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961.
Hybrid quantum mechanics/molecular mechanics simulations, coupled to the recently introduced metadynamics method, performed on the adenosine triphosphate (ATP) of the bovine Hsc70 ATPase protein, show which specific water molecule of the solvation shell of the Mg2+ metal cation acts as a trigger in the initial phase of the ATP hydrolysis reaction in ATP synthase. Furthermore, we provide a detailed picture of the reaction mechanism, not accessible to experimental probes, that allows us to address two important issues not yet unraveled: (i) the pathway followed by a proton and a hydroxyl anion, produced upon dissociation of a putative catalytic H2O molecule, that is crucial in the selection of the reaction channel leading to the hydrolysis; (ii) the unique and cooperative role of K+ and Mg2+ metal ions in the reaction, acting as co-catalysts and promoting the release of the inorganic phosphate via an exchange of the OH- hydroxyl anion between their respective solvation shells. This is deeply different from the proton wire mechanism evidenced, for instance, in actin and lowers significantly the free energy barrier of the reaction. 相似文献
962.
Ono M Oishi K Abe H Masuoka C Okawa M Ikeda T Nohara T 《Chemical & pharmaceutical bulletin》2006,54(10):1421-1424
Two new iridoid glucosides, verbenabraside A (1) and verbenabraside B (2), were isolated from the aerial parts of Verbena brasiliensis VELL., along with six known iridoid glucosides, gelsemiol 3-O-beta-D-glucoside (3), verbraside (4), 9-hydroxysemperoside (5), griselinoside (6), aralidioside (7), and 6alpha-hydroxyforsythide dimethyl ester (8), three known phenylethanoid glycosides, 2-phenylethyl O-beta-D-xylopyranosyl-(1-->2)-beta-D-glucopyranoside (9), acteoside (10), and leucosceptoside A (11), two known lignan glucosides, dihydroxymethyl-bis(3,5-dimethoxy-4-hydroxyphenyl) tetrahydrofuran-9 (or 9')-O-beta-glucopyranoside (12) and (+)-lyoniresinol 3alpha-O-beta-D-glucopyranoside (13), a known methyl salicylate glucoside, methyl 2-O-beta-D-glucopyranosylbenzoate (14), and two known sterols, beta-sitosterol 3-O-beta-D-glucopyranoside (15) and beta-sitosterol (16). Their chemical structures were determined on the basis of spectroscopic data. Compound 1 exhibited stronger scavenging effect on the stable free radical 1,1-diphenyl-2-picrylhydrazyl than that of alpha-tocopherol. 相似文献
963.
The mechanism of the deprotonation of 2-butanone (1) with methoxide anion (2) was studied by ab initio molecular orbital (MO) methods. Calculations of the thermodynamic stabilities of each complex and the regioselectivity of the reaction were performed using a static isodensity surface polarized continuum model (IPCM) which takes the solvent effect into consideration. The calculated energies of the complexes lead ultimately to the conclusion that the major deprotonation pathway in protic solvents is dependent upon thermodynamically stable complexes with small activation energies under equilibrium control. 相似文献
964.
Li D Ikeda T Matsuoka N Nohara T Zhang H Sakamoto T Nonaka G 《Chemical & pharmaceutical bulletin》2006,54(10):1425-1428
From the unripe fruits of Lo Han Kuo (Siraitia grosvenori), a Chinese medicinal plant, two new cucurbitane triterpene glycosides, 20-hydroxy-11-oxomogroside IA(1) (1) and 11-oxomogroside IIE (2), were isolated along with five known cucurbitane glycosides, 11-oxomogroside IA(1) (3), mogroside IIE (4), mogroside III (5), mogroside IVA (6), and mogroside V (7), and two flavonoid glycosides, kaempferol 7-O-alpha-L-rhamnopyranoside (8) and kaempferol 3,7-di-O-alpha-L-rhamnopyranoside (9). Their structures were determined on the basis of detailed analyses of 1D, 2D-NMR spectroscopic methods and by comparing with literature values. This paper describes the first investigation of unripe bitter Lo Han Kuo fruits. 相似文献
965.
Kluger R Ikeda G Hu Q Cao P Drewry J 《Journal of the American Chemical Society》2006,128(49):15856-15864
Mandelylthiamin (MT) is formally the conjugate of thiamin and benzoylformate. It is the simplified analogue of the first covalent intermediate in benzoylformate decarboxylase. Although MT is the functional equivalent of the enzymic intermediate, it is 106-fold less reactive in decarboxylation. Furthermore, upon loss of carbon dioxide, it undergoes a fragmentation reaction that is about 102-fold faster than the enzymic reaction. While Br?nsted acids in general can suppress the fragmentation to some extent, they do not accelerate the decarboxylation. Surprisingly, the conjugate acid of pyridine accelerates decarboxylation; it also blocks fragmentation with particularly high efficiency. These results are consistent with the conjugate acid of pyridine acting as a "spectator" catalyst, associating with MT prior to decarboxylation. In the absence of catalyst, carbon dioxide formed upon carbon-carbon bond breaking overwhelmingly reverts to the carboxylate. Association of pyridine (and its conjugate acid) with MT permits trapping of the nascent carbanion by protonation, while nonassociated acids must arrive by the relatively slow process of diffusion. C-Alkyl pyridine acids provide similar catalysis while other acids have no effect. This suggests that an enzyme that generates an aldehyde from a 2-ketoacid should have functional Br?nsted acids in their active sites that would trap the carbanion, as does benzoylformate decarboxylase. Enzymes that give nonaldehydic products from decarboxylation of thiamin diphosphate conjugates containing an associated electron acceptor or electrophilic substrate would also be able to prevent the reversal of decarboxylation. 相似文献
966.
Ikeda A Irisa T Hamano T Kitahashi T Sasaki Y Hashizume M Kikuchi J Konishi T Shinkai S 《Organic & biomolecular chemistry》2006,4(3):519-523
A new molecular design of fullerene derivatives exhibiting trigger-responsive self-aggregation in organic solvents has been established. Calix[4]arene was covalently connected with fullerene in order to apply host-guest interaction to the aggregation control. The self-assembly behaviour was studied in organic solvents by UV-vis absorption spectroscopy, dynamic light scattering and transmission electron microscopy. Results show that the bisfullerene formed self-aggregations with a low polydispersity index due to the fullerenes' tendency to aggregate in polar organic solvents. Furthermore, the aggregate sizes can be changed readily by solvent composition and the addition of guest cations. Especially, disaggregation of the bisfullerene was induced by addition of LiClO4 or NaClO4. 相似文献
967.
[reaction: see text] Solid-phase Staudinger ligation of small peptides was performed on a novel core-shell-type resin. Solid-phase Staudinger ligation was mediated by synthetic solid-supported phosphinothiol, which was readily prepared by a straightforward synthetic route. This protocol afforded final peptide products in excellent yields and purities and thus could provide the opportunity to facilitate a simple manipulation for condensation of peptide fragments. In particular, the resulting resin could be recycled in a successful manner. 相似文献
968.
We have used a mechanochemical high-speed vibration milling (HSVM) technique to solubilize single-walled carbon nanotubes (SWNTs) in organic solvents through the formation of complexes between the SWNTs and a polythiophene (PT) derivative. This HSVM approach is superior to the conventional sonication method used to solubilize SWNTs in organic solvents. Moreover, we found that in these supramolecular complexes the PT chains were ordered one-dimensionally on the SWNT surfaces in organic solvents. [structure: see text]. 相似文献
969.
Moosa Hatami Gerrit E.W. Bauer Saburo Takahashi Sadamichi Maekawa 《Solid State Communications》2010,150(11-12):480-484
We present a semiclassical theory of spin diffusion in a ferromagnetic metal subject to a temperature gradient. Spin-flip scattering can generate pure thermal spin currents by short-circuiting spin channels while suppressing spin accumulations. A thermally induced spin density is locally generated when the energy dependence of the density of states is spin polarized. 相似文献
970.
N. Ikeda F. Nakamura Y. Inotani K. Koga M. Koga S. Koto T. Sugimitsu H. Fujita S. Morinobu 《The European Physical Journal A - Hadrons and Nuclei》2000,9(2):213-220
The 16O(11B,12C)15N reaction at 41.25 MeV has been investigated using the kinematical coincidence method. Polarization tensors t20 and t40 of 12C[2+
1] for the quantization axis taken along the direction of propagation have been measured at center-of-mass angles (Θc.m.) between 48° and 62° by analyzing the energy spectrum of 12C[2+
1] modulated by the effect of γ-ray emission. The cross-sections of the transfer reactions leading to the 12C[g.s.]+15N[g.s.], 12C[2+
1]+15N[g.s.] and 12C[g.s.]+15N[3/2-
1] final states have also been measured in the range 48°≤Θc.m.≤ 120°. The polarization tensor terms of 12C[2+
1] largely deviating from zero have been observed, contrary to the prediction by the distorted-wave Born approximation (DWBA).
The one-step DWBA calculation also fails in describing the transfer reaction cross-sections. It is shown that the coupled
channel model calculation including excitations and reorientations in 11B and 12C satisfactorily reproduces both the tensor terms and the cross-sections of the transfer reactions. The multi-step processes
passing through the excited states of 11B are found to significantly contribute to the reaction.
Received: 3 July 2000 / Accepted: 15 September 2000 相似文献