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Potential difficulties associated with background silver salt clusters during matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of nonpolar polymers are reported. Silver salt cluster ions were observed from m/z 1500 to 7000 when acidic, polar matrices, such as 2,5-dihydroxybenzoic acid (DHB), all-trans-retinoic acid (RTA) or 2-(4-hydroxyphenylazo)benzoic acid (HABA), were used for the analysis of nonpolar polymers. These background signals could be greatly reduced or eliminated by the use of nonpolar matrices such as anthracene or pyrene. Representative examples of these background interferences are demonstrated during the analysis of low molecular weight nonpolar polymers including polybutadiene and polystyrene. Nonpolar polymers analyzed with acidic, polar matrices (e.g., RTA) and silver cationization reagents can yield lower quality mass spectral results when interferences due to silver clusters are present. Replacing the polar matrices with nonpolar matrices or the silver salts with copper salts substantially improved the quality of the analytical results. In addition, it was found that silver contamination cannot be completely removed from standard stainless steel sample plates, although the presence of silver contamination was greatly reduced after thorough cleaning of the sample plate with aluminum oxide grit. Carry-over silver may cationize polymer samples and complicate the interpretation of data obtained using nonpolar matrices in the absence of added cationization reagents.  相似文献   
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It is shown that every regular 3-valent polyhedral graph whose faces are all 5-gons and 6-gons contains a cycle through at least 4/5 of its vertices.  相似文献   
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The oxidation of alkylsilane monolayers on Au has been studied by X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact-angle measurements, and scanning tunneling microscopy. Exposure of the monolayers at 298 K to pure O(2) or H(2)O (>5 x 10(-5) Torr and >150 000 L) does not cause oxidation. Ambient atmosphere only causes oxidation if direct sight lines are maintained to the sample. Ozone exposure results in rapid monolayer oxidation. Oxidation initially occurs only at the Si atom, resulting in formation of a cross-linked siloxane monolayer that retains alkyl surface termination. Prolonged ozone exposures result in the oxidation and subsequent loss of the alkyl chain.  相似文献   
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[reaction: see text] The Passerini reaction of N-protected amino aldehydes, isonitriles, and TFA using pyridine-type bases proceeds under mild conditions and directly affords alpha-hydroxy-beta-amino amide derivatives in moderate to high yields. These adducts are readily hydrolyzed to alpha-hydroxy-beta-amino carboxylic acids. Application of these key intermediates to concise syntheses of P(1)-alpha-ketoamide protease inhibitors is illustrated.  相似文献   
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Abstract— Single-strand breaks are produced in the phosphodiester backbone of ultraviolet-light-irradiated 5–bromodeoxyuridine-containing DNA (BU-DNA) after treatment with alkali. No radiation dependent breakage is observed in thymine-containing DNA (thy-DNA). The relative yields of breaks terminated by 5'-hydroxyl and 5'-phosphate groups was determined by measuring the rate of phosphorylation achieved with polynucleotide kinase in BU-DNA single strands before and after treatment with alkaline phosphatase. The ratio of 5'-phosphate to 5' hydroxyl groups ranged from 2.3 to 2.9 in different experiments. When cysteamine was present during irradiation no new end groups were produced.
In order to identify the nucleoside(s) at the 5'-termini, phosphate groups were removed with alkaline phosphatase and the 5'-hydroxyl groups were phosphorylated with polynucleotide kinase. Electrophoresis of enzymatic digests showed a single 32P-labeled component migrating more rapidly than any of the four usual 5'-mononucleotides. Upon column chromatography this component resolved into a major peak coincident with 5'-dUMP and a lesser unidentified constituent. No 5'-dBUM32P was observed among these nucleotides.  相似文献   
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