Abstract L-cysteine is a stimulating starting product for the generation of transient sulfenic acids that add to suitable acceptors, allowing formation of sulfoxides showing a biologically active residue. For instance, N-(tert-butoxycarbonyl)-L-cysteine methyl ester furnished in few steps (R)-2-tert-butoxycarbonylamino-2-methoxycarbonyl-ethanesulfenic acid, which was readily converted into (R,SS)-(2-tert-butoxycarbonylamino-2-methoxycarbonyl-ethylsulfinyl)ethene, the methyl ester of Boc-protected nor-alliin. 相似文献
LptA is a periplasmic protein involved in the transport of lipopolysaccharide (LPS) from the inner membrane (IM) to the outer membrane (OM) of Gram-negative bacteria. Growing evidence supports a model in which LptA assembles into oligomers, forming a physical bridge connecting IM and OM. This work investigates assembly and architecture of LptA oligomers. Circular dichroism and “native” electrospray-ionization ion-mobility mass spectrometry (ESI-IM-MS) are employed to test concentration dependence of LptA structural features and to analyze the morphology of higher-order aggregates. The results show that LptA progressively assembles into rod-like oligomers without fixed stoichiometry, and grows by an n + 1 mechanism up to at least the pentamer. The oligomerization process induces disorder-to-order transitions in the polypeptide chain. Comparison with crystallographic and computational data suggests that these conformational changes likely involve short disordered regions at the N- and C-termini of monomeric LptA. The protein response to thermal denaturation displays strong concentration dependence, indicating that oligomerization increases protein stability. LptA conformational stability can also be enhanced by in vitro LPS binding. The genesis of these fibrillar structures could be relevant for the correct transport of LPS across the bacterial periplasm. 相似文献
The aim of this work is to search for innovative solutions to avoid physical migration of the stabilizers from plastic films. New families of macromolecular additives bearing tuned amounts of a selected functionality were explored, in particular, novel random copolymers of ethylene with 1‐olefin co‐units bearing an efficient antioxidant and/or anti‐UV moiety covalently bonded to a mono‐ or disubstituted olefinic bond. Polyolefin blends containing the novel macromolecular additives showed higher degradation temperatures with respect to the neat matrix and its blends with 2,6‐t‐butyl‐4‐methoxyphenol (BHA). Aging tests showed that the novel antioxidants also constitute a protection against photo‐oxidation.
In a previous article, Rigano et al. established a new linear retention index system for the identification of triacylglycerols by liquid chromatography methods only on the basis of the retention behavior and independently from many experimental parameters. In that work, a database of 209 compounds was built, but only 54 of them, typical of vegetable oils, were confirmed by mass spectrometry. The aim of the present research is to extend the applicability of the novel approach to more complex samples, such as fish lipid extracts, and assess the complementarity between mass spectromtery and retention information to achieve univocal identification. With this purpose, a new software was implemented to make the identification process easy and automatic as in gas chromatography‐mass spectrometry where the retention index filter is added in the spectral search to discriminate between compounds with similar mass spectrometry spectra. A total of 69 species were identified and, thanks to their baseline separation obtained by an ultra high performance liquid chromatography method, a semiquantification was also performed. The species under investigation were Dicentrarchus labrax, coming from aquaculture and the wild. Some differences in their native lipid composition were observed, probably related to a different diet. A major number of samples would be necessary to confirm such a preliminary finding. 相似文献
A novel approach is presented to determine four bisphenols in water and urine samples, employing magnetic dispersive solid‐phase extraction combined with liquid chromatography and diode array detection. A modified zeolite‐based magnetic composite was used as an efficient sorbent, combining the advantages of magnetic materials with the remarkable properties of zeolites. A multivariate optimization design was employed to optimize some experimental factors affecting magnetic dispersive solid‐phase extraction. The method was evaluated under optimized conditions (i.e., amount of sorbent, 50 mg; sample pH, unadjusted; NaCl concentration, 1.25%; extraction and elution time, 2 min; eluent solvent, ethanol; eluent solvent volume, 400 µL), obtaining good linearity with correlation coefficients ranging between 0.995 and 0.999 (N = 5) (from 2 to 250 µg/L for bisphenol A, bisphenol AP, and bisphenol P and from 5 to 250 µg/L for bisphenol AF). Method repeatability was assessed obtaining coefficients of variation between 3 and 11% (n = 6). Finally, the method was applied to spiked real samples, obtaining for water samples relative recoveries between 83 and 105%, and for urine samples between 81 and 108% for bisphenol A, bisphenol AP, and bisphenol AF, and between 47 and 59% for bisphenol P. 相似文献
We apply Wigner-transform techniques to the analysis of difference methods for Schr?dinger-type equations in the case of
a small Planck constant. In this way we are able to obtain sharp conditions on the spatial-temporal grid which guarantee convergence
for average values of observables as the Planck constant tends to zero. The theory developed in this paper is not based on local and global error estimates and does not depend on whether caustics develop or not. Numerical test examples are presented to help interpret the theory.
Received April 17, 1997 / Revised version received February 23, 1998 相似文献