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61.
Kanai M Porfyrakis K Briggs GA Dennis TJ 《Chemical communications (Cambridge, England)》2004,(2):210-211
We report the purification of the nitrogen-containing incar-fullerenes iNC(60) and iNC(70), and their characterisation by UV-Vis absorption spectroscopy. 相似文献
62.
Addition of stabilized Horner-Wadsworth-Emmons (HWE) phosphonates to substituted 1,2-dioxines leads to diastereomerically pure di- and trisubstituted cyclopropanes in high yields and represents a viable alternative to ylides in the cyclopropanation reaction involving 1,2-dioxines. While yields are comparable, reaction times with these stabilized phosphonates were accelerated and the diastereoselectivity for this cyclopropanation reaction was significantly greater than for the previously reported examples employing ylides. 相似文献
63.
The classical simplex method is extended into the Semiglobal Simplex (SGS) algorithm. Although SGS does not guarantee finding the global minimum, it affords a much more thorough exploration of the local minima than any traditional minimization method. The basic idea of SGS is to perform a local minimization in each step of the simplex algorithm, and thus, similarly to the Convex Global Underestimator (CGU) method, the search is carried out on a surface spanned by local minima. The SGS and CGU methods are compared by minimizing a set of test functions of increasing complexity, each with a known global minimum and many local minima. Although CGU delivers substantially better success rates in simple problems, the two methods become comparable as the complexity of the problems increases. Because SGS is generally faster than CGU, it is the method of choice for solving optimization problems in which function evaluation is computationally inexpensive and the search region is large. The extreme simplicity of the method is also a factor. The SGS method is applied here to the problem of finding the most preferred (i.e., minimum free energy) solvation sites on a streptavidin monomer. It is shown that the SGS method locates the same lowest free energy positions as an exhaustive multistart Simplex search of the protein surface, with less than one-tenth the number of minizations. The combination of the two methods, i.e.. multistart simplex and SGS, provides a reliable procedure for predicting all potential solvation sites of a protein. 相似文献
64.
The structure of p,p'-dibenzene (PDB) has been investigated by full geometry optimizations using the empirical force field (EFF) and MINDO/3 methods. While other structural parameters are in good agreement, the central bond length calculated by MINDO/3 (1.595 Å), as confirmed by an ab initio (STO-3G basis set) optimization (1.596 Å), is in striking contrast to the corresponding length calculated by EFF (1.543 Å). A detailed analysis of the electronic structure of PDB based on a quantitative perturbational molecular orbital treatment reveals that through-bond coupling of the four π systems is responsible for an elongation of the σ bond which mediates this interaction. Further studies using the EFF and MINDO/3 approaches demonstrate that extended C-C single bonds can arise even in structures with fewer than four π systems. The effect of substituents on the central bond length in PDB has been briefly investigated. (MINDO/3). A variety of other structures have been identified in which bond lengthening may result from through-bond coupling. 相似文献
65.
GABA(C) (rho) receptors are members of the Cys-loop superfamily of neurotransmitter receptors, which includes nicotinic acetylcholine (nACh), 5-HT(3), and glycine receptors. As in other members of this family, the agonist binding site of GABA(C) receptors is rich in aromatic amino acids, but while other receptors bind agonist through a cation-pi interaction to a tryptophan, the GABA(C) binding site has tyrosine at the aligning positions. Incorporating a series of tyrosine derivatives at position 198 using unnatural amino acid mutagenesis reveals a clear correlation between the cation-pi binding ability of the side chain and EC(50) for receptor activation, thus demonstrating a cation-pi interaction between a tyrosine side chain and a neurotransmitter. Comparisons among four homologous receptors show variations in cation-pi binding energies that reflect the nature of the cationic center of the agonist. 相似文献
66.
Riboflavin was irradiated anaerobically in aqueous EDTA solutions over the pH range 2.5–10. In other dye systems (Bonneau and Pereyre, 1975), only the trivalent anion of EDTA was found to have significant reactivity for photoreduction. For riboflavin, the reactivity begins with monoanionic EDTA, and the reactivity is markedly increased as the charge increases. This suggests that the charge on the reductant is more important to the electron transfer process for riboflavin than the formation of a nonhydrogen bonded nitrogen site on EDTA. At high concentrations of EDTA in the pH range 4–8, quenching of the photoreduction occurs, which can be explained by an energy transfer between the excited singlet state of riboflavin and trianionic EDTA, possibly as an association complex. The rate constants for the photoreduction of riboflavin by the monovalent, divalent, and trivalent anions of EDTA are 1.0 times 107M-1 s-l, 4.8 times 10′M-1 s-l, and 2.0 times 108M-1s-1, respectively. The rate constant for the singlet state quenching by trianionic EDTA is 3 times 109M-l s-1, and the limiting quantum yield for intersystem crossing for riboflavin in aqueous solution is 0.50 ± 0.05. 相似文献
67.
It is shown that in several cases where planar delocalisation in organic cations would result in the formation of an anti-aromatic system, polyhedral delocalisation is the form of bonding actually preferred. This explains, for instance, why organic cations in such cases adopt cage-like structures. A full graph-theoretical analysis, similar to one previously published12 for polyhedral boranes, carboranes and metal clusters, indicates that the nido structure for (CH)5+ may readily be accounted for. Moreover, in the case of the dication (CH)62+ the fact that its energy minimum also corresponds to a nido structure is explained. In fact, no closo- or arachno-type structures appear to be possible for organic cations. A number of structural predictions concerning these species are given in the conclusion. 相似文献
68.
The synthesis of a novel motif for threonine protease inhibition is described. The desired E-1,2-dichlorovinyl ethers are obtained from alcohols and trichloroethylene as single diastereomers. Aqueous treatment at pH 11 unmasks the hidden α-chloroacetate, which is required for the reaction with the active site of the protease. 相似文献
69.
The specific rates of solvolysis of N,N,N',N'-tetramethyldiamidophosphorochloridate have been measured at 25.0 degrees C in 31 solvents. Analysis with the extended Grunwald-Winstein equation leads to sensitivities toward changes in solvent nucleophilicity (l) of 1.20 +/- 0.07 and toward changes in solvent ionizing power (m) of 0.69 +/- 0.04. The correlation is improved by omission of the four data points for 2,2,2-trifluoroethanol-ethanol mixtures (F-test value from 155 to 320) with very small reductions in both l and m values. Activation parameters are reported for eight of the solvolyses. The l and m values are very similar to those previously reported for solvolyses of several arenesulfonyl chlorides, consistent with a concerted substitution process. This assignment is supported by a large k(Cl)/k(F) ratio for hydrolysis and a corresponding ratio for hydroxide-assisted hydrolysis of 178. The stereochemistry of nucleophilic attack at tetracoordinate phosphorus(V) is discussed. 相似文献
70.
Abstract— The kinetic factors which determine the rate at which Na+ channels in nerve membranes become photochemically modified were studied on giant axons from lobsters using the double sucrose gap voltage clamp technique. Axons were bathed in artificial sea water containing sensitizing dyes and illuminated from a Xe are source with light in the visible region while being repetitively step depolarized. Successive values of peak Na+ current and time-to-peak were monitored and rate constants for their change served as the assay for magnitude of modification. Action spectra for four sensitizers in the fluorescein series exhibited red shifts of roughly 17nm demonstrating that sensitizing species are not simply free in solution. Eosin Y diffuses to its sensitization sites with a half time of 70s indicating the existence of a major diffusion barrier which may mean that dye must penetrate to the interior of the membrane to be effective. Eosin Y is removed from sensitization sites by rinse with the same half time but shows two fractions: a faster fraction comprising 80% of sensitizing effectiveness and a slower fraction comprising 20%. The concentration dependence for Eosin Y is linear below 10 μ M and shows a progressive saturation at higher values, where the relationship is difficult to determine because of shielding. Different sensitizers vary in their ability to sensitize block of channels vs disruption of inactivation, demonstrating separate processes for the two modifications. It is suggested that both modifications proceed from single photon absorption events by individual sensitizer molecules bound or located close to the modification sites on the channels. 相似文献