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31.
Yin H Chacon A Porter NA Masterson DS 《Journal of the American Society for Mass Spectrometry》2007,18(5):807-816
Protein identification is routinely accomplished by peptide sequencing using mass spectrometry (MS) after enzymatic digestion.
Site-specific chemical modification may improve peptide ionization efficiency or sequence coverage in mass spectrometry. We
report herein that amino group of lysine residue in peptides can be selectively modified by reaction with a peroxycarbonate
and the resulting lysine peroxycarbamates undergo homolytic fragmentation under conditions of low-energy collision-induced
dissociation (CID) in electrospray ionization (ESI) and matrix-assisted laser desorption and ionization (MALDI) MS. Selective
modification of lysine residue in peptides by our strategy can induce specific peptide cleavage at or near the lysine site.
Studies using deuterated analogues of modified lysine indicate that fragmentation of the modified peptides involves apparent
free-radical processes that lead to peptide chain fragmentation and side-chain loss. The formation of a-, c-, or z-types of
ions in MS is reminiscent of the proposed free-radical mechanisms in low-energy electron capture dissociation (ECD) processes
that may have better sequence coverage than that of the conventional CID method. This site-specific cleavage of peptides by
free radical- promoted processes is feasible and such strategies may aid the protein sequencing analysis and have potential
applications in top-down proteomics. 相似文献
32.
A series of air- and water-stable tertiary phosphine-stabilized arsenium salts of the type R(3)P-->AsR(2)(+)PF(6)(-) has been isolated. In the crystal structures of two chiral triarylphosphine complexes of prochiral methylphenylarsenium hexafluorophosphate, the stereochemistry around arsenic is trigonal pyramidal with the phosphorus atom occupying the apical position, the As-P bond being orthogonal to the plane of the trigonal (lone-pair included) arsenium ion: Ph(3)P-->AsMePh(+) PF(6)(-), P2(1)/c, a = 10.7775(2) A, b = 17.7987(3) A, c = 13.3797(2) A, beta = 109.066(1) degrees, V = 2425.78(7) A(3), T = 200 K, Z = 4; Ph(2)(2-MeOC(6)H(4))P-->AsMePh(+) PF(6)(-), P1, a = 10.8077(2) A, b = 10.9741(2) A, c = 13.5648(2) A, alpha = 99.0162(9) degrees, beta = 105.2121(9) degrees, gamma = 116.4717(9) degrees, V = 1318.11(5) A(3), T = 200 K, Z = 2. The arsenium ion in each case appears to be further stabilized by conjugation of the lone pair with the phenyl group, with which the arsenic and methyl-carbon atoms are almost coplanar. In the crystal structure of the 2-(methoxymethylphenyl)diphenylphosphine adduct of methylphenylarsenium hexafluorophosphate, there operates a counteractive chelate effect in which anchimeric oxygen coordination to arsenic destabilizes the arsenic-phosphorus bond in the six-membered chelate ring. Although they are stable, phosphine-stabilized arsenium salts undergo rapid phosphine exchange and attack at arsenic by anionic carbon and oxygen nucleophiles to give tertiary arsines and arsinous acid esters, respectively, with liberation of the phosphine. 相似文献
33.
We exploit the approximate (broken) symmetries of time translation, time reversal, and Hamiltonian structure to obtain general scaling laws governing the process of pattern formation in weakly damped Faraday waves. Using explicit parameter symmetries we determine, for the case of two-frequency forcing, how the strength of observed three-wave interactions depends on the frequency ratio and on the relative phase of the two driving terms. These symmetry-based predictions are verified for numerically calculated coefficients, and help explain the results of recent experiments. 相似文献
34.
35.
Experimental data for chromium(III) complex ions on the spectroscopic properties, photoreactions, and intermolecular energy transfer are analyzed in terms of the primary photophysical and photochemical processes. It is suggested that the lowest excited quartet state,4T2g inO
h
symmetry, is the immediate precursor to photoaquation and that the lowest doublet state,2E, is substitution inert except via the4
T
2g
state reached by back intersystem crossing.
Dedicated to the memory of Professor Hans-Ludwig Schläfer. 相似文献
Zusammenfassung Experimentelle Daten von Chrom-(III)-Komplex-Ionen in bezug auf spektroskopische Eigenschaften, Photoreaktionen und intermolekularen Energieübertragung wurden mit dem Begriffssystem der photophysikalischen und photochemischen Primärprozesse analysiert. Und zwar wird vorgeschlagen, daß der tiefste angeregte Quartettzustand,4 T 2g inO h -Symmetrie, der unmittelbare Vorläufer der Photoaquatisierung ist und daß der tiefste Doublettzustand,2E, inert gegenüber Substitutionen ist, außer über den4 T 2g Zustand, wenn er durch Re-intersystem crossing erreicht wird.
Dedicated to the memory of Professor Hans-Ludwig Schläfer. 相似文献
36.
37.
Jeou-Shyong Wang Jack R. Knox Roger S. Porter 《Journal of Polymer Science.Polymer Physics》1978,16(10):1709-1719
The steady-state and dynamic melt rheology for a series of poly(1-olefins) has been investigated. The series includes poly(1-butene), poly(1-hexene), poly(1-heptene), poly(1-octene), Poly(1-undecene), poly(1-tridecene), poly(1-hexadecene), and poly(1-octadecene). The flow behavior was investigated by use of a Weissenberg rheogoniometer. Measurements on poly(1-butene) were also made using an Instron capillary rheometer. The empirical relationship developed by Cox and Merz was obeyed for the entire series of poly(1-olefins) at all temperatures investigated. Graessley's theory was used to calculate the flow curves for the poly(1-olefins) from the measured molecular weight distributions. The purpose was to investigate the effect of polymer composition on the shear rate dependence of viscosity. It was found that all experimental flow curves except those for poly(1-hexene) can be fitted with the calculated curves from the individual molecular weight distributions. The conclusion is made that flow curves of poly(1-olefins) depend predominately on molecular weight distribution and are essentially independent of side-chain length even for poly(1-olefins) with pendant groups as long as 16 carbon atoms. The low-shear limiting Newtonian viscosity η0 for all poly(1-olefins) was expressed by, η0 = KM? or by η0 = K′P? where M?w is the weight-average molecular weight and P?w is the weight-average degree of polymerization. The K and K′ values obtained decrease systematically as the side chain is increased. 相似文献
38.
Tetsuo Kanamoto Anagnostis E. Zachariades Roger S. Porter 《Journal of Polymer Science.Polymer Physics》1982,20(8):1485-1496
The aliphatic polyamides nylon 6 and 6,6 have been cold drawn by using the new concept of reversible plasticization with NH3. Two aspects were investigated: the effect of NH3 plasticization on the stress-strain behavior of nylon 6,6, and the extent of NH3 absorption as determined by x-ray analysis for the structurally simpler nylon 6. X-ray analysis shows that ammonia is absorbed preferentially by the amorphous phase and that the plasticization has a significant effect on the unit-cell orientation, the small-angle void scattering, and the interplanar spacings between sheets of hydrogen-bonded chains. The effect of plasticization was seen in the lower yield stress, the higher draw ratio (up to 8X) and tensile modulus (10 GPa) not attainable with unplasticized specimens. 相似文献
39.
40.