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111.
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Methyl 3-O-acetyl-2-deoxy-α-d-ribofuranoside, 1,3-di-O-acetyl-2-deoxy-α-d-ribofuranose and 1,2,3-tri-O-acetyl-α-d-arabinofuranose were diastereoselectively prepared (de = 100%) from anomeric mixtures of the corresponding 5-acetylated compounds through Candida antarctica B lipase (CAL B)-catalysed alcoholysis. 相似文献
114.
115.
Tabares LC Cortez N Agalidis I Un S 《Journal of the American Chemical Society》2005,127(16):6039-6047
Two different temperature dependences of the manganese(II) high-field electron paramagnetic resonance spectrum of manganese superoxide dismutase from E. coli were observed. In the 25-200 K range, the zero-field interaction steadily decreased with increasing temperature. This was likely due to the thermal expansion of the protein. From these results, it was possible to deduce an approximately r(-)(2.5) dependence of Mn(II) zero-field interaction on ligand-metal distance. At temperatures above 240 K, a distinct six-line component was detected, the amplitude of which decreased with increasing temperature. On the basis of similarities to the six-line spectrum observed for the azide-complexed E. coli manganese superoxide dismutase, the newly detected six-line spectrum was assigned to a hexacoordinate Mn(II) center resulting from the coordination of a nearby water molecule to the normally five-coordinate center. The changes in enthalpy and entropy characterizing the hexacoordinate-pentacoordinate equilibrium in the 240-268 K range were -5 kcal/mol and -24 cal/mol.K, respectively. The structural implications of the zero-field parameters of the newly found hexacoordinate form in comparison to those of the Mn(II) centers in concanavalin-A and manganese-containing R. spheroides photosynthetic reaction centers and the values predicted by the superposition model are discussed. 相似文献
116.
Ciprian?BorceaEmail author Ileana?StreinuEmail author 《Discrete and Computational Geometry》2004,31(2):287-303
Rigid frameworks in some Euclidean space are embedded graphs
having a unique local realization (up to Euclidean motions) for
the given edge lengths, although globally they may have several.
We study the number of distinct planar embeddings of minimally
rigid graphs with $n$ vertices. We show that, modulo planar rigid
motions, this number is at most ${{2n-4}\choose {n-2}} \approx
4^n$. We also exhibit several families which realize lower bounds
of the order of $2^n$, $2.21^n$ and $2.28^n$.
For the upper bound we use techniques from complex algebraic
geometry, based on the (projective) Cayley--Menger variety
${\it CM}^{2,n}(C)\subset P_{{{n}\choose {2}}-1}(C)$ over the complex
numbers $C$. In this context, point configurations are represented
by coordinates given by squared distances between all pairs of
points. Sectioning the variety with $2n-4$ hyperplanes yields at
most $deg({\it CM}^{2,n})$ zero-dimensional components, and one finds
this degree to be $D^{2,n}=\frac{1}{2}{{2n-4}\choose {n-2}}$.
The lower bounds are related to inductive constructions of
minimally rigid graphs via Henneberg sequences.
The same approach works in higher dimensions. In particular, we
show that it leads to an upper bound of $2 D^{3,n}=
{({2^{n-3}}/({n-2}})){{2n-6}\choose{n-3}}$ for the number of
spatial embeddings with generic edge lengths of the $1$-skeleton
of a simplicial polyhedron, up to rigid motions.
Our technique can also be adapted to the non-Euclidean case. 相似文献
117.
The conformation in solution of new modified nucleosides bearing branched-chain sugars used as building blocks for antisense oligonucleotides is investigated by NMR spectroscopy. Within a classical approach to the conformational study of these molecules, a new methodology is adopted, which is capable of exploring all the possibilities, for a given conformational model, that are compatible with the experimental data. The relationship between conformation of the monomers and hybridization properties of the corresponding oligonucleotides is examined. 相似文献
118.
Here we present some distribution function inequalities between certain functionals defined relative to a convolution approximation procedure. Such inequalities are best known when the approximation is made using dilations of the Gaussian or Cauchy kernels. In these cases, classical differential equations, the heat equation or Laplace's equation, provide the basis for comparisons; in the latter case, the quadratic functional is known as the Lusin area integral. The kernels we consider are compactly supported, and satisfy a dilation equation, rather than a differential equation. For these kernels, there is an intrinsic quadratic variation, defined from the dilation structure. We obtain good lambda distribution function inequalities between a maximal function and the quadratic variation functional. 相似文献
119.
Steven J. Pachuta Ileana Isern-Flecha R. Graham Cooks 《Journal of mass spectrometry : JMS》1986,21(1):1-5
The site of protonation, methylation and ethylation of anilines, phenols and thiophenols can be determined by tandem mass spectrometry (MS/MS). The extent to which the cation attachment products formed from these compounds undergo charge stripping is related to the site of cation attachment. The formation of doubly charged ions for phenols and anilines is favored when cation attachment occurs on the ring. For thiophenols alkylation occurs almost exclusively on the substituent, and yet an abundant stripping peak is recorded. This behavior is opposite to that seen for anilines and phenols and is accounted for in terms of orbital energies in the three systems. The site of cation attachment, as deduced by charge stripping, agrees with conclusions drawn from the fragmentation behavior of these ions upon collision-induced dissociation. 相似文献
120.
Renate Drozdzak Nele Ledoux Bart Allaert Ileana Dragutan Valerian Dragutan Francis Verpoort 《Central European Journal of Chemistry》2005,3(3):404-416
A two step procedure for the synthesis of a novel family of homogeneous and immobilized Ru-complexes containing Schiff bases
as O,N-bidentate ligands is described. The new Ru-complexes have been structurally characterized by IR, Raman,1H-,13C-NMR spectroscopy. The Schiff bases were associated with a diversity of inorganic and organic ligands such as chloride, phosphane,
arenes, various carbenes (alkylidene, vinylidene, indenylidene and allenylidene as well as N-heterocyclic carbenes) and cyclodienes.
By choosing a selective range of substituents for the Schiff base, useful physical and chemical properties of the prepared
Rucomplexes can be induced. This synthetic approach is promising in creating a valuable and diverse selection of Ru-complexes,
valuable for future applications. 相似文献