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61.
Compounds 4a-4d, 4f and 10 were prepared and their ring-opening reactions with N-bromosuccinimide (NBS) investigated. Compounds 4a and 4b gave a mixture of products which did not contain any significant quantity of the corresponding aldehydes 5a and 5b whereas compounds 4c, 4d and 4f gave exclusively the aldehydes 5c, 5d and 5f respectively. Compound 10 similarly gave the aldehyde 11 when treated with NBS. 相似文献
62.
Calcium ions have been proposed to serve as important regulatory elements in stimulus-response coupling for phytochrome responses. An important test of this hypothesis will be to identify specific targets of calcium action that are required for some growth or development process induced by the photoactivated form of phytochrome (Pfr). Initial studies have revealed that there are at least two enzymes in pea nuclei that are stimulated by Pfr in a Ca(2+)-dependent fashion, a calmodulin-regulated nucleoside triphosphatase and a calmodulin-independent but Ca(2+)-dependent protein kinase. The nucleoside triphosphatase appears to be associated with the nuclear envelope, while the protein kinase co-purifies with a nuclear fraction highly enriched for chromatin. This short review summarizes the latest findings on these enzymes and relates them to what is known about Pfr-regulated nuclear metabolism. 相似文献
63.
Stanley R. Hobart Charles H. Mack Darrell J. Donaldson Julius P. Neumeyer 《Thermochimica Acta》1976,16(2):149-171
Differential thermal analyses (DTA) were made on a series of polyester/cotton blend fabrics before and after treatment with Thpc—urea—poly(vinyl bromide). This flame retardant did not affect the polyester melting endotherm, which was proportional to the polyester content and appeared at approximately 250°C. In nitrogen atmosphere, DTA of the treated blends showed exothermic peaks at 285°C for the cotton decomposition. and at 415°C for the polyester decomposition. In air, DTA of the treated blends showed exothermic peaks at 333°C for cellulose decomposition, at 431°C for polyester decomposition and at 490°C for char decomposition. The Thpc-urea component of the flame retardant is effective on the cotton cellulose portion of the blend; the poly(vinyl bromide) appears to decompose and act in the vapor state on the polyester. 相似文献
64.
Wang YN Topol IA Collins JR Burt SK 《Journal of the American Chemical Society》2003,125(43):13265-13273
Phosphate hydrolysis by GTPases plays an important role as a molecular switch in signal transduction and as an initiator of many other biological processes. Despite the centrality of this ubiquitous reaction, the mechanism is still poorly understood. As a first step to understand the mechanisms of this process, the nonenzymatic hydrolysis of mono-phosphate and tri-phosphate esters were systematically studied in gas phase and aqueous solution using hybrid density functional methods. The dielectric effect of the environment on the energetics of these processes was also explored. Theoretical results show that for mono-phosphate ester, the dissociative pathway is much more favorable than the associative pathway. However, the reaction barriers for the dissociative and associative pathways of tri-phosphate hydrolysis are very close in aqueous solution, though the dissociative pathway is more favorable in the gas phase. High dielectric solvents, such as water, significantly lower the activation barrier of the associative pathway due to the greater solvation energy of the associative transition states than that of the reactant complex. By contrast, the barrier of the dissociative pathway, with respect to the gas phase, is less sensitive to the surrounding dielectric. In the associative hydrolysis pathway of the tri-phosphate ester, negative charge is transferred from the gamma-phosphate to beta-phosphate through the bridging ester oxygen and results in Pgamma-O bond dissociation. No analogous charge transfer was observed in the dissociative pathway, where Pgamma-O bond dissociation resulted from proton transfer from the gamma-phosphate to the bridge oxygen. Finally, the active participation of local water molecules can significantly lower the activation energy of the dissociative pathway for both mono-phosphate and tri-phosphate. 相似文献
65.
Stanley HE Buldyrev SV Goldberger AL Hausdorff JM Havlin S Mietus J Peng C-K Sciortino F Simons M 《Physica A》1992,191(1-4):1-12
Here we discuss recent advances in applying ideas of fractals and disordered systems to two topics of biological interest, both topics having common the appearance of scale-free phenomena, i.e., correlations that have no characteristic length scale, typically exhibited by physical systems near a critical point and dynamical systems far from equilibrium. (i) DNA nucleotide sequences have traditionally been analyzed using models which incorporate the possibility of short-range nucleotide correlations. We found, instead, a remarkably long-range power law correlation. We found such long-range correlations in intron-containing genes and in non-transcribed regulatory DNA sequences as well as intragenomic DNA, but not in cDNA sequences or intron-less genes. We also found that the myosin heavy chain family gene evolution increases the fractal complexity of the DNA landscapes, consistent with the intron-late hypothesis of gene evolution. (ii) The healthy heartbeat is traditionally thought to be regulated according to the classical principle of homeostasis, whereby physiologic systems operate to reduce variability and achieve an equilibrium-like state. We found, however, that under normal conditions, beat-to-beat fluctuations in heart rate display long-range power law correlations. 相似文献
66.
What is the most number of vectors inR d such that anyk+1 contain an orthogonal pair? The 24 positive roots of the root systemF 4 inR 4 show that this number could exceeddk. 相似文献
67.
68.
The dipeptide sweetener, aspartame, has been prepared in high yield via the coupling of L-phenylalanine methyl ester and L-aspartic acid N-thiocarboxyanhydride. 相似文献
69.
Wing W. Lam Gary E. Martin Vincent M. Lynch Stanley H. Simonsen Charles M. Lindsay Keith Smith 《Journal of heterocyclic chemistry》1986,23(3):785-791
The synthesis of 2-azathianthrene ([1,4]benzodithiino[2,3-c]pyridine), the only remaining monoazathianthrene yet to be reported, is described. Attempts at the direct condensation of disubstituted pyridines with the dianion of 1,2-dimercaptobenzene were generally unsuccessful requiring that the alternative condensation of the dianion with disubstituted pyridine 1-oxides be employed. The title compound was characterized by physical means including 13C-nmr spectroscopy. One analog, 4-nitro-2-azathianthrene was also studied by X-ray crystallographic means; the molecule crystallized with two molecules in the asymmetric unit P21/n, a = 20.712(3), b = 7.8109(13), c = 13.720(2)Å, β = 107.880(11)°, Z = 8, the data refined to a final R = 0.051 for 3061 reflections. Dihedral angles between the planes of the phenyl rings were 135.00(13) and 132.52(13)° for the two independent molecules contained in the crystal. Close non? bonded S ?O intramolecular contacts were observed in both molecules between the sulfur and nitro-group oxygens. Both nitro groups are twisted out of the plane of the pyridine ring and are oriented at angles of 28.75 and 38.82° respectively. 相似文献
70.
When bromoform (CHBr3) is photolyzed at 266 or 303 nm in the presence of O2 and NO, the formation of secondary Br atoms is observed. By following the rate of growth of this secondary Br atom signal as a function of conditions, rate constants have been determined for the reactions CHBr2 + O2, CHBr2 + NO (both pressure-dependent), and CHBr2O2 + NO (k(2a) = (1.74 +/- 0.16) x 10(-11) cm3 molecule(-1) s(-1) at 23 degrees C). By measuring the amplitude of the secondary Br signal compared to the primary Br formed in the initial photolysis, it is established that the CHBr2O radical spontaneously decomposes to form CHBrO + Br at least 90%, and probably 100%, of the time, in agreement with previous work and with recent ab initio calculations. A survey of four other polybrominated methanes, CH2Br2, CHClBr2, CF2Br2, and CBr4, shows that they all generate secondary Br atoms when photolyzed at 266 nm in the presence of O2 and NO, suggesting that their reaction sequences are similar to that of bromoform. 相似文献