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Distribution by Scientific Domains

Life Sciences	50%
Chemistry	50%

Selected Abstracts

Anaerobic transformation of compounds of technical toxaphene.

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2003

Abstract Technical toxaphene (Melipax) and the single compounds of technical toxaphene (CTTs) 2,2,5- endo, 6- exo, 8,8,9,10- octachlorobornane (B8-806), 2,2,5- endo, 6- exo, 8,9,9,10-octachlorobornane (B8-809), 2,2,5,5,8,9,9,10,10-nonachlorobornane (B9- 1025), 2- endo, 3- exo, 5- endo, 6- exo, 8,8,9,10,10-nonochlorobornane (B9-1679), 2- endo, 3- exo, 5- endo, 6- exo, 8,9,10,10-octachlorobornane (B8-1414), 2- endo, 3- exo, 5- endo, 6- exo, 8,8,9,10-octachlorobornane (B8-1412), and 2- exo, 3- endo, 5- exo, 9,9,10,10-heptachlorobornane (B7-1453) were treated with suspensions of the anaerobic bacterium Dehalospirillum multivorans. After 7 d, more than 50% of technical toxaphene was transformed, and the relative amount of early eluting CTTs increased. After 16 d, only 2- exo, 3- endo, 6- exo, 8,9,10-hexachlorobornane (B6-923), 2- endo, 3- exo, 5- endo, 6- exo, 8,9,10-heptachlorobornane (B7-1001), and a few minor penta- and hexachloro-CTTs were detected in the samples. The result of the transformation was comparable with observations in naturally contaminated sediments and soil. However, the performance with D. multivorans was more simple and reproducible, as well as faster, than use of soil, sediment, or anaerobic sewage sludge. In agreement with reports in the literature, reductive dechlorination at geminal chlorine atoms (gem -Cls) was found to be the major CTT transformation pathway. Experiments conducted with CTTs and gem -Cls at both primary and secondary carbons clarified that the initial Cl -> H substitution takes place at the secondary carbon C2. Furthermore, the 2- endo -Cl position was preferably substituted with hydrogen. In the case of B8-806, the dechlorination at the secondary carbon C2 was approximately 20-fold faster than the subsequent, slow reduction at the primary carbon C8. The three different formerly unknown heptachloro-CTTs, 2- exo, 3- endo, 6- exo, 8,9,9,10-heptachlorobornane (B7-1473), 2- exo, 3- endo, 6- endo, 8,9,9,10-hepatchlorobornane (B7-1461), and 2- exo, 3- endo, 6- exo, 8,8,9,10-heptachlorobornane (B7-1470) were found as intermediates of the B8-806/809 transformation. Treatment of B9-1679 with D. multivorans indicated that gem -Cls on the bridge (C8 and C9) are dechlorinated faster than gem -Cls on the bridgehead (C10). [source]

Photoregulation of DNA transcription by using photoresponsive T7 promoters and clarification of its mechanism

FEBS JOURNAL, Issue 6 2010
Xingguo Liang
With the use of photoresponsive T7 promoters tethering two 2,-methylazobenzenes or 2,,6,-dimethylazobenzenes, highly efficient photoregulation of DNA transcription was obtained. After UV-A light irradiation (320,400 nm), the rate of transcription with T7 RNA polymerase and a photoresponsive promoter involving two 2,,6,-dimethylazobenzenes was 10-fold faster than that after visible light irradiation (400,600 nm). By attaching a nonmodified azobenzene and 2,,6,-dimethylazobenzene at the two positions, respectively, and by utilizing the different cis,trans thermal stability between cis -nonmodified azobenzene and cis- 2,,6,-dimethylazobenzene, four species of T7 promoter (cis,cis, trans,cis, cis,trans, and trans,trans) were obtained. The four species showed transcriptional activity in the order of cis,cis > cis,trans > trans,cis > trans,trans. Kinetic analysis revealed that the Km for the cis,cis promoter (both of the introduced azobenzene derivatives were in the cis form) and T7 RNA polymerase was 68 times lower than that for the trans,trans form, indicating that high photoregulatory efficiency was mainly due to a remarkable difference in affinity for RNA polymerase. The present approach is promising for the creation of biological tools for artificially controlling gene expression, and as a photocontrolled system for supplying RNA fuel for RNA-powered molecular nanomachines. [source]

The Lifetimes of Pharaonis Phoborhodopsin Signaling States Depend on the Rates of Proton Transfers,Effects of Hydrostatic Pressure and Stopped Flow Experiments,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008
Takashi Kikukawa
Pharaonis phoborhodopsin (ppR), a negative phototaxis receptor of Natronomonas pharaonis, undergoes photocycle similar to the light-driven proton pump bacteriorhodopsin (BR), but the turnover rate is much slower due to much longer lifetimes of the M and O intermediates. The M decay was shown to become as fast as it is in BR in the L40T/F86D mutant. We examined the effects of hydrostatic pressure on the decay of these intermediates. For BR, pressure decelerated M decay but slightly affected O decay. In contrast, with ppR and with its L40T/F86D mutant, pressure slightly affected M decay but accelerated O decay. Clearly, the pressure-dependent factors for M and O decay are different in BR and ppR. In order to examine the deprotonation of Asp75 in unphotolyzed ppR we performed stopped flow experiments. The pH jump-induced deprotonation of Asp75 occurred with 60 ms, which is at least 20 times slower than deprotonation of the equivalent Asp85 in BR and about 10-fold faster than the O decay of ppR. These data suggest that proton transfer is slowed not only in the cytoplasmic channel but also in the extracellular channel of ppR and that the light-induced structural changes in the O intermediate of ppR additionally decrease this rate. [source]

The effect of temperature and lipid on the conformational transition of gramicidin A in lipid vesicles,

BIOPOLYMERS, Issue 4 2005
Ta-Hsien Lin
Abstract The present study investigated the effect of temperature and lipid/peptide molar ratio on the conformational changes of the membrane peptide gramicidin A from a double-stranded helix to a single-stranded helical dimmer in 1,2-dimyristoyl-glycerol-3-phosphochloine (DMPC) vesicles. Tryptophan fluorescence spectroscopy results suggested that the conformational transition fitted a three-state (two-step) "folding" model. Rate constants, k1 and k2, were determined for each of the two steps. Since k1 and k2 increased with an increase in temperature, we hypothesized that the process corresponded to the breakage and formation of the backbone hydrogen bonds. The k1 was from 10 to 45 folds faster than k2, except for lipid/peptide molar ratios above 89.21, where k2 increased rapidly. At molar ratios below 89.21, k2 was insensitive to changes in lipid concentration. To account for this phenomenon, we proposed that while the driving interaction at high molar ratios is between the indole rings of the tryptophan residues and the lipid head groups, at low molar ratios there may be an intermolecular interaction between the tryptophan residues that causes gramicidin A to form an organized aggregated network. This aggregated network, caused by the tryptophan,tryptophan interaction, may be the main effect responsible for the slow down of the conformation change. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 179,186, 2005 [source]