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51.
James Gardiner Raveendra I. Mathad Berhard Jaun Jürg V. Schreiber Oliver Flögel Dieter Seebach 《Helvetica chimica acta》2009,92(12):2698-2721
β3‐Peptides consisting of six, seven, and ten homologated proteinogenic amino acid residues have been attached to an α‐heptapeptide (all d‐ amino acid residues; 4 ), to a hexaethylene glycol chain (PEGylation; 5c ), and to dipicolinic acid (DPA derivative 6 ), respectively. The conjugation of the β‐peptides with the second component was carried out through the N‐termini in all three cases. According to NMR analysis (CD3OH solutions), the (M)‐314‐helical structure of the β‐peptidic segments was unscathed in all three chimeric compounds (Figs. 2, 4, and 5). The α‐peptidic section of the α/β‐peptide was unstructured, and so was the oligoethylene glycol chain in the PEGylated compound. Thus, neither does the appendage influence the β‐peptidic secondary structure, nor does the latter cause any order in the attached oligomers to be observed by this method of analysis. A similar conclusion may be drawn from CD spectra (Figs. 1, 3, and 5). These results bode well for the development of delivery systems involving β‐peptides. 相似文献
52.
Peter C Rueping M Wörner HJ Jaun B Seebach D van Gunsteren WF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(23):5838-5849
Folding properties of beta-peptides were investigated by means of NMR experiments and MD simulations of beta-dipeptides, which serve as small test systems to study the influence of stereocenters and side chains on hydrogen-bond and consequently on secondary-structure formation. Two stereoisomers, SR and SS, of a Val-Phe dipeptide, and of the corresponding Ala-Ala dipeptide, and a Gly-Gly dipeptide were simulated in methanol for 40 ns. In agreement with experiment, the isomers of the Val-Phe dipeptide adopt quite different conformers at 298 K, the differences being reduced at 340 K. Interestingly, the SR isomer shows enhanced hydrogen bonding at the higher temperature. The adopted conformations are primarily determined by the R or S side chain substitution, and less by the type of side chain. Back-calculation of (1)H ROESY spectra and (3)J coupling constants from the MD simulations and comparison with the experimental data for the Val-Phe dipeptides shows good agreement between simulation and experiment, and reveals possible problems and pitfalls, when deriving structural properties of a small and extremely flexible molecule from NMR data only. Inclusion of all aspects of internal dynamics is essential to the correct prediction of the NMR spectra of these small molecules. Cross comparison of calculated with experimental spectra for both isomers shows that only a few out of many ROESY peaks reflect the sizeable conformational differences between the isomers at 298 K. 相似文献
53.
Ronald Breslow David A. Cortés Bernhart Jaun R. David Mitchell 《Tetrahedron letters》1982,23(8):795-798
Chlorination of tricyanocylopropane is observed to lead directly to trichlorotricyanocyclopropane which undergoes a remarkable reductive conversion to hexacyanobenzene. The synthesis of mono- and dichlorotricyanocyclopropane ( and ) and their use as potential precursors for tricyanocyclopropenyl anion is described. 相似文献
54.
Photoelastic coatings were bonded to limestone and granite plates which were then struck on edge with a steel projectile. The resulting fracture zone growth was observed with a reflecting polariscope and recorded with an image converter camera. It is shown that the maximum velocity at which cracks propagate under these circumstances is very much higher than predicted by quasi-steady theories. ‘Apparent’ supervelocity cracks can be generated by shock reflection from bounding interfaces, producing tensile waves of sufficient amplitude to activate and join isolated flaw. Photoelastic coatings are also shown to very much enhance surface-crack visualization subsequent to impact. 相似文献
55.
The photochemical formation of bicyclooctenes b from 12,13-didehydrosqualene ( 1 ) upon irradiation at wavelengths longer than 290 nm is reported. The same photoreaction is also observed with trienes 7 and 8 which like 1 have a remote double bond two C-atoms removed from the triene system. A configurational assignment for bicyclooctene 10 obtained from 8 and a mechanism for the observed photochemical reaction are proposed. 相似文献
56.
A methylnickel(II) derivative of coenzyme F430 ( 1 ) was proposed as an intermediate in the enzymic process catalyzed by methyl-CoM reductasc. Indirect evidence points to formation of CH3–F430MII in the reaction of F30M1 (obtained from F430MII ( 2 )) with eleclrophilic methyl donors. The results presented here show, that such a compound does exist. A paramagnetic CD3–NiII derivative 5b of the pentamethyl ester 2 (F430M) of coenzyme F430 was prepared by in situ methylation with (CD3)2Mg and characterized by its isotropically shifted 2H-NMR spectrum. At ?40°, the very broad D-signal of the axially coordinated CD3 group is found at ?490 ppm. Comparison with the 2H- and 1H-NMR spectra of mcthyl(tetramethylcyclam)nickel(II) derivatives 4 ([NiII(CH3))(tmc)]CF3SO3 ( 4a ) is the only isolated CH3–Ni derivative of a N4macrocyclic NiII complex' shows that the large isotropic shift to high field is characteristic for a Me group axially bound to the Ni center. The temperature dependence of the isotropic shift of the CD3–Ni group in both 4b and 5b follows Curie's law and yields 2H hyperfine coupling constants of ?0.65 ( 4b ) and ?0.85 MHz ( 5b ), respectively. The 1H-NMR spectrum indicates that, in contrast to the five-coordinate monochloro complex [NiIICl(tmc)]+, intermolecular exchange of the axial ligand in [NiII(CH3)(tmc)]+ 4a is either slow at the NMR time scale or does not occur at all. 相似文献
57.
Rueping M Mahajan YR Jaun B Seebach D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(7):1607-1615
Two different strategies have been employed for the synthesis of Fmoc-protected beta(3)-homoarginine; the Arndt-Eistert homologation of alpha-arginine and the guanidinylation of beta(3)-homoornithine. Solid-phase beta-peptide synthesis was used for the preparation of beta-heptapeptide 1, which was designed to form a helix stabilized by electrostatic interactions through positively (beta(3)hArg) and negatively charged (beta(3)hGlu) amino acid residues. CD measurements and corresponding NMR investigations in MeOH and aqueous solutions do indeed show that the beta-peptidic 3(14)-helix can be stabilized by salt-bridge formation. 相似文献
58.
Dieter Seebach Karl Gademann JüRg V. Schreiber Jennifer L. Matthews Tobias Hintermann Bernhard Jaun Lukas Oberer Ulrich Hommel Hans Widmer 《Helvetica chimica acta》1997,80(7):2033-2038
β-Hexapeptides 1–5 and a β-dodecapeptide 6 with sequences containing two different types of β-amino acids (aliphatic proteinageous side chains in the 2- or in the 3-position) have been prepared. CD (Fig. 1) and NMR measurements indicate that, with one exception, the secondary structures formed by these new β-peptides differ from those of isomers studied previously. Detailed NMR analysis of the β-hexapeptide 5 (with alternating β2,β3-building blocks) and molecular-dynamics simulations have produced a minimum energy conformation (Fig. 2,b)which might be described as a novel irregular helix containing ten- and twelve-membered H-bonded rings. This demonstrates the great structural variability of β-peptides, since three different helical secondary structures have been discovered to date. 相似文献
59.
Auffrant A Jaun B Jarowski PD Houk KN Diederich F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(12):2906-2911
A variety of 1,1,4,4-tetraal kynylbutatrienes and 1,4-dialkynylbutatrienes was synthezized by dimerization of the corresponding gem-dibromoolefins. Both (1)H and (13)C NMR spectroscopy indicated that the di- and tetraalkynylated butatrienes are formed as a mixture of cis and trans isomers. Variable temperature NMR studies evidenced a facile cis-trans isomerization, thus preventing the separation of these isomers by gravity or high-performance liquid chromatography (HPLC). For 1,1,4,4-tetraalkynylbutatrienes, the activation barrier deltaG( not equal ) was measured by magnetization transfer to be around 20 kcal mol(-1), in the range of the barrier for internal rotation about a peptide bond. Unlike the tetraalkynylated [3]cumulenes, 1,4-dialkynylbutatrienes are more difficult to isomerize and could, in one case, be obtained isomerically pure. Based on experimental data, the rotational barrier DeltaG( not equal ) for 1,4-dialkynylbutatrienes is estimated to be around 25 kcal mol(-1). The hypothesis of a stabilizing effect of the four alkynyl substituents on the proposed but-2-yne-1,4-diyl singlet diradical transition state of this cis-trans isomerization is further supported by a computational study. 相似文献
60.
Andreas Pfaltz Bernhard Jaun Alexander Fassler Albert Eschenmoser Rolf Jaenchen Hans Harald Gilles Gabriele Diekert Rudolf K. Thauer 《Helvetica chimica acta》1982,65(3):828-865
Factor F430 from Methanogenic Bacteria: Structure of the Porphninoid Ligand System A structure is proposed for F430M, a non-cristalline methanolysis product of isolates of the nickel-containing, porphinoid factor F430 from Methanobacterium thermoautotrophicum. Crucial to the structure determination are five incorporation experiments with M. thermoautotrophicum (strain Marburg) in which the specifically mono-13C-labeled biosynthetic precursors (2-13C), (3-13C), (4-13C)-, (5-13C) ALA (ALA = δ-amino-levulinic acid) and L-(methyl-13C)methionine were incorporated into F430 with high efficiency. The 13C-NMR,-spectra of the specifically labeled F430M samples derived therefrom, together with the UV./VIS. spectral data of F430M, contain all the information necessary for the deduction of the constitution of the F430M chromophore, assuming the established pattern of porphinoid biosynthesis to be operative in F430 biosynthesis. 1H-NMR. spectroscopy and, in particular, 1H-NMR.-NOE-difference spectroscopy corroborates and completes the constitutional assignments and, furthermore, makes possible an almost complete derivation of the molecule's relative configuration. Schemes 3 and 4 summarize the results of 1H-NMR. spectroscopy, presenting them within the context of the proposed structure for F430M. The assignment of absolute configuration implied in the formula is given preference because of F430M's very close structural and (assumed) biosynthetic relationship to sirohydrochlorin and vitamin B12 (with respect to ring C, the assignment is based on degradative evidence). According to the proposed structure, the nickel complex F430M possesses an uroporphinoid (Type III) ligand skeleton with an additional carbocyclic ring and a chromophore system not previously encountered among natural porphinoids. It can be considered to be a (tetrahydro) derivative of the corphin system, combining structural elements of both porphyrins and corrins. 相似文献