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41.
42.
Denisov AY Hannoush RN Gehring K Damha MJ 《Journal of the American Chemical Society》2003,125(38):11525-11531
We have recently shown that hairpins containing 2',5'-linked RNA loops exhibit superior thermodynamic stability compared to native hairpins comprised of 3',5'-RNA loops [Hannoush, R. N.; Damha, M. J. J. Am. Chem. Soc. 2001, 123, 12368-12374]. A remarkable feature of the 2',5'-r(UUCG) tetraloop is that, unlike the corresponding 3',5'-linked tetraloop, its stability is virtually independent of the hairpin stem composition. Here, we determine the solution structure of unusually stable hairpins of the sequence 5'-G(1)G(2)A(3)C(4)-(U(5)U(6)C(7)G(8))-G(9)(U/T(10))C(11)C(12)-3' containing a 2',5'-linked RNA (UUCG) loop and either an RNA or a DNA stem. The 2',5'-linked RNA loop adopts a new fold that is completely different from that previously observed for the native 3',5'-linked RNA loop. The 2',5'-RNA loop is stabilized by (a). U5.G8 wobble base pairing, with both nucleotide residues in the anti-conformation, (b). extensive base stacking, and (c). sugar-base and sugar-sugar contacts, all of which contribute to the extra stability of this hairpin structure. The U5:G8 base pair stacks on top of the C4:G9 loop-closing base pair and thus appears as a continuation of the stem. The loop uracil U6 base stacks above U5 base, while the cytosine C7 base protrudes out into the solvent and does not participate in any of the stabilizing interactions. The different sugar pucker and intrinsic bonding interactions within the 2',5'-linked ribonucleotides help explain the unusual stability and conformational properties displayed by 2',5'-RNA tetraloops. These findings are relevant for the design of more effective RNA-based aptamers, ribozymes, and antisense agents and identify the 2',5'-RNA loop as a novel structural motif. 相似文献
43.
L. M. Kustov L. A. Surina V. B. Kazanskii P. A. Zhdan B. K. Nefedov 《Russian Chemical Bulletin》1988,37(4):633-636
1. | By means of diffuse scattering IR spectroscopy and x-ray photoelectron spectroscopy, it has been shown that thermal treatment leads to dealuminization of the lattice of TsVM zeolite and to a substantial decrease in the total number of acid centers with an increase in the relative number of Lewis acid centers, related in particular to extralattice aluminum. |
2. | The decrease in the total number of acid centers results in a decrease in pseudo-cumene conversion, but an increase in selectivity of isomerization. It is concluded that the reaction takes place in the mouths of the pores or on the outer surface of the zeolite. |
44.
45.
Synthesis and Catalytic Properties of Di- and Trinuclear Palladium Complexes with PCP-Pincer Ligands
Beletskaya I. P. Chuchuryukin A. V. van Koten G. Dijkstra H. P. van Klink G. P. M. Kashin A. N. Nefedov S. E. Eremenko I. L. 《Russian Journal of Organic Chemistry》2003,39(9):1268-1281
Linear and branched conjugated pincer ligands having Ph2P groups were synthesized: 3,3',5,5'- tetrakis(diphenylphosphinomethyl)diphenylacetylene, 3,3',5,5'-tetrakis(diphenylphosphinomethyl)diphenyldi- acetylene, 1,3,5-tris[3,5-bis(diphenylphosphinomethyl)phenylethynyl]benzene, and hexakis[3,5-bis(diphenylphosphinomethyl)phenylethynyl]benzene. Palladation of these ligands by heating with Pd(BF4)2(MeCN)4 in boiling acetonitrile gave the corresponding di- and trinuclear ionic pincer palladium complexes. No individual complex was obtained from hexakis[3,5-bis(diphenylphosphinomethyl)phenylethynyl]benzene. The ionic com- plexes were converted into the corresponding chloride complexes by treatment with sodium chloride in a mixture of water with methylene chloride. The structure of the ionic palladium complex with 3,3',5,5'-tetrakis(diphenylphosphinomethyl)diphenylacetylene was established by X-ray analysis. The obtained palladium complexes exhibited a considerable catalytic activity in the Heck reaction of iodobenzene with ethyl acrylate and in the Michael addition of ethyl cyanoacetate with methyl vinyl ketone. The catalytic activity per palladium atom decreases as the number of palladium atoms in the complex increases. 相似文献
46.
The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)(3)] (OTf = (-)OS(O)(2)CF(3)) as a function of at 25 degrees C. Plots of the pseudo-first-order rate constants (k(obs)) for methanolysis as a function of [La(OTf)(3)](total) were obtained under buffered conditions from 5.15 to 10.97, and the slopes of the linear parts of these were used to determine the second-order rate constants (k(2)(obs)) for the La(3+)-catalyzed methanolysis of 2. Detailed analysis of the potentiometric titration data of La(OTf)(3) in methanol through fits to a multicomponent equilibrium mixture of dimers of general stoichiometry La(3+)(2)((-)OCH3)n, where n assumes values of 1-5, gives the equilibrium distribution of each as a function of. These data, when fit to a second expression describing k(2)(obs) in terms of a linear combination of individual rate constants k(2)(2:1), k(2)(2:2).k(2)(2:)n for the dimers, allow one to describe the overall catalytic profile in terms of the individual contributions. The most catalytically important species are the three dimers La(3+)(2)((-)OCH3)1, La(3+)(2)((-)OCH3)2, and La(3+)(2)((-)OCH3)3. The catalysis of the methanolysis of 2 is spectacular: a 2 x 10(-3) M solution of [La(3+)](total), at neutral, affords a 10(9)-fold acceleration relative to the base reaction (t(1/2) approximately 20 s at 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La(3+)(2)((-)OCH3)2 species is proposed. 相似文献
47.
48.
Balandina A Kalinin A Mamedov V Figadère B Latypov S 《Magnetic resonance in chemistry : MRC》2005,43(10):816-828
(1)H, (13)C and (15)N NMR chemical shifts for a variety of novel quinoxalines were determined by different 2D methods and were calculated using the GIAO DFT approach. Comparison with experimental data shows good correlations in the case of (1)H, (13)C and (15)N chemical shifts. Different combinations of basis sets were tested. In non-polar solvents quinoxalines exist as dimers owing to strong hydrogen bonding. Calculations for dimers improve the correlation between experiment and theory. Additive empirical methods for estimating chemical shifts have drawbacks and have to be used with a great care for this type of compound. 相似文献
49.
Sidorov A. A. Fomina I. G. Talismanov S. S. Aleksandrov G. G. Novotortsev V. M. Nefedov S. E. Eremenko I. L. 《Russian Journal of Coordination Chemistry》2001,27(8):548-559
Transformations of polymeric trimethylacetate complexes [M(OH)
n
(OOCCMe3)2 – n
]
m
(M = Ni (I) and Co (II)) and clusters Ni9(4-OH)3(3-OH)3(-O,O-OOCCMe3)(-O,O"-OOCCMe3)7(3-O,O,O"-OOCCMe3)3(4-O,O,O",O"-OOCCMe3)(HOOCCMe3)4(III) and Co6(3-OH)2(-OOCCMe3)10(HOOCCMe3)4(VIII), which are formed from Iand IIupon their recrystallization from nonpolar solvents, were studied. It was shown that the action of N-phenyl-o-phenylenediamine (L) on Ior IIIresults, depending on the solvent, in different tetranuclear clusters with the hydroxo bridges. For example, in benzene, the L2Ni4(3-OH)2(HOOCCMe3)4(-OOCCMe3)6complex (IX) is formed; its L molecules are coordinated in a monodentate way, whereas in acetonitrile, they chelate to give the {[o-C6H4(NH2)(NHPh)]2Ni4(3-OH)2(MeCN)2(OOCCMe3)2(-OOCCMe3)4} compound (X). Heating of Xin the presence of atmospheric oxygen yields IX, the mononuclear bissemiquinonediimine [o-C6H4(NH)(NPh)]2Ni complex (XI), and water. It was noted that the use of aniline in these reactions affords, independent of the nature of the solvent, only one (NH2C6H5)2Ni4(3-OH)2(HOOCCMe3)4(-OOCCMe3)6cluster (VI); in acetonitrile, this cluster is formed as the solvate VI· 2HOOCCMe3(VIa). When treated with ethanol, Iand IIIgive the Ni4(EtOH)6(3-OH)2(2-OOCCMe3)4(OOCCMe3)2cluster (V), which is structurally close to the known cobalt-containing analog IV. Thermolysis of IVin decalin at 170° causes its dimerization, giving the octanuclear Co8(4-O)2(
n
-OOCCMe3)12complex (VII) with the tetradentate oxo bridges. 相似文献
50.
E. A. Shapiro G. V. Lun'kova A. Ya. Shteinshneider I. E. Dolgii O. M. Nefedov 《Russian Chemical Bulletin》1981,30(5):923-925
Conclusions The oxidative dimerization of a mixture of the trans and cis isomers of 2-methyl-2-ethynylcyclopropanecarboxylic acid in the presence of CuCl and NH4Cl gives, along with bis(trans-1-methyl-2-carboxyl-1-cyclopropyl)diacetylene (53% yield), also the corresponding dilactone, and specifically bi(1-methyl-4-oxo-3-oxabicyclo[3.1.0]-2-hexalidene)methyl (14% yield), which is probably formed via the intramolecular cyclization of the intermediately formed bis(cis-1-methyl-2-carboxy-1-cyclopropyl) diacetylene.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1164–1166, May, 1981. 相似文献