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991.
Friscić T Fábián L Burley JC Reid DG Duer MJ Jones W 《Chemical communications (Cambridge, England)》2008,(14):1644-1646
Comparison of structure and hydration stability of pairs of chiral and racemic binary cocrystals indicates that the racemic solid is more stable than the chiral one; we illustrate that this difference might arise from intermolecular (crystal packing) factors in one case, while intramolecular (molecular conformation) factors are more significant in the other. 相似文献
992.
Charged donor-acceptor [3]catenanes comprising the pi-accepting cyclobis(paraquat-4,4'-biphenylene) and pi-donating aromatic crown ether macrocycles have been prepared in high yields using thermodynamically controlled dynamic nucleophilic substitution. 相似文献
993.
B. Janković B. Adnađević J. Jovanović 《Journal of Thermal Analysis and Calorimetry》2008,92(3):821-827
An isothermal dehydration of equilibrium swollen poly(acrylic acid) hydrogel in the temperature range from 306 to 361 K was
investigated. The specific parameters connected with shape of the conversion curves were defined. The activation parameters
(E, lnA) of the isothermal dehydration of equilibrium swollen poly(acrylic acid) hydrogel were calculated, using Johnson-Mehl-Avrami
(JMA), ‘initial rate’ and ’stationary point’ methods. The reaction models for the investigated dehydration are determined
using the ‘model-fitting’ method. It was established that both, the reaction model and activation parameters of the hydrogel
dehydration were completely different for the isothermal process than for the non-isothermal one. It was found that the increase
in dehydration temperature lead to the changes in isothermal kinetic model for the investigated hydrogel dehydration. It was
established that the apparent activation energy (E) of hydrogel dehydration is similar to the value of the molar enthalpy
of water evaporation. 相似文献
994.
The essential oil of Helichrysum italicum (Roth) G. Don (everlasting or Immortelle essential oil) was isolated by hydrodistillation and analysed by GC and GCMS. Forty four compounds were identified. The main components were alpha-pinene(12.8%), 2-methyl-cyclohexyl pentanoate (11.1 %), neryl acetate (10.4%), 1,7-di-epi-alpha-cedrene (6.8%) and other compounds. The oil was fractionated and ester-containing fraction was hydrolysed with KOH/H(2)SO(4). The liberated volatiles were analysed by GC and GC-MS: three phenols and twenty seven volatile carboxylic acids were identified[70% low fatty acids (C(2)-C(5)), 15% C(10)-C(12) acids and 15% other acids]. The main acids were acetic acid (24.3%) propanoic acid (17.2%), 2-methylpropanoic acid (11.4%),dodecanoic acid (8.7%), 2-methylbutanoic acid (8.3%), (Z)-2-methylbutenoic acid(5.1%) and decanoic acid (4.6%). With respect to the identified bonded carboxylic acids,the minimal number of esters in the oil was twenty seven, but their overall quantity was probably larger due to different possible combinations of alcohols with acids to form esters. On the other hand, only six main esters were identified in the oil before fractionation and hydrolysis. 相似文献
995.
996.
997.
Petković H Sandmann A Challis IR Hecht HJ Silakowski B Low L Beeston N Kuscer E Garcia-Bernardo J Leadlay PF Kendrew SG Wilkinson B Müller R 《Organic & biomolecular chemistry》2008,6(3):500-506
The production of epothilone mixtures is a direct consequence of the substrate tolerance of the module 3 acyltransferase (AT) domain of the epothilone polyketide synthase (PKS) which utilises both malonyl- and methylmalonyl-CoA extender units. Particular amino acid motifs in the active site of AT domains influence substrate selection for methylmalonyl-CoA (YASH) or malonyl-CoA (HAFH). This motif appears in hybrid form (HASH) in epoAT3 and may represent the molecular basis for the relaxed specificity of the domain. To investigate this possibility the AT domains from modules 2 and 3 of the epothilone PKS were examined in the heterologous DEBS1-TE model PKS. Substitution of AT1 of DEBS1-TE by epoAT2 and epoAT3 both resulted in functional PKSs, although lower yields of total products were observed when compared to DEBS1-TE (2% and 11.5% respectively). As expected, epoAT3 was significantly more promiscuous in keeping with its nature during epothilone biosynthesis. When the mixed motif (HASH) of epoAT3 within the hybrid PKS was mutated to HAFH (indicative of malonyl-CoA selection) it resulted in a non-productive PKS. When this mixed motif was converted to YASH (indicative of methylmalonyl-CoA selection) the selectivity of the hybrid PKS for methylmalonyl-CoA showed no statistically significant increase, and was associated with a loss of productivity. 相似文献
998.
Iliyas A Eić M Zahedi-Niaki MH Vasenkov S 《The journal of physical chemistry. B》2008,112(12):3821-3825
The tracer zero-length column (ZLC) method has been employed to study the diffusion of toluene in one-dimensional ZSM-12 and SAPO-5 zeolites. A significant deviation in the shape of the measured tracer exchange curves from monoexponential behavior was observed for toluene diffusion in both adsorbents in the limit of long-time asymptotes. In contrast, water/ZSM-12 and acetylene/SAPO-5 systems exhibit tracer exchange curves that are close to monoexponential behavior. Monoexponential curves are usually observed for systems obeying normal (Fickian) diffusion. Such diffusion is expected for the latter two systems because the diameters of both sorbates are less than the radii of their corresponding host channels. The differences in the shape of the tracer exchange curves for large and small sorbates can be explained by assuming the occurrence of anomalous, single-file diffusion for large sorbates in narrow, one-dimensional channels. 相似文献
999.
An interpretative strategy (factorial design experimentation+total resolution analysis+chromatogram simulation) was employed to optimize the separation of 16 polycyclic aromatic hydrocarbons (PAHs) (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benzo(a)anthracene, benzo(k)fluoranthene, benzo(b)fluoranthene, benzo(a)pyrene, indeno(1,2,3-c,d)pyrene, dibenzo(a,h)anthracene, benzo(g,h,i)perylene) in temperature-programmed gas chromatography (GC). Also, the retention behavior of PAHs in the same GC system was studied by a feed-forward artificial neural network (ANN). GC separation was investigated as a function of one (linear temperature ramp) or two (linear temperature ramp+the final hold temperature) variables. The applied interpretative approach resulted in rather good agreement between the measured and the predicted retention times for PAHs in both one and two variable modeling. The ANN model, strongly affected by the number of input experiments, was shown to be less effective for one variable used, but quite successful when two input variables were used. All PAHs, including difficult to separate peak pairs (benzo(k)fluoranthene/benzo(b)fluoranthene and indeno(1,2,3-c,d)pyrene/dibenzo(a,h)anthracene), were separated in a standard (5% phenyl-95% dimethylpolysiloxane) capillary column at an optimum temperature ramp of 8.0 degrees C/min and final hold temperature in the range of 260-320 degrees C. 相似文献
1000.
D. Krstić M. Çolović M. B. Kralj P. Trebše K. Krinulović V. Vasić 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2008,82(4):663-668
The influence of malathion and its four main degradation products found in irradiated solutions (malaoxon, isomalathion, diethyl
maleate and O,O-dimethyl phosphate) on acetylcholinesterase (AChE) of free and immobilized bovine erythrocytes was investigated.
The concentration-dependent responses to malathion and related organophosphates, malaoxon and isomalathion, of both AChE bioassays
used were obtained. The IC
50 values for free and immobilized AChE (3.7 ± 0.2) × 10−4 M/(1.6 ± 0.1) × 10−4, (2.4 ± 0.3) × 10−6/(3.4 ± 0.1) × 10−6 M, and (3.2 ± 0.3) × 10−6 M/(2.7 ± 0.2) × 10−6 M were obtained in the presence of malathion, malaoxon and isomalathion, respectively. However, diethyl maleate inhibited
AChE activity at concentrations ≥ 10 mM, while O,O-dimethyl phosphate did not noticeably affect enzyme activity at all investigated
concentrations. The relation between the structure of the compounds and their ability to inhibit enzyme activity was discussed.
The article is published in the original. 相似文献