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71.
Yeung BK Nakao Y Kinnel RB Carney JR Yoshida WY Scheuer PJ Kelly-Borges M 《The Journal of organic chemistry》1996,61(20):7168-7173
Four cyclic peptides, kapakahines A-D, were isolated from the marine sponge Cribrochalina olemda. Their structures including complete stereochemistry were elucidated by spectral analysis and chemical degradation. The unique structural feature of these peptides is the lack of an amide linkage between two tryptophan residues. Instead the ring is closed by a bond from the indole nitrogen of Trp-1 to the beta-carbon of Trp-2. 相似文献
72.
S. M. Frank D. J. Bunnell K. P. Carney S. G. Johnson 《Journal of Radioanalytical and Nuclear Chemistry》1995,194(1):35-39
A time-of-flight mass spectrometer has been designed and is presently being installed within a radiological controlled hot cell at Argonne National Laboratory-West. Direct solid sampling is performed by laser ablation or glow discharge sputtering followed by supersonic expansion into the TOFMS source chamber. Once the atom/ion beam enters the source chamber, enhanced ionization can be accomplished by laser ionization or electron impact. An assortment of samples may be analyzed ranging from irradiated nuclear fuel elements and cladding materials to eutectic salts, radioactive waste materials and environmental samples. 相似文献
73.
A new front-end electronics architecture with sub-nanosecond intrinsic timing resolution has recently been incorporated into a 16 slice LSO PET/CT scanner for imaging applications in oncology. The new electronics are designed to work optimally with the lutetium orthosilicate (LSO) scintillator. Clinical performance of the LSO PET/CT is examined before and after upgrading to the new PICO 3D electronics, and compared with results using the NEMA NU 2 standard for evaluating scanner performance. Improved noise-equivalent count rates are seen in clinical studies, and reduced scatter fractions are observed, consistent with the increased lower-level energy threshold used to reject scatter events in the upgraded configuration. 相似文献
74.
75.
M. Baubillier I. J. Bloodworth A. Burns J. N. Carney G. F. Cox U. Dore J. B. Kinson K. Knudson F. Levy P. J. Negus B. Y. Oh E. Quercigh J. M. Scarr G. A. Smith O. Villalobos Baillie M. F. Votruba J. Whitmore R. Zitoun 《Zeitschrift fur Physik C Particles and Fields》1983,17(4):309-317
A partial wave analysis of theK \(\bar K\) system produced by 8.25 GeV/cK ? mesons in the reaction \(K^ - p \to K\bar K\Lambda ^{ 0} \) has been performed, taking into account the information provided by the Λ0 decay. Thef′ region is dominated byD 0 (?) andD 1 (+) waves. We see no evidence for the production of new 0++ states in the mass region 1.05 to 1.75 GeV. 相似文献
76.
This report describes the synthesis, structural characterization, and polymerization behavior of a series of chromium(II) and chromium(III) complexes ligated by tris(2-pyridylmethyl)amine (TPA), including chromium(III) organometallic derivatives. For instance, the combination of TPA with CrCl(2) yields monomeric (TPA)CrCl(2) (1). A similar reaction of CrCl(2) with TPA, followed by chloride abstraction with NaBPh(4) or NaBAr(F)(4) (Ar(F) = 3,5-(CF(3))(2)C(6)H(3)), provides the weakly associated cationic dimers [(TPA)CrCl](2)[BPh(4)](2) (2A) and [(TPA)CrCl](2)[BAr(F)(4)](2) (2B), respectively. X-ray crystallographic analysis reveals that each chromium(II) center in 1, 2A, and 2B is a tetragonally elongated octahedron; such Jahn-Teller distortions are consistent with the observed high spin (S = 2) electronic configurations for these chromium(II) complexes. Likewise, reaction of CrCl(3)(THF)(3) with TPA, followed by anion metathesis with NaBPh(4) or NaBAr(F)(4), yields the monomeric, cationic chromium(III) complexes [(TPA)CrCl(2)][BPh(4)] (4A) and [(TPA)CrCl(2)][BAr(F)(4)] (4B), respectively. Treatment of 4A with methyl and phenyl Grignard reagents produces the cationic chromium(III) organometallic derivatives [(TPA)Cr(CH(3))(2)][BPh(4)] (5) and [(TPA)CrPh(2)][BPh(4)] (6), respectively. Similar reactions of 4A with organolithium reagents leads to intractable solids, presumably due to overreduction of the chromium(III) center. X-ray crystallographic analysis of 4A, 5, and 6 confirms that each possesses a largely undistorted octahedral chromium center, consistent with the observed S = (3)/(2) electronic ground states. Compounds 1, 2A, 2B, 4A, 4B, 5, and 6 are all active polymerization catalysts in the presence of methylalumoxane, producing low to moderate molecular weight high-density polyethylene. 相似文献
77.
Tang L Ward S Chung L Carney JR Li Y Reid R Katz L 《Journal of the American Chemical Society》2004,126(1):46-47
The epothilones, originally isolated from the myxobacterium Sorangium cellulosum, are macrocyclic compounds that are synthesized by a modular polyketide synthase, an enzyme complex composed of six large, multifunctional proteins. The penultimate intermediates in epothilone production, and the products of the PKS-catalyzed reactions, are epothilones D and C, which contain a 12,13-cis-double bond. The 12 and 13 positions of epothilones are generated during the fourth elongation step that is governed by module 4. Module 4 does not contain a dehydratase (DH) domain, which is required for dehydration to create the double bond. A DH domain, present in module 5 and presumed to act in the fifth elongation step at the 10 and 11 positions, was proposed to act as well to generate the 12,13-cis-double bond. Inactivation of the DH domain in module 5 resulted in the production of 10,11-dehydro-13-hydroxyepothilone D as the major product, confirming that DH5 is required for 12,13 dehydration. A mechanistic model based on domain skipping and modular stuttering is presented to explain the basis for the iterative DH5 activity observed. 相似文献
78.
T. A. Armstrong I. J. Bloodworth J. N. Carney R. Childs B. R. French B. Ghidini J. B. Kinson V. Lenti A. Palano G. Vassiliadis O. Villalobos Baillie M. F. Votruba G. Zito R. Zitoun WA Collaboration 《Zeitschrift fur Physik C Particles and Fields》1987,35(2):167-171
In a search for glueballs and exotic states decaying into baryons and antibaryons we have investigated the production of baryon antibaryon pairs produced centrally in the reactions π+/pp → π+/p(X 0)p at 85 GeV/c. In particular, channels whereX 0 goes to \(p\bar p,p\bar p\pi ^ + \pi ^ - ,p\bar p2\pi ^ + 2\pi ^ - \) and \(\Lambda \bar \Lambda \) have been observed. No significant new structure is observed in the mass spectra. 相似文献
79.
Schultz AW Oh DC Carney JR Williamson RT Udwary DW Jensen PR Gould SJ Fenical W Moore BS 《Journal of the American Chemical Society》2008,130(13):4507-4516
Two new diketopiperazine dipeptides, cyclomarazines A and B, were isolated and characterized along with the new cyclic heptapeptide cyclomarin D from the marine bacterium Salinispora arenicola CNS-205. These structurally related cyclic peptides each contain modified amino acid residues, including derivatives of N-(1,1-dimethylallyl)-tryptophan and delta-hydroxyleucine, which are common in the di- and heptapeptide series. Stable isotope incorporation studies in Streptomyces sp. CNB-982, which was first reported to produce the cyclomarin anti-inflammatory agents, illuminated the biosynthetic building blocks associated with the major metabolite cyclomarin A, signifying that this marine microbial peptide is nonribosomally derived largely from nonproteinogenic amino acid residues. DNA sequence analysis of the 5.8 Mb S. arenicola circular genome and PCR-targeted gene inactivation experiments identified the 47 kb cyclomarin/cyclomarazine biosynthetic gene cluster (cym) harboring 23 open reading frames. The cym locus is dominated by the 23 358 bp cymA, which encodes a 7-module nonribosomal peptide synthetase (NRPS) responsible for assembly of the full-length cyclomarin heptapeptides as well as the truncated cyclomarazine dipeptides. The unprecedented biosynthetic feature of the megasynthetase CymA to synthesize differently sized peptides in vivo may be triggered by the level of beta oxidation of the priming tryptophan residue, which is oxidized in the cyclomarin series and unoxidized in the cyclomarazines. Biosynthesis of the N-(1,1-dimethyl-2,3-epoxypropyl)-beta-hydroxytryptophan residue of cyclomarin A was further illuminated through gene inactivation experiments, which suggest that the tryptophan residue is reverse prenylated by CymD prior to release of the cyclic peptide from the CymA megasynthetase, whereas the cytochrome P450 CymV installs the epoxide group on the isoprene of cyclomarin C post-NRPS assembly. Last, the novel amino acid residue 2-amino-3,5-dimethylhex-4-enoic acid in the cyclomarin series was shown by bioinformatics and stable isotope experiments to derive from a new pathway involving condensation of isobutyraldehyde and pyruvate followed by S-adenosylmethionine methylation. Assembly of this unsaturated, branched amino acid is unexpectedly related to the degradation of the environmental pollutant 3-(3-hydroxyphenyl)propionic acid. 相似文献
80.