1,041 research outputs found
Reconciling aging and slip state evolutions from laboratory-derived canons of rate-and-state friction
The aging law and the slip law are two representative evolution laws of the
rate- and state-dependent friction (RSF) law, based on canonical behaviors in
three types of laboratory experiments: slide-hold-slide (SHS), velocity-step
(VS), and steady-state (SS) tests. The aging law explains the SHS canon but
contradicts the VS canon, and vice versa for the slip law. The later proposed
composite law, which switches these two laws according to the slip rate ,
explains both canons but contradicts the SS canon. The present study constructs
evolution laws satisfying all three canons throughout the range of variables
where experiments have confirmed the canons. By recompiling the three canons,
we have derived constraints on the evolution law and find that the evolution
rates in the strengthening phases of the SHS and VS canons are so different
that complete reconciliation throughout the entire range of variables is
mathematically impossible. However, for the limited range of variables probed
by experiments so far, we have found that the SHS and VS canons can be
reconciled without violating the SS canon by switching the evolution function
according to , the ratio of the state to its steady-state
value for the instantaneous slip rate. We could generally
show that, as long as the state evolution rate depends only on
the instantaneous values of and , simultaneous reproduction of the
three canons, throughout the experimentally confirmed range, requires the
aging-law-like evolution for sufficiently below a threshold
and the slip-law-like evolution for sufficiently above . The
validity of the canons in existing experiments suggests .Comment: 90pages, 13 figure
Exploring the Base of the Volcano: A Case Study of an Active Stratovolcano, Mt. Zao, NE Japan
It is very important to explore the base of large volcanoes because older volcanoes with distinct petrological characteristics are sometimes hidden behind them. Such older volcanoes provide keys to investigate the change of magma genesis and tectonic setting during geological time. We newly found an older volcano in southern part of Zao volcano, located in Japan. We have investigated in detail the eruptive products outcropping in its southern part and found that some eruptive rocks with peculiar features form a new stratovolcano which is different from Zao volcano. We call this newly found volcano, the Hiyamizuyama volcano. We have performed K-Ar dating on the representative rocks, obtaining an old age of approximately 1.45 My. The rocks are calcalkaline andesites to dacites, having distinct chemical compositional features with respect to any other calcalkaline rock of the stages 2â6 of Zao volcano. Megacrystals and plutonic intrusions represent a distinct character of the eruptive rocks of the Hiyamizuyama volcano. The finding of this older volcano is also important in order to consider the long-term temporal variation of volcanism and magmatism in the northeastern sector of Japan
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SAM-dependent enzyme-catalysed pericyclic reactions in natural product biosynthesis.
Pericyclic reactions-which proceed in a concerted fashion through a cyclic transition state-are among the most powerful synthetic transformations used to make multiple regioselective and stereoselective carbon-carbon bonds. They have been widely applied to the synthesis of biologically active complex natural products containing contiguous stereogenic carbon centres. Despite the prominence of pericyclic reactions in total synthesis, only three naturally existing enzymatic examples (the intramolecular Diels-Alder reaction, and the Cope and the Claisen rearrangements) have been characterized. Here we report a versatile S-adenosyl-l-methionine (SAM)-dependent enzyme, LepI, that can catalyse stereoselective dehydration followed by three pericyclic transformations: intramolecular Diels-Alder and hetero-Diels-Alder reactions via a single ambimodal transition state, and a retro-Claisen rearrangement. Together, these transformations lead to the formation of the dihydropyran core of the fungal natural product, leporin. Combined in vitro enzymatic characterization and computational studies provide insight into how LepI regulates these bifurcating biosynthetic reaction pathways by using SAM as the cofactor. These pathways converge to the desired biosynthetic end product via the (SAM-dependent) retro-Claisen rearrangement catalysed by LepI. We expect that more pericyclic biosynthetic enzymatic transformations remain to be discovered in naturally occurring enzyme 'toolboxes'. The new role of the versatile cofactor SAM is likely to be found in other examples of enzyme catalysis
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9-Hydroxyellipticine inhibits telomerase activity in human pancreatic cancer cells
AbstractThere is increasing interest in identifying potent inhibitors of telomerase because the enzyme plays a crucial role in the development of cellular immortality and carcinogenesis. We hypothesized that 9-hydroxyellipticine (9-HE), an antitumor alkaloid, would inhibit telomerase activity because the drug has a unique mechanism of inhibiting phosphorylation of mutant p53 protein via inhibition of protein kinases, thereby restoring wild-type p53 function. This study was conducted to examine the effect of 9-HE on telomerase activity in human pancreatic cancer cells with differing p53 gene status. 9-HE treatment at relatively high concentrations resulted in rapid, complete inhibition of telomerase activity, irrespective of the p53 status. We conclude that 9-HE may exert a strong inhibitory effect on telomerase activity possibly through inhibition of protein kinases rather than through restoration of functional wild-type p53
The Ionized Stellar Wind in Vela X-1 During Eclipse
We present a first analysis of a high resolution X-ray spectrum of the
ionized stellar wind of Vela X-1 during eclipse. The data were obtained with
the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray
Observatory. The spectrum is resolved into emission lines with fluxes between
0.02 and 1.04x10^4 ph/cm^2/s. We identify lines from a variety of charge
states, including fluorescence lines from cold material, a warm photoionized
wind. We can exclude signatures from collisionally ionized plasmas. For the
first time we identify fluorescence lines from L-shell ions from lower Z
elements. We also detect radiative recombination continua from a kT = 10 eV
(1.2 x 10^5 K) photoionized optically thin gas. The fluorescence line fluxes
infer the existence of optically thick and clumped matter within or outside the
warm photoionized plasma.Comment: 4 pages, 2 figures, accepted by ApJ letter
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