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91.
[formula: see text] The synthesis and characterization of prototype alpha-helical peptide nucleic acid (alpha PNA) modules 1-3 as well as disulfide dimers 4 and 5 are reported. These molecules combine an alpha-helical peptidyl scaffold with well-defined nucleobase molecular recognition patterns and could serve as a basis for novel antisense and/or antigene agents. Structure assignments for these alpha PNAs were supported by MALDI-TOF mass spectrometry, and the alpha-helical nature of 4 in water was confirmed by circular dichroism (CD) spectroscopy. 相似文献
92.
Benisvy L Blake AJ Collison D Davies ES Garner CD McInnes EJ McMaster J Whittaker G Wilson C 《Chemical communications (Cambridge, England)》2001,(18):1824-1825
A new N,O-bidentate pro-ligand (HL), [ML2] (M = Cu, Zn) and [CuL2][BF4] have been synthesised; [CuL2].4DMF and [CuL2][BF4].2CH2Cl2 have been crystallographically and spectroscopically characterised; these data indicate that [CuL2]+ cations are constituted as [Cu2+(L.)(L-)]+ and involve the phenoxyl radical L.. 相似文献
93.
Davies SG Garner AC Nicholson RL Osborne J Savory ED Smith AD 《Chemical communications (Cambridge, England)》2003,(17):2134-2135
Asymmetric [3,3]-sigmatropic aza-Claisen rearrangement of the (Z)-N-allyl-N,O-silylketene aminal of (3S,4E,alphaR)-1-benzyloxy-3-(N-propionyl-N-alpha-methylbenzylamino)hex-4-ene furnishes (2S,3R,4E,alphaR)-N-alpha-methylbenzyl-2,3-dimethyl-7-benzyloxyhept-4-enamide in > 92% d.e.; rearrangement of the diastereomeric (3R,4E,alphaR)-(Z)-N,O-silylketene aminal proceeds with low diastereoselectivity. 相似文献
94.
We consider the semipositone problem $${\matrix {-\Delta u (x)= \lambda f (u(x))\ \ \; \ \ \ \ \ x \in \Omega \cr \qquad \qquad \qquad u(x)=0 \ \ \ \;\ \ \ \ x \in \partial \Omega \cr}}$$ where λ > 0 is a constant, Ω is a bounded region in Rn with a smooth boundary, and f is a smooth function such that f ′(u) is bounded below, f (0) < 0 and \({\rm lim}_{u \rightarrow}+\infty {f(u)\over u}=0. \) We prove under some additional conditions the existence of a positive solution (1) for λ ∈ I where I is an interval close to the smallest eigenvalue of —Δ with Dirichlet boundary condition and (2) for λ large. We also prove that our solution u for λ large is such that∥u∥ ? supx∈Ω ¦u(x)¦ → ∞ as A → ∞. Our methods are based on sub and super solutions. In particular, we use an anti maximum principle to obtain a subsolution for our existence result for λ ∈ I. 相似文献
95.
Circular DNA of more than 1,400 bp in size is known not to migrate into polyacrylamide gels. The migration of supercoiled plasmid pBR322 DNA (4,363 by) into uncrosslinked polyacrylamide (Mw 5 x 10(6)) solutions and its separation, on the basis of conformation, from its nicked form is demonstrated in this study. Migration of the supercoiled, nicked circular and linear forms of the plasmid DNA is retarded in proportion to the concentration of uncrosslinked polyacrylamide, the degree of retardation being highest for the nicked circular form. Decreasing the level of supercoiling of the covalently closed circular form by decreasing the concentration of the intercalating dye (ethidium homodimer) shows that the degree of retardation decreases in proportion to the superhelix density. 相似文献
96.
The kinetics of precipitation reactions with110AgNO3 of some di (β-chlorethyl) amine derivates and hydrochlorides with esters of N-(p-aminobenzoyl)-L-aspartic acid as carriers in dimethylformamide-water mixture, were studied. The rate constants of these reactions were of the order of 10?4 1 · mol?1 · min?1. The concentrations of the corresponding hydrochloride solutions were measured by radiometric titration with110AgNO3 solution of given concentration. 相似文献
97.
98.
Freya J. Hine Adam J. Taylor C. David Garner 《Coordination chemistry reviews》2010,254(13-14):1570-1579
The development of the coordination chemistry of dithiolene ligands is summarised, together with a consideration of the electronic structure of complexes of these ‘non-innocent’ ligands. This information provides a context for a consideration of the role of dithiolenes in natural systems, i.e. as the ligand that binds molybdenum (or tungsten) at the catalytic centre of an extensive series of enzymes. These enzymes catalyse the transfer of an oxygen atom to or from the substrate: e.g. the sulfite oxidases catalyse the conversion of sulfite to sulfate and the nitrate reductases catalyse the conversion of nitrate to nitrite. The nature of the catalytic centres of several of these enzymes has been determined and each involves one or two ‘molybdopterin’ (MPT) cofactors bound to a mononuclear metal centre via their dithiolene group. The biosynthesis of MPT is described and, given its nature, possible roles for this moiety in the function of the oxotransferase enzymes are discussed. The review concludes with a consideration of the coordination chemistry that has been stimulated by the present knowledge of the nature and function of the catalytic centres of these enzymes. 相似文献
99.
An improved synthesis of lithium phenyltris(methimazolyl)borate, Li[PhTm(Me)], (methimazole = 1-methylimidazole-2-thione) is described, and the structure of the methanol-solvated [Li(OHMe)4][PhTm(Me)] has been determined. The syntheses and characterization of complexes [M(PhTm(Me))(PR3)] (M = Cu, Ag, Au; R = Et, Ph;) are reported, and the complexes [Cu(PhTm(Me))(PPh3)], [Ag(PhTm(Me))(PEt3)] and [Au(PhTm(Me))(PEt3)] are crystallographically characterized, showing a progression from pseudo-tetrahedral geometry (copper, S3P coordination) to trigonal planar geometry (silver, S2P coordination) to linear geometry (gold, SP coordination). In addition, the copper(I) and silver(I) triphenylphosphine complexes of the adventitiously formed phenylhydrobis(methimazolyl)borate ligand, [M(PhBm(Me))(PPh3)], have been crystallographically characterized, showing both species to have a trigonal planar primary coordination sphere, with a secondary M...H-B interaction. Finally, reaction of copper(II) chloride with Li[PhTm(Me)] results in formation of a compound analyzing as [Cu(II)(PhTm(Me))Cl], although its extreme insolubility and marked instability have precluded its complete characterization. Attempts to prepare this by ultra-slow diffusion of the reactants through solvent blanks has led to isolation of a mixed-valence copper(I/II) methimazolate cluster, [Cu(I)10Cu(II)2(mt)12Cl2] and a copper(I) dimeric complex [Cu2(PhTm(Me))2], indicating that copper(II) ions oxidatively decompose the phenyltris(methimazolyl)borate anion. 相似文献
100.
[reaction: see text] Highly functionalized pyrrolidines are obtained in a single chemical step via a mild, efficient, and selective Ag(I)-catalyzed asymmetric [C+NC+CC] coupling process. Oppolzer's camphorsultam enables the desired reaction cascade and provides a reliable means to control the developing stereochemistry and purify the products. This three-component reaction provides unprecedented access to structurally diverse pyrrolidines for both target- and diversity-oriented syntheses. 相似文献