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991.
Ivleva VB Sapp LM O'Connor PB Costello CE 《Journal of the American Society for Mass Spectrometry》2005,16(9):1552-1560
Metastable decomposition of ions generated in matrix-assisted laser desorption/ionization (MALDI) mass spectrometers complicates analysis of biological samples that have labile bonds. Recently, several academic laboratories and manufacturers of commercial instruments have designed instruments that introduce a cooling gas into the ion source during the MALDI event and have shown that the resulting vibrational cooling stabilizes these labile bonds. In this study, we compared stabilization and detection of desorbed gangliosides on a commercial orthogonal time-of-flight (oTOF) instrument with results we reported previously that had been obtained on a home-built Fourier transform mass spectrometer. Decoupling of the desorption/ionization from the detection steps resulted in an opportunity for desorbing thin-layer chromatography (TLC)-separated gangliosides directly from a TLC plate without compromising mass spectral accuracy and resolution of the ganglioside analysis, thus coupling TLC and oTOF mass spectrometry. The application of a declustering potential allowed control of the matrix cluster and matrix adduct formation, and, thus, enhanced the detection of the gangliosides. 相似文献
992.
993.
Wind JJ Papp L Happel M Hahn K Andriole EJ Poutsma JC 《Journal of the American Society for Mass Spectrometry》2005,16(7):1151-1161
A new version of the single-reference-extended kinetic method is presented in which direct entropy correction is incorporated. Results of calibration experiments with the monodentate base pyridine and the bidentate base ethylenediamine are presented for which the method provides proton affinities in excellent agreement with published values and reasonable predictions for the protonation entropies. The method is then used to determine the proton affinity and protonation entropy of the non-protein amino acid beta-oxalylaminoalanine (BOAA). The PA of BOAA is found to be 933.1 +/- 7.8 kJ/mol and a prediction for the protonation entropy of -39 J mol(-1) K(-1) is also obtained, indicating a significant degree of intramolecular hydrogen bonding in the protonated form. These results are supported by hybrid density functional theory calculations at the B3LYP/6-311++G**//B3LYP/6-31+G* level. They indicate that the preferred site of protonation is the alpha-nitrogen atom (PA = 935.0 kJ/mol) and that protonated BOAA has a strong hydrogen bond between the hydrogen on the alpha-amino group and one of the carbonyl oxygen atoms on the side chain. 相似文献
994.
在密度泛函理论的框架下, 采用嵌入点电荷簇模型研究了O2在MgO(001)完整和缺陷表面上的吸附.用电荷自洽的方法确定了点电荷的值.计算结果表明, O2倾向吸附在低配位的角Mg2+端.并且发现, 当O2为平躺吸附时,键长有较大的拉伸,将有利于O2的解离.同时,分别计算了使用裸簇和嵌入表观±2.0 e点电荷簇模型时的吸附能,并与采用电荷自洽方法的计算值进行了比较.结果表明,电荷自洽方法更能有效反映簇周围的环境,得到的计算结果能够较好地与实验值吻合.最后,分别计算了不同吸附情况下O2的振动频率. 相似文献
995.
Görög S 《Analytical and bioanalytical chemistry》2003,377(5):852-862
A system is proposed for the classification of related organic impurities in drugs and drug products including among others (separated and non-separated) intermediates, various kinds of by-products, among them products of different side reactions, epimeric/diastereomeric, enantiomeric impurities, impurities in natural products, and finally degradation products. Examples are taken mainly from the author's own experience and from among the named impurities in the European Pharmacopoeia with focus on impurities in hydrocortisone, prednisolone, enalapril maleate, lisinopril, ethynodiol diacetate, pipecuronium bromide, cimetidine, and ethynylsteroids. The methodological aspects of impurity profiling from the detection to the identification/structure elucidation and quantitative determination of impurities are briefly summarized.This paper is Part 23 in the series "Estimation of impurity profiles of drugs and related materials". For Part 22 see ref. [1]. 相似文献
996.
The suitability of high-resolution two-dimensional gel electrophoresis for barley cultivar discrimination and for classification with respect to their malting properties was studied. Seed proteins of 14 barley cultivars with different malting qualities were extracted with urea/dithiothreitol/Nonidet P-40 buffer and subjected to two-dimensional gel electrophoresis with immobilized pH gradients in the first dimension (IPG-DALT). The results of IPG-DALT were compared to the protein patterns obtained by a standard technique, sodium dodecyl sulfate polyacrylamide gel electrophoresis of hordeins. Sodium dodecyl sulfate-gel electrophoresis yielded seven different "B" and four different "C" hordein patterns; "A" and "D" hordein patterns were uniform in all cultivars tested. Four cultivars could be distinguished unequivocally, the others were classified into three groups containing between two and five cultivars. In contrast to these findings. IPG-DALT yielded three different "A", eight different "B", four different "C" and two different "D" hordein patterns. When the "A", "B", "C" and "D" hordein patterns were combined, ten cultivars exhibited unique hordein patterns whereas the remaining ones were classified into two groups containing two cultivars each. Moreover, when albumin and globulin proteins were used for evaluation in addition to the hordeins, all cultivars could be discriminated by IPG-DALT. IPG-DALT, performed on small-scale and/or ready-made gels, proved to be an ideal complementary system to one-dimensional electrophoretic methods for routine seed testing purposes because of its speed, reliability, and simplicity. IPG-DALT was also applied to study the relationship between the different polypeptide patterns and the malting quality. Although cultivars with identical one-dimensional protein patterns but different malting quality could be successfully differentiated by IPG-DALT, a direct correlation between specific protein spots or protein patterns to the malting quality was not found within the cultivars tested. 相似文献
997.
Olga?V.?DorofeevaEmail author Igor?F.?Shishkov Lev?V.?Vilkov István?Hargittai 《Structural chemistry》2005,16(6):617-628
The conformational composition of gaseous MTMNB and the molecular structures of the rotational forms have been studied by
electron diffraction at 130∘C aided by results from ab initio and density functional theory calculations. The conformational potential energy surface
has been investigated by using the B3LYP/6-31G(d,p) method. As a result, six minimum-energy conformers have been identified.
Geometries of all conformers were optimized using MP2/6-31G(d,p), B3LYP/6-31G(d,p), and B3LYP/cc-pVTZ methods. These calculations
resulted in accurate geometries, relative energies, and harmonic vibrational frequencies for all conformers. The B3LYP/cc-pVTZ
energies were then used to calculate the Boltzmann distribution of conformers. The best fit of the electron diffraction data
to calculated values was obtained for the six conformer model, in agreement with the theoretical predictions. Average parameter
values (ra in angstroms, angle α in degrees, and estimated total errors given in parentheses) weighted for the mixture of six conformers
are r(C–C) = 1.507(5), r(C–C)ring, av = 1.397(3), r(C–S)av = 1.814(4), r(C–N) = 1.495(4), r(N–O)av = 1.223(3), ∠(C–C–C)ring = 116.0–122.5, ∠ C6–C4–C7 = 118.2(4), ∠ C–C–S = 113.6(6), ∠ C–S–C = 98.5(12), ∠ N–C–C4 = 121.9(3), ∠(O–N–C)av = 116.8(3), ∠ O–N–O = 127.0(4). Torsional angles could not be refined. Theoretical B3LYP/cc-pVTZ torsional angles for the
rotation about C–N bond, φC−N, were found to be 30.5–36.5∘ for different conformers. As to internal rotation about C–C and C–S bonds, values of φC−C = 68–118∘ and φC−S = 66–71∘ were obtained for the three most stable conformers with gauche orientation with respect to these bonds.
Some conclusions of this work were presented in a short communication in Russ. J. Phys. Chem. 2005, 79, 1701. 相似文献
998.
Le‐Qing Fan Ji‐Huai Wu 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(11):m548-m549
The title compound, [ZnI2(bipy)]n (bipy is 4,4′‐bipyridine, C10H8N2), has been prepared by the hydrothermal reaction of ZnI2 and bipy at 433 K. Each Zn atom is coordinated by two N atoms from two different bipy ligands and by two I atoms in a distorted tetrahedral geometry, with Zn—N distances ranging from 2.068 (7) to 2.101 (8) Å and Zn—I distances ranging from 2.5471 (13) to 2.5673 (13) Å. The molecular structure features a zigzag polymeric chain. Face‐to‐face π–π stacking interactions between adjacent bipy ligands stabilize the structure. 相似文献
999.
Namieśnik J Zabiegała B Kot-Wasik A Partyka M Wasik A 《Analytical and bioanalytical chemistry》2005,381(2):279-301
The current state-of-the-art of passive sampling and/or extraction methods for long-term monitoring of pollutants in different environmental compartments is discussed in this review. Passive dosimeters that have been successfully used to monitor organic and inorganic contaminants in air, water, sediments, and soil are presented. The application of new approaches to the determination of pollutants at the sampling stage is discussed. The main milestones in the development of passive techniques for sampling and/or extraction of analytes, and in biomonitors used in environmental analysis, are summarized in this review. Passive samplers and biomonitors are compared. 相似文献
1000.
Chun-Long?Chen Andrea?M.?Goforth Mark?D.?Smith William?R.?Gemmill Cheng-Yong?Su Hans-Conrad?zur?LoyeEmail author 《Journal of Cluster Science》2005,16(4):477-487
A new flexible ligand, 1,5-bis(8-oxaquinoline)-3-methylpentane (C5MeOQ), was synthesized and used in the preparation of the
new dinuclear copper(II) complex 1: [Cu2(μ-OMe)2(μ-C5MeOQ)(NO3)2]. Single crystal X-ray analysis revealed that complex 1 contains dinuclear Cu(II) units with bridging methoxo groups. The ligand, C5MeOQ, takes on a bent conformation in order to
connect to and complete the square planar coordination environment of both copper centers; and one of the two Cu(II) centers
adopts a distorted square planar configuration as a result of the ligand conformation. The magnetic properties of complex
1 were investigated by variable-temperature magnetic susceptibility measurements in the 100–300 K range. Interestingly, the
susceptibility data for complex 1 reveals that the compound is diamagnetic, suggesting the presence of very strong antiferromagnetic coupling interactions
between the adjacent Cu(II) centers. 相似文献