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71.
Summary The Vendian-Cambrian interval on the Siberian Platform contains thick carbonate and evaporite sequences formed in extensive
shallow-water basins. The carbonate sequences are characterized by a cyclic composition. Finegrained dolomites, undulated
algal dolomites, flat pebble conglomerates, storm breccias and solution-collapse breccias form the base of each cycle. These
rocks are characterized by an increased clay content which can be high enough to form argillites. Short sedimentation breaks
reflected by mudcracks or silicification horizons are present as well as small cross-bedded tidal channels. Peloidal grainstones
with algal overgrowth dominate in the central parts of the cycles. These members are often recrystallized and dolomitized.
Micritic dolomites, undulous laminated dolomites, storm breccias and columnar stromatolites with abundant mud cracks form
the upper members of the cycles. These sequences are free of clay but contain abundant anhydrite crystals and nodules. In
the uppermost parts of some cycles massive layered anhydrite beds are present. The cycles vary in thickness, but usually they
are between 15 and 20 m thick.
The lower cycle member documents extreme shallow-water deposits. They formed in tidal and partially also supratidal zones
not far from the mainland, from where fine clayey material was washed in. These parts of the sequence reflect a slow transgression.
The central member of a cycle was deposited during the maximal transgression in a shallow basin with normal salinity and rather
active hydrodynamics.
Sedimentation of the upper part of the cycles reflects a regression stage (tidal and especially sabkha environments). The
final layered anhydrite beds formed most probably in relict lakes on the sabkha plain. During sea-level falls some sequences
of the central parts of the cycles were subaerially exposed and underwent partial dolomitization.
The Vendian-Cambrian sabkhas are partly comparable with their recent counterparts. 相似文献
72.
Quantitative understanding of the kinetics of lymphocyte proliferation and death upon activation with an antigen is crucial for elucidating factors determining the magnitude, duration and efficiency of the immune response. Recent advances in quantitative experimental techniques, in particular intracellular labeling and multi-channel flow cytometry, allow one to measure the population structure of proliferating and dying lymphocytes for several generations with high precision. These new experimental techniques require novel quantitative methods of analysis. We review several recent mathematical approaches used to describe and analyze cell proliferation data. Using a rigorous mathematical framework, we show that two commonly used models that are based on the theories of age-structured cell populations and of branching processes, are mathematically identical. We provide several simple analytical solutions for a model in which the distribution of inter-division times follows a gamma distribution and show that this model can fit both simulated and experimental data. We also show that the estimates of some critical kinetic parameters, such as the average inter-division time, obtained by fitting models to data may depend on the assumed distribution of inter-division times, highlighting the challenges in quantitative understanding of cell kinetics. 相似文献
73.
Separation of optical isomers obtainable from trans‐norborn‐5‐ene‐2,3‐dicarboxylic acid methyl and tert‐butyl monoesters was performed by crystallization of the respective salts prepared with (R)‐ and (S)‐1‐phenylethylamine. Starting from racemic endo‐monomethyl ester of trans‐norborn‐5‐ene‐2,3‐dicarboxylic acid, prepared by partial hydrolysis of the cyclopentadiene‐dimethyl fumarate adduct, the corresponding (2R,3R)‐endo‐monoester was isolated in 97% enantiomeric excess (ee) yield after seven repeated crystallizations from tetrachloromethane. Starting from exo‐mono‐tert‐butyl ester of the same acid, prepared by alcoholysis of the cyclopentadiene‐maleic anhydride adduct followed by isomerization, (2R,3R)‐exo‐monoester was isolated in >98% ee yield after four repeated crystallizations from ethanol. Crystallization of the acids from the mother liquor containing (S)‐1‐phenylethylamine yielded products with inverse stereochemical configuration. Chirality 27:151–155, 2015. © 2014 Wiley Periodicals, Inc. 相似文献
74.
Nikolay I. Zhurilo Mikhail V. Chudinov Andrey V. Matveev Olga S. Smirnova Irina D. Konstantinova Anatoly I. Miroshnikov Alexander N. Prutkov Lyubov E. Grebenkina Natalya V. Pulkova Vitaly I. Shvets 《Bioorganic & medicinal chemistry letters》2018,28(1):11-14
The novel isosteric ribavirin analogues were synthesized by two different ways. Some of them showed significant antiviral action against hepatitis C virus (HCV), herpes simplex (HCV-1) and influenza A virus comparable to that of ribavirin itself. The data obtained confirm the proposed theory of the ribavirin possible antiviral activity mechanism related with bioisosterism. 相似文献
75.
Vitaly N. Nikandrov Oleg N. Murashko Galina V. Vorobyova Nelly S. Pyzhova Natalie V. Kvyatkovskaya Oksana A. Bartalevich 《International journal of peptide research and therapeutics》1997,4(4-6):497-502
Summary The formation of stable equimolar complexes of streptokinase or plasminogen with muscle lactate dehydrogenase or pyruvate
kinase, heart mitochondrial malate dehydrogenase and hepatic catalase at pH 7.4, 3.0 and 10.0 was first detected by differential
spectroscopy methods. All complexes, except those of plasminogen with dehydrogenases, were resistant to 6 M urea. Judging
from circular dichroism spectra, tertiary and secondary structures were considerably changed in the complexes. These changes
were significantly dependent upon the nature of interacting proteins; in some cases their structures were more ordered. NAD
(but not NADH) hampered the formation of streptokinase complexes with dehydrogenases. The plasminogen-activating function
of streptokinase and the ability of plasminogen to be activated by streptokinase in the complexes with oxidoreductases were
essentially unchanged. Pyruvate kinase induced a moderate (by 35%) increase in the streptokinase activating function. It is
assumed that the formation of complexes of streptokinase or plasminogen with enzymes may serve as a link in metabolic regulation
and/or intercellular interactions. 相似文献
76.
Raf kinases: function, regulation and role in human cancer 总被引:3,自引:0,他引:3
Leicht DT Balan V Kaplun A Singh-Gupta V Kaplun L Dobson M Tzivion G 《Biochimica et biophysica acta》2007,1773(8):1196-1212
The Ras-Raf-MAPK pathway regulates diverse physiological processes by transmitting signals from membrane based receptors to various nuclear, cytoplasmic and membrane-bound targets, coordinating a large variety of cellular responses. Function of Raf family kinases has been shown to play a role during organism development, cell cycle regulation, cell proliferation and differentiation, cell survival and apoptosis and many other cellular and physiological processes. Aberrations along the Ras-Raf-MAPK pathway play an integral role in various biological processes concerning human health and disease. Overexpression or activation of the pathway components is a common indicator in proliferative diseases such as cancer and contributes to tumor initiation, progression and metastasis. In this review, we focus on the physiological roles of Raf kinases in normal and disease conditions, specifically cancer, and the current thoughts on Raf regulation. 相似文献
77.
78.
Efficient design of ionic compounds requires a systematic understanding of cation–anion interactions. Weakening of electrostatic attraction is essential to increase the liquid range of the ionic compound and decrease its melting point. Here, we report simulations of the closest-approach cation–anion distances in a variety of ion pairs containing the tetrakis(pentafluorophenyl)borate (TFPB–) anion. Small alkali cations (Li+, Na+) penetrate the TFPB– core, whereas K+ and larger organic cations do not. In the latter case, the shortest possible distance from the cations to the boron atom of TFPB– ranges from 0.50 nm to 0.63 nm. TFPB– was shown to be substantially rigid, providing a steric hindrance to thermodynamically efficient cation–anion coordination. Our results prove that TFPB– is more efficient for electrostatic charge confinement than the tetraoctylammonium cation, whereas the perfluorophenyl group is more efficient than linear alkyl chains. These simulations will motivate development of TFPB–-based ionic liquids with low phase transition points. 相似文献
79.
Erling S. Nordøy Lars P. Folkow Vladimir Potelov Vitaly Prischemikhin Arnoldus Schytte Blix 《Polar Biology》2008,31(9):1119-1135
Eight adult female harp seals (Pagophilus
groenlandicus) of the White Sea–Barents Sea stock were tagged with satellite-linked dive recorders during the nursing period and followed
from breeding in late February 1995 until moulting in late April 1995. Another ten adult harp seals of both sexes were tagged
and followed from moult in early May 1996 until breeding in late February the following year. Between breeding and moult the
seals were distributed along the coasts of Kola of Russia and eastern Finnmark of Norway, coinciding in time and space with
the spawning capelin (Mallotus villosus). Between moulting and breeding they encircled the entire Barents Sea, mostly in open water, using the water column from
20 to 300 m, and in so doing by and large reflecting the annual migrations of the capelin. Capelin is therefore assumed to
be the main source of prey for the White Sea–Barents Sea stock of harp seals, to be substituted, in part, by amphipods (e.g.
Themisto libellula) in mid-summer and polar cod (Boreogadus saida) and herring (Clupea pallasii) in late autumn and winter. These data provide a baseline for the evaluation of the effects of future climatic change in
the rich Barents Sea ecosystem. 相似文献
80.
Manu Svetlana Surkova Alexander V. Spirov Vitaly V. Gursky Hilde Janssens Ah-Ram Kim Ovidiu Radulescu Carlos E. Vanario-Alonso David H. Sharp Maria Samsonova John Reinitz 《PLoS computational biology》2009,5(3)
The variation in the expression patterns of the gap genes in the blastoderm of the fruit fly Drosophila melanogaster reduces over time as a result of cross regulation between these genes, a fact that we have demonstrated in an accompanying article in PLoS Biology (see Manu et al., doi:10.1371/journal.pbio.1000049). This biologically essential process is an example of the phenomenon known as canalization. It has been suggested that the developmental trajectory of a wild-type organism is inherently stable, and that canalization is a manifestation of this property. Although the role of gap genes in the canalization process was established by correctly predicting the response of the system to particular perturbations, the stability of the developmental trajectory remains to be investigated. For many years, it has been speculated that stability against perturbations during development can be described by dynamical systems having attracting sets that drive reductions of volume in phase space. In this paper, we show that both the reduction in variability of gap gene expression as well as shifts in the position of posterior gap gene domains are the result of the actions of attractors in the gap gene dynamical system. Two biologically distinct dynamical regions exist in the early embryo, separated by a bifurcation at 53% egg length. In the anterior region, reduction in variation occurs because of stability induced by point attractors, while in the posterior, the stability of the developmental trajectory arises from a one-dimensional attracting manifold. This manifold also controls a previously characterized anterior shift of posterior region gap domains. Our analysis shows that the complex phenomena of canalization and pattern formation in the Drosophila blastoderm can be understood in terms of the qualitative features of the dynamical system. The result confirms the idea that attractors are important for developmental stability and shows a richer variety of dynamical attractors in developmental systems than has been previously recognized. 相似文献