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991.
Louis A. Carpino Eberhard Krause Calin Dan Sferdean Heinz Fabian Michael Beyermann 《Tetrahedron letters》2004,45(40):7519-7523
Recently a 26-mer peptide 1 incorporating Ser and Thr was described as a ‘difficult’ sequence that could not be synthesized by standard methods. If the first Ser residue was used to incorporate a depsipeptide unit, the resulting hybrid was readily assembled. The 26-mer ester was then converted to the native peptide by an O→N acyl shift. The technique may be general for other systems containing appropriate Ser and Thr units and was demonstrated here for the case of the amyloid peptide Aβ(1-42). 相似文献
992.
The conformation and aggregation behavior of synthetic Alzheimer's amyloid peptides (Abeta) has been investigated using hydrogen-deuterium exchange measured by electrospray ionization mass spectrometry and matrix-assisted laser desorption/ionization mass spectrometry. Mass spectrometric fragmentation of deuterated Abeta peptides was carried out by collision-induced dissociation, inlet fragmentation, and post-source decay. In contrast to the C-terminally truncated peptides Abeta(1-40) and Abeta(1-36) showing full hydrogen-deuterium exchange, Abeta(1-42) and the pyroglutamyl peptide Pyr(3)-Abeta(3-42) produced more complex signal patterns resulting from the formation of beta-sheet-structured oligomers having 18-20 strongly protected protons. Using mass spectrometric fragmentation the results show that the reduced isotope exchange of Abeta(1-42) can be attributed to the central part of the chain comprising residues 8-23. This confirms involvement of the hydrophobic binding domain LVFFA in the course of Abeta aggregation and demonstrates that hydrogen-deuterium exchange in combination with mass spectrometry is well suited for structural analysis of monomeric and reversibly associated amyloid peptides using picomole quantities of material. 相似文献
993.
Abstract— A method is described to determine spectral properties of phytochrome in vivo. For photochrome in 7-day-old dark-grown Cucurbita pepo L. seedlings the mole fraction of the far-red-absorbing form (Pfr) present at photoequilibrium at 664 nm was found to be 0.76 ± 0.02 in vivo. Based on reflectance measurements, the photon fluence rate just below the surface of the cotyledons was calculated. Local rates of photoconversion for known local fluence rates were measured across cotyledons after non-saturating irradiations with wavelengths between 544 and 781 nm and in situ molar photoconversion coefficients were obtained. In contrast to purified oat phytochrome, the in situ molar photoconversion coefficients for Pfr show a strong shoulder between 660 and 700 nm. The maximum of Pfr absorption is at 726 nm. An isosbestic point of phytochrome is found at 686 nm. The mole fraction of Pfr present at photoequilibrium with 686 nm light is 0.58. The ratio of photoconversion quantum yields (that for Pr→ Pfr divided by that for Pfr→ Pr) is 1.38 ± 0.06. 相似文献
994.
Blanchard S Neese F Bothe E Bill E Weyhermüller T Wieghardt K 《Inorganic chemistry》2005,44(10):3636-3656
The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H(2)[L(1-6)], with Ni(OAc)(2).4H(2)O in ethanol in the presence of air yields six four-coordinate species [Ni(L(1-6)(*))(2)] (1-6) where (L(1-6)(*))(1-) represent the monoanionic pi-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-[Ni(L(1)(-3)(*))(2)], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of [Ni(L(5)(*))(2)] (5) and [Ni(L(6)(*))(2)] (6). Both series of mononuclear complexes possess a diamagnetic ground state. The electronic structures of both series have been elucidated experimentally (electronic spectra magnetization data). The square planar complexes 1-3 consist of a diamagnetic central Ni(II) ion and two strongly antiferromagnetically coupled ligand pi-radicals as has been deduced from correlated ab initio calculations; they are singlet diradicals. The tetrahedral complexes 4-6 consist of a paramagnetic high-spin Ni(II) ion (S(Ni) = 1), which is strongly antiferromagnetically coupled to two ligand pi-radicals. This is clearly revealed by DFT and correlated ab initio calculations. Electrochemically, complexes 1-6 can be reduced to form stable, paramagnetic monoanions [1-6](-) (S = (1)/(2)). The anions [1-3](-) are square planar Ni(II) (d,(8) S(Ni) = 0) species where the excess electron is delocalized over both ligands (class III, ligand mixed valency). In contrast, one-electron reduction of 4, 5, and 6 yields paramagnetic tetrahedral monoanions (S = (1)/(2)). X-band EPR spectroscopy shows that there are two different isomers A and B of each monoanion present in solution. In these anions, the excess electron is localized on one ligand [Ni(II)(L(4-6)(*))(L(4-6))](-) where (L(4-6))(2-) is the closed shell dianion of the ligands H(2)[L(4-6)] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes [Ni(II)(L(1,ox))(2)I(2)] (7), [Ni(II)(L(1,ox))(3)](I(3))(2) (8), and trans-[Ni(II)(L(5,ox))(2)(I(3))(2)] (9), which have been characterized by X-ray crystallography; (L(1-)(6,ox)) represent the neutral, two-electron oxidized forms of the corresponding dianions (L(1-6))(2-). The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1-5. 相似文献
995.
Abstract— Slow destruction of the far-red-absorbing form of phytochrome (Pfr ), which has been observed in light-grown oat and maize, occurs in light- and dark-grown Amaranthus, Pharbitis , and Brassica seedlings as well. Destruction of Pfr in these seedlings shows two phases: if a high level of Pfr is produced in dark-grown seedlings, the destruction is fast in the beginning and then slows after a low Pfr level has been reached. Slow Pfr destruction is predominant in light-grown tissue. 相似文献
996.
W. Trombik W. Dünnweber W. Trautmann W. Dahme K. A. Eberhard W. Hering D. Konnerth R. Singh 《Zeitschrift für Physik A Hadrons and Nuclei》1980,296(2):187-188
The spin alignment Pzz of12C(2 1 + ) produced in12C+12C inelastic scattering at bombarding energies in the region of known resonances between Ecm=18.5 and 32.8 MeV has been determined with a γ-ray detector positioned in the direction of the scattering normal. The values obtained in the centers of three resonances as well as those obtained off-resonance are spread over a range between ?.05 and +.80 with no clear indication in favour of simple pictures of molecular resonances. 相似文献
997.
tBu2P–PLi–PtBu2·2THF reacts with [cis‐(Et3P)2MCl2] (M = Ni, Pd) yielding [(1,2‐η‐tBu2P=P–PtBu2)Ni(PEt3)Cl] and [(1,2‐η‐tBu2P=P–PtBu2)Pd(PEt3)Cl], respectively. tBu2P– PLi–PtBu2 undergoes an oxidation process and the tBu2P–P–PtBu2 ligand adopts in the products the structure of a side‐on bonded 1,1‐di‐tert‐butyl‐2‐(di‐tert‐butylphosphino)diphosphenium cation with a short P–P bond. Surprisingly, the reaction of tBu2P–PLi–PtBu2·2THF with [cis‐(Et3P)2PtCl2] does not yield [(1,2‐η‐tBu2P=P–PtBu2)Pt(PEt3)Cl]. 相似文献
998.
Lucien Eberhard Jean-Pierre Lampin Francois Mathey 《Journal of organometallic chemistry》1974,80(1):109-118
The reaction of n-BuLi with phenyldiferrocenylphosphine oxide yields a mixture of two isomeric dianions. This mixture reacts with Me3SiCl, Br2, CO2, PhCHO and PhCHNPh to give bifunctional products. With CoCl2, Me2SiCl2, Ph2SiCl2, Bu2SnCl2 and PhCOOEt, the same mixture gives cyclic products. In almost every case each of these compounds is a mixture of two isomers corresponding to the original compound. The characteristic patterns of the PMR spectra are in full agreement with the structures proposed for the isomeric dianions. 相似文献
999.
ANALYSIS OF THE BINDING OF PHYTOCHROME TO PARTICULATE FRACTIONS 总被引:2,自引:0,他引:2
Abstract— The binding of phytochrome to receptor sites in a particulate fraction of maize coleoptiles has been studied as a function of the level of far-red-absorbing phytochrome (Pfr ) offered in vivo and in vitro. Evidence is presented that the binding is cooperative. The degree of cooperativity expressed by the Hill coefficient of the binding function is the same (1–6) both in vivo and in vitro , whereas the Hill coefficient of the state function in vivo is significantly higher (2-1). The highest Hill coefficient (3–5) was found for the in vitro binding function in squash hooks. 相似文献
1000.
Klemm C Schröder S Glückmann M Beyermann M Krause E 《Rapid communications in mass spectrometry : RCM》2004,18(22):2697-2705
The identification of phosphorylation sites is essential for a full understanding of the cellular functions of proteins. However, mass spectrometric analysis is often hampered by the low abundance of phosphoproteins, the difficulty of obtaining full sequence coverage by specific proteolysis reactions, and the low ionization efficiency of phosphopeptides compared with their non-phosphorylated analogs. In the present work a beta-elimination/Michael addition was used to replace the phosphate groups of pSer or pThr by a group which gives rise to an enhanced ionization efficiency. In order to find optimum reaction conditions, beta-elimination/Michael addition was examined using phosphorylated model peptides. Whereas complete elimination of phosphate could be achieved by treatment with barium hydroxide in organic solvents such as ethanol or acetonitrile, the yield of the Michael adduct strongly depended on the nucleophile and the peptide sequence. Reaction with 2-phenylethanethiol, p-bromophenethylamine and ethylenediamine clearly resulted in products showing higher matrix-assisted laser desorption/ionization (MALDI) signal intensities compared with those of the corresponding phosphorylated precursors. The method was successfully used to identify phosphorylated sequences of ovalbumin and human Stat1 by in-gel derivatization with 2-phenylethanethiol and subsequent peptide mass fingerprint analysis of the trypsin digests. 相似文献