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1.
Q M Weng W M Hammargren D Slowikowski K H Schram K Z Borysko L L Wotring L B Townsend 《Analytical biochemistry》1989,178(1):102-106
The effect of trimethylsilyl (TMS) derivatization on detection limits of mononucleotides in fast atom bombardment-mass spectrometry (FAB-MS) was examined. FAB-MS methods were developed to optimize sensitivity using adenosine 5'-monophosphate as a model compound and then applied to reference standards of two clinically important nucleotides: tricyclic nucleoside-5'-monophosphate (TCNMP) and 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP). The detection limit for the TMS derivative of TCNMP was 2.5-5 ng/microliters and less than 2.5 ng/microliters for FdUMP as its TMS derivative. This is greater than two orders of magnitude more sensitive than the FAB-MS analysis of the corresponding free compounds. These low detection limits for the TMS derivatives were obtained using a narrow scan range, signal averaging, detection in the negative ion mode, and 3-nitrobenzyl alcohol as the matrix. Hydrolysis of one or more of the labile TMS groups did occur, with the extent of hydrolysis being greatest in the more protic matrices. 相似文献
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Synthesis of the tricyclic nucleoside 8-amino-6-N-methyl-2-(beta-D-ribo-furanosyl)-1,2,3,5,6,7-hexaazaacena phthylene (5) has been accomplished by the ring closure of an appropriately substituted pyrazolo[3,4-d]-pyrimidine nucleoside followed by the requisite chemical conversions. The formation, isolation and structural elucidation of two unexpected nucleosides formed by a reductive ring cleavage of the hexaazaacenaphthylene ring system is discussed. A comparison of the antitumor and biological activity of 5 with the structurally related tricyclic pentaazaacenaphthylene nucleoside which is currently in phase II clinical trials at the 5'-phosphate pro-drug is also presented. 相似文献
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Background
Distance-based methods are popular for reconstructing evolutionary trees thanks to their speed and generality. A number of methods exist for estimating distances from sequence alignments, which often involves some sort of correction for multiple substitutions. The problem is to accurately estimate the number of true substitutions given an observed alignment. So far, the most accurate protein distance estimators have looked for the optimal matrix in a series of transition probability matrices, e.g. the Dayhoff series. The evolutionary distance between two aligned sequences is here estimated as the evolutionary distance of the optimal matrix. The optimal matrix can be found either by an iterative search for the Maximum Likelihood matrix, or by integration to find the Expected Distance. As a consequence, these methods are more complex to implement and computationally heavier than correction-based methods. Another problem is that the result may vary substantially depending on the evolutionary model used for the matrices. An ideal distance estimator should produce consistent and accurate distances independent of the evolutionary model used. 相似文献8.
Background
Profile hidden Markov model (HMM) techniques are among the most powerful methods for protein homology detection. Yet, the critical features for successful modelling are not fully known. In the present work we approached this by using two of the most popular HMM packages: SAM and HMMER. The programs' abilities to build models and score sequences were compared on a SCOP/Pfam based test set. The comparison was done separately for local and global HMM scoring. 相似文献9.
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Charge scan reveals an extended region at the intracellular end of the GABA receptor pore that can influence ion selectivity 总被引:2,自引:0,他引:2 下载免费PDF全文
Selective permeability is a fundamental property of ion channels. The Cys-loop receptor superfamily is composed of both excitatory (ACh, 5-HT) and inhibitory (GABA, glycine) neurotransmitter-operated ion channels. In the GABA receptor, it has been previously shown that the charge selectivity of the integral pore can be altered by a single mutation near the intracellular end of the second transmembrane-spanning domain (TM2). We have extended these findings and now show that charge selectivity of the anionic rho1 GABA receptor can be influenced by the introduction of glutamates, one at a time, over an 8-amino acid stretch (-2' to 5') in the proposed intracellular end of TM2 and the TM1-TM2 intracellular linker. Depending on the position, glutamate substitutions in this region produced sodium to chloride permeability ratios (P(Na)+(/Cl)-) varying from 0.64 to 3.4 (wild type P(Na)+(/Cl)- = 0). In addition to providing insight into the mechanism of ion selectivity, this functional evidence supports a model proposed for the homologous nicotinic acetylcholine receptor in which regions of the protein, in addition to TM2, form the ion pathway. 相似文献