(η(2)-H-SiRR'))] (2a-c; previously reported for 2a,b). Reactions of 1a-c with XylNC were further investigated to assess how metal complexes with multiple M-H-Si bonds can mediate transformations of unsaturated substrates. Complex 2a eliminates an N-methylsilacycloindoline product (3a) that results from hydrosilylation, hydrogenation, and benzylic C-H activation of XylNC. Turnover was achieved in a pseudocatalytic manner by careful control of the reaction conditions. Complex 1c mediates a catalytic isocyanide reductive coupling to furnish an alkene product (4) in a transformation that has precedent only in stoichiometric processes. The formations of 3a and 4 were investigated with deuterium labeling experiments, KIE and other kinetic studies, and by examining the reactivity of XylNC with an η(3)-H2SiMeMes complex (1d) to form a C-H activated complex (6). Complex 6 serves as a model for an intermediate in the formation of 3a, and NMR investigations at -30 °C reveal that 6 forms via a carbene complex (1d) that isomerizes to aminomethyl complex 7d. These investigations reveal that the formations of 3a and 4 involve multiple 4-, 5-, and 6-coordinate silicon species with 0, 1, 2, or 3 Ru-H-Si bonds. These mechanisms demonstrate exceptionally intricate roles for silicon in transition-metal-catalyzed reactions with a silane reagent.
A series of monomeric palladium amido complexes of the form trans-(PPh3)2Pd(Ar)(NAr'2) and (DPPF)Pd(Ar)(NAr'2) (DPPF = 1,1'-bis(diphenylphosphino)ferrocene) and dimeric palladium amido complexes of the form {(PPh3)Pd(Ar)(μ-NHR)}2 (R = Ph, t-Bu) have been prepared by the reaction of lithium and potassium amides with palladium aryl halide complexes. An X-ray crystal structure of (DPPF)Pd(p-NMe2C6H4)[N(p-CH3C6H4)2] was obtained. Upon thermolysis in the presence of PPh3, serving as a trapping agent, both the monomeric and dimeric palladium amido complexes underwent C−N-bond-forming reductive elimination to form arylamines in high yields along with a Pd(0) species. Reductive elimination was also observed from azametallacycle (PPh3)Pd(η2-C6H4C6H4NH), to form carbazole and Pd(PPh3)4 at room temperature. Mechanistic studies on the reductive elimination reactions of the monomeric PPh3-ligated amido complexes indicated the presence of two competing pathways for the formation of amine. At low [PPh3], reductive elimination occurs via phosphine dissociation to form a three-coordinate intermediate; however, as [PPh3] is increased, a pathway of reductive elimination from a four-coordinate complex becomes dominant. The DPPF-ligated palladium amido complexes directly eliminated amine from the four-coordinate complex. The mechanism of the reductive elimination from dimeric palladium amido complexes was also studied. These complexes undergo reductive elimination of amine via dimer dissociation to generate a three-coordinate intermediate analogous to those formed by the PPh3-ligated monomeric amido complexes. The C−N-bond forming reductive elimination reactions were accelerated by electron-withdrawing groups on the Pd bound aryl group and by electron-donating groups on the amido ligand, suggesting that the aryl group acts as an electrophile and the amido ligand acts as a nucleophile.
