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51.
[Reaction: see text] We report a one-step, racemization-free method for the diversification of peptide thiazoles via direct lithiation of the thiazole ring. The method is compatible with N-Boc, N-trityl, carboxylic ester, and carboxamide protecting groups and has been used to directly functionalize the thiazole ring of cyclopeptide natural products. 相似文献
52.
Summary The rhodium(I) carboxylates,trans-RhO2CR(CO)(PPh3)2 (R = C6F5, C6Cl5,p-HC6F4,m-HC6F4,o-HC6F4,p-McOC6F4, 4,5-H2C6F3, 3,5-H2C6F3, or 2,6-F2C6H3, have been prepared by reaction of RhH(CO)(PPh3)3 with the appropriate polyhalogenobenzoic acids in ethanol and/or by reaction oftrans-RhCl(CO)(PPh3)2 with the appropriate thallous carboxylates in benzene. Decarboxylations with formation of polyhalogenoarylrhodium(I) compounds,trans-RhR(CO)(PPh3)2 (R = C6F5, C6Cl5,p-HC6F4,m-HC6F4,p-MeOC6F4, 4,5-H2C6F3 or 3,5-H2C6F3), have been achieved either by decomposition of the corresponding rhodium(I) carboxylates in pyridine or by reaction oftrans-RhCl(CO)(PPh3)2 and the thallous carboxylates in pyridine, but the derivatives R =o-HC6F4 or 2,6-F2C6H3 could not be obtained by this method. The rate of decarboxylation decreased in the sequence R = C6F5 >p-MeOC6F4 >p-HC6F4 >m-HC6F4 > 4,5-H2C6F3 > 3,5-H2C6F3.Part 1, ref. 10.Preliminary communication, ref. 9. 相似文献
53.
54.
Xuejun Pan Dan Xie Neil Gilkes David J. Gregg Jack N. Saddler 《Applied biochemistry and biotechnology》2005,124(1-3):1069-1079
Pretreatment of Douglas-fir by steam explosion produces a substrate containing approx 43% lignin. Two strategies were investigated
for reducing the effect of this residual lignin on enzymatic hydrolysis of cellulose: mild alkali extraction and protein addition.
Extraction with cold 1% NaOH reduced the lignin content by only approx 7%, but cellulose to glucose conversion was enhanced
by about 30%. Before alkali extraction, addition of exogenous protein resulted in a significant improvement in cellulose hydrolysis,
but this protein effect was substantially diminished after alkali treatment. Lignin appears to reduce cellulose hydrolysis
by two distinct mechanisms: by forming a physical barrier that prevents enzyme access and by non-productively binding cellulolytic
enzymes. Cold alkali appears to selectively remove a fraction of lignin from steam-exploded Douglas-fir with high affinity
for protein. Corresponding data for mixed softwood pretreated by organosolv extraction indicates that the relative importance
of the two mechanisms by which residual lignin affects hydrolysis is different according to the pre- and post-treatment method
used. 相似文献
55.
56.
Towards the aim of creating a functional mimic of isopenicillin N synthase, a small molecule designed to coordinate around iron(II) and model the enzyme active site has been prepared in nine synthetic steps from 2,6-bis(hydroxymethyl)pyridine, (S)-(+)-mandelic acid and pivaldehyde. One aspartate, two histidines and a water ligand in the natural enzyme are replaced by an α-hydroxy acid, pyridine and aniline in the model compound. Additionally, a free thiol designed to simulate the enzyme substrate, δ-(l-α-aminoadipoyl)-l-cysteinyl-d-valine, is linked to the ligand by a three carbon chain. We postulate that in the presence of molecular oxygen, the complex formed between this synthetic ligand and iron(II) will display oxidative chemistry similar to that observed in the active site of isopenicillin N synthase. 相似文献
57.
Adams RD Captain B Herber RH Johansson M Nowik I Smith JL Smith MD 《Inorganic chemistry》2005,44(18):6346-6358
The reaction of Re2(CO)8[mu-eta2-C(H)=C(H)Bu(n)](mu-H) with Ph3SnH at 68 degrees C yielded the new compound Re2(CO)8(mu-SnPh2)2 (10) which contains two SnPh2 ligands bridging two Re(CO)(4) groups, joined by an unusually long Re-Re bond. Fenske-Hall molecular orbital calculations indicate that the bonding in the Re2Sn2 cluster is dominated by strong Re-Sn interactions and that the Re-Re interactions are weak. The 119Sn M?ssbauer spectrum of 10 exhibits a doublet with an isomer shift (IS) of 1.674(12) mm s(-1) and a quadrupole splitting (QS) of 2.080(12) mm s(-1) at 90 K,characteristic of Sn(IV) in a SnA2B2 environment. The IS is temperature dependent, -1.99(14) x 10(-4) mm s(-1) K(-1); the QS is temperature independent. The temperature-dependent properties are consistent with the known Gol'danskii-Kariagin effect. The germanium compound Re2(CO)8(mu-GePh2)2 (11) was obtained from the reaction of Re2(CO)8[mu-eta2-C(H)=C(H)Bu(n)](mu-H) with Ph3GeH. Compound 11 has a structure similar to that of 10. The reaction of 10 with Pd(PBu(t)3)2 at 25 degrees C yielded the bis-Pd(PBu(t)3) adduct, Re2(CO)8(mu-SnPh2)2[Pd(PBu(t)3)]2 (12); it has two Pd(PBu(t)3) groups bridging two of the four Re-Sn bonds in 10. Fenske-Hall molecular orbital calculations show that the Pd(PBu(t)3) groups form three-center two-electron bonds with the neighboring rhenium and tin atoms. The mono- and bis-Pt(PBu(t)3) adducts, Re2(CO)8(mu-SnPh2(2)[Pt(PBu(t)3)] (13) and Re2(CO)8(mu-SnPh2)2[Pt(PBu(t)3)]2 (14), were formed when 10 was treated with Pt(PBu(t)3)2. A mono adduct of 11, Re2(CO)8(mu-GePh2)2[Pt(PBu(t)3)] (15), was obtained similarly from the reaction of 11 with Pt(PBu(t)3)2. 相似文献
58.
Focant JF Reiner EJ Macpherson K Kolic T Sjödin A Patterson DG Reese SL Dorman FL Cochran J 《Talanta》2004,63(5):1231-1240
Comprehensive two-dimensional gas chromatography with isotope-dilution time-of-flight mass spectrometry (GC × GC-IDTOFMS) was used to measure polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated dibenzofuran (PCDF), and coplanar polychlorinated biphenyl (cPCB) concentrations in ash, sediment, vegetation, and fish samples. The GC × GC capability was achieved by using a quad jet, dual stage, thermal modulator. Zone compression of the GC peaks from modulation resulted in a significant increase of the signal intensity over classical GC-IDTOFMS. The GC × GC column set used an Rtx-Dioxin 2 phase as the first dimension (1D) and an Rtx-500 as the second dimension (2D). The chromatographic separation of the 17 PCDD/Fs and the 4 cPCBs was attained in 1D except for 2,3,7,8-TCDD and CB126 for which deconvoluted ion currents (DIC) were required to be reported separately. The Rtx-500 phase separated the bulk matrix interfering compounds from the target analytes in 2D. The instrumental limit of detection (iLODs) was 0.5 pg for 2,3,7,8-TCDD. The calibration curves showed good correlation coefficients for all the compounds investigated in the concentration range of 0.5–200 pg. GC × GC-IDTOFMS results compared favorably to those from conventional isotope-dilution one-dimensional gas chromatography-high resolution mass spectrometry (GC-IDHRMS). The comprehensive mass analysis of the TOFMS further permitted the identification of other contaminants of concern in the samples. 相似文献
59.
A fully automated nanoelectrospray tandem mass spectrometric method for analysis of Caco-2 samples 总被引:1,自引:0,他引:1
Van Pelt CK Zhang S Fung E Chu I Liu T Li C Korfmacher WA Henion J 《Rapid communications in mass spectrometry : RCM》2003,17(14):1573-1578
Caco-2 cells offer a means to rapidly screen permeability of drug candidates, allowing pharmaceutical companies to eliminate candidates unable to cross the intestinal barrier early in the discovery process. This screening process is typically performed by conventional liquid chromatography/tandem mass spectrometry (LC/MS/MS), which can require time-consuming method development. An alternative to LC/MS/MS, automated nanoelectrospray tandem mass spectrometry (nanoESI-MS/MS), is introduced. This novel approach requires an off-line ZipTip desalting step followed by automated nanoESI-MS/MS, using the NanoMate 100 and ESI Chip. In addition to reduced method development time, automated nanoESI-MS/MS also offers no carry-over between samples, low sample consumption, and ease-of-use as compared with conventional pulled-capillary nanoelectrospray. Furthermore, the infusion system described has the potential to be high-throughput. A comparison of Caco-2 samples analyzed both by LC/MS/MS and by automated nanoESI-MS/MS is presented. The permeability and recovery data of the two compounds analyzed in this study obtained from conventional LC/MS/MS and by automated nanoESI-MS/MS were in excellent agreement. 相似文献
60.
The aim of the present work was to improve the discriminatory potential, and hence the probative value, of Y-STR-based testing by extending the set of Y chromosome STR loci available for forensic casework. In accordance with the requirements of a Y chromosome multiplex analytical system developed specifically for forensic casework use, we have sought to maximize the number of loci able to be co-amplified, ensure appropriate assay sensitivity (1–2 ng of input genomic DNA), balance inter-locus signals and minimize confounding female DNA artifacts. Two Y chromosome STR systems, multiplex I (MPI) and multiplex II (MPII), have been developed which permit the robust co-amplification of 18 Y-STRs. The loci include DYS19, DYS385(a) and (b), DYS388, DYS389I and II, DYS390, DYS391, DYS392, DYS393, DYS425, DYS434, DYS437, DYS438, DYS439, Y-GATA-C4, Y-GATA-A7.1 (DYS460) and Y-GATA-H4. The two multiplex systems are robust over a wide range of primer, magnesium, and DNA polymerase concentrations and perform well under a variety of cycling conditions. Complete male haplotypes can be obtained with as little as 100–250 pg of template DNA. Although a limited number of female DNA artifacts are observed in mixed stains in which the male DNA comprises 1/100 of the total, the male profile is easily discernible. Slightly modified versions of MPI and MPII demonstrate a significant reduction in female artifacts. Thus, it may not be necessary to employ a differential extraction strategy to obtain a male haplotype (or haplotypes in the case of multiple male donors) in cases of sexual assault. The potential utility of MPI and MPII for forensic casework is exemplified by their ability to dissect out the male haplotype in post-coital vaginal swabs and to determine the number of male donors in mixed semen stains.This study has emphasized the need for novel Y-STR multiplexes developed for forensic use to undergo a series of validation exercises that go beyond simply optimizing the PCR reaction conditions. Specifically, stringent performance checks on their efficacy need to be carried out using casework-type specimens in order to determine potential confounding effects from female DNA. 相似文献