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91.
We present a new device for quantifying gases or gas mixtures based on the simple principle of bubble counting. With this device, we can follow reaction kinetics down to volume step sizes of 8–12 μL. This enables the accurate determination of both time and size of these gas quanta, giving a very detailed kinetic analysis. We demonstrate this method and device using ammonia borane hydrolysis as a model reaction, obtaining Arrhenius plots with over 300 data points from a single experiment. Our device not only saves time and avoids frustration, but also offers more insight into reaction kinetics and mechanistic studies. Moreover, its simplicity and low cost open opportunities for many lab applications.  相似文献   
92.
To test the effect of NH−C=S groups (Scheme 1) on the stability of β-peptide secondary structures, we have synthesized three β-thiohexapeptide analogues of H-(β-HVal-β-HAla-β-HLeu)2-OH ( 1 ) with one, two, and three C=S groups in the N-terminal positions (cf. 2 – 4 and model in Fig. 1). The first C=S group was introduced selectively by treatment with Lawesson reagent of Boc-β-dipeptide esters ( 6 and 8 ). A series of fragment-coupling steps (with reagents as for the corresponding sulfur-free building blocks) and another thionation reaction led to the title compounds with a C=S group in residues 1, 1, and 3, as well as 1, 2, and 3 of the β-hexapeptide (Schemes 2 and 3). The sulfur derivatives, especially those with three C=S groups, were much more soluble in organic media than the sulfur-free analogues (>1000-fold in CHCl3; Table 1). The UV and CD spectra (in CHCl3, MeOH, and H2O) of the new compounds were recorded and compared with those of the parent β-hexapeptide 1 (Figs. 2 – 4); they indicate the presence of more than one secondary structure under the various conditions. Most striking is a pronounced exciton splitting (Δλ ca. 20 nm, amplitude up to +121000) of the ππ*C=S band near 270 nm with the β-trithiohexapeptide (with and without terminal protecting groups), and strong, so-called `primary solvent effects', in the CD spectra. The CD spectrum of the β-dithiohexapeptide 3 undergoes drastic changes upon irradiation with 266-nm laser light of a MeOH solution (Fig. 5). The NMR structure in CD3OH of the unprotected β-trithiohexapeptide 4 was determined to be an (M)-314-helix (Fig. 7), very similar to that of the non-thionated analogue (cf. 1 ). NMR and mass spectra of the β-hexapeptides with C=S and with C=O groups are compared (Figs. 6 and 8).  相似文献   
93.
Herein, the direct C-5 fluoroalkylselenolation of 8-aminoquinolines is described. The Pd-catalyzed reaction is performed with the in situ generated fluoroalkylselenyl chloride and various 8-aminoquinoline amides or sulfonamides. The desired products were formed in moderate to excellent yields.  相似文献   
94.
95.
Polymer‐solvent compounds can be formed from homogeneous solutions obtained at high temperature but also by solvent diffusion at the solid interface of a bulk polymer at room temperature. This phenomenon we call “physical” corrosion as it modifies the surface structure without chemical change. Here we report preliminary experiments indicating that the thermodynamics seems to be little dependent upon the path used while the morphology is.  相似文献   
96.
UV‐induced formation of pyrimidine dimers in DNA is a major deleterious event in both eukaryotic and prokaryotic cells. Accumulation of cyclobutane pyrimidine dimers and pyrimidine (6‐4) pyrimidone photoproducts can lead to cell death or be at the origin of mutations. In skin, UV induction of DNA damage is a major initiating event in tumorigenesis. To counteract these deleterious effects, all cell types possess DNA repair machinery, such as nucleotide excision repair and, in some cell types, direct reversion. Different analytical approaches were used to assess the efficiency of repair and decipher the enzymatic mechanisms. We presently review the information provided by chromatographic methods, which are complementary to biochemical assays, such as immunological detection and electrophoresis‐based techniques. Chromatographic assays are interesting in their ability to provide quantitative data on a wide range of damage and are also valuable tools for the identification of repair intermediates.  相似文献   
97.
98.
Starting from (R)-(+)- and from (S)-(?)-camphor the chiral alcohols 1, 2 and 3 have been prepared; their acrylates II underwent TiCl2(OR)2-promoted Diels-Alder additions to cyclopentadiene giving efficiently in a predictable manner either the (2R)- or the (2S)-adducts III with up to virtually quantitative asymmetric induction.  相似文献   
99.
100.
On doubling metric measure spaces endowed with a strongly local regular Dirichlet form, we show some characterisations of pointwise upper bounds of the heat kernel in terms of global scale-invariant inequalities that correspond respectively to the Nash inequality and to a Gagliardo–Nirenberg type inequality when the volume growth is polynomial. This yields a new proof and a generalisation of the well-known equivalence between classical heat kernel upper bounds and relative Faber–Krahn inequalities or localised Sobolev or Nash inequalities. We are able to treat more general pointwise estimates, where the heat kernel rate of decay is not necessarily governed by the volume growth. A crucial role is played by the finite propagation speed property for the associated wave equation, and our main result holds for an abstract semigroup of operators satisfying the Davies–Gaffney estimates.  相似文献   
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