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51.
Janos P 《Electrophoresis》2003,24(12-13):1982-1992
The separation of lanthanide and actinide elements belongs to one of the most challenging tasks of the separation science, due to a great similarity in their physical and chemical properties. The electrophoretic separation can be accomplished in the presence of suitable complex-forming agents, from which alpha-hydroxyisobutyric acid (HIBA) has been used most often. In the most effective capillary electrophoretic mode--capillary zone electrophoresis (CZE)--a complete separation of lanthanide ions can be accomplished within a few minutes. Various electrophoretic methods can be relatively easily adopted for the determinations of individual lanthanide elements in certain kinds of technical materials, concentrates, precursors, etc., where the high speed and low costs of analysis characteristics of capillary electrophoresis (CE) may be advantageously exploited. Electrophoretic techniques may also be employed for speciation studies, especially for examinations of the behavior of actinides in the environment. 相似文献
52.
The thermodynamic behavior of films of hydrated purple membranes from Halobacterium salinarum and the water confined in it was studied by Fourier transform infrared spectroscopy in the 180–280 K range. Unlike bulk water, water in the thin layers sandwiched between the biological membranes does not freeze at 273 K but will be supercooled to ~256 K. The melting point is unaffected, leading to hysteresis between 250 and 273 K. In its heating branch, a gradually increasing light‐scattering by ice is observed with rate‐limiting kinetics of tens of minutes. Infrared (IR) spectra decomposition provided extinction coefficients for the confined water vibrational bands and their changes upon freezing. Because of the hysteresis, at any given temperature in the 255–270 K range, the interbilayer water could be either liquid or frozen, depending on thermal history. We find that this difference affects the dynamics of the bacteriorhodopsin photocycle in the hysteresis range: the decay of the M and N states and the redistribution between them are different depending on whether or not the water was initially precooled to below the freezing point. However, freezing of interbilayer water does block the M to N transition. Unlike the water, the purple membrane lipids do not undergo any IR‐detectable phase transition in the 180–280 K range. 相似文献
53.
Xanthorhodopsin (XR), the light-driven proton pump of the halophilic eubacterium Salinibacter ruber, exhibits substantial homology to bacteriorhodopsin (BR) of archaea and proteorhodopsin (PR) of marine bacteria, but unlike them contains a light-harvesting carotenoid antenna, salinixanthin, as well as retinal. We report here the pH-dependent properties of XR. The pKa of the retinal Schiff base is as high as in BR, i.e. > or =12.4. Deprotonation of the Schiff base and the ensuing alkaline denaturation cause large changes in the absorption bands of the carotenoid antenna, which lose intensity and become broader, making the spectrum similar to that of salinixanthin not bound to XR. A small redshift of the retinal chromophore band and increase of its extinction, as well as the pH-dependent amplitude of the M intermediate indicate that in detergent-solubilized XR the pKa of the Schiff base counterion and proton acceptor is about 6 (compared to 2.6 in BR, and 7.5 in PR). The protonation of the counterion is accompanied by a small blueshift of the carotenoid absorption bands. The pigment is stable in the dark upon acidification to pH 2. At pH < 2 a transition to a blueshifted species absorbing around 440 nm occurs, accompanied by loss of resolution of the carotenoid absorption bands. At pH < 3 illumination of XR with continuous light causes accumulation of long-lived photoproduct(s) with an absorption maximum around 400 nm. The photocycle of XR was examined between pH 4 and 10 in solubilized samples. The pH dependence of recovery of the initial state slows at both acid and alkaline pH, with pKas of 6.0 and 9.3. The decrease in the rates with pKa 6.0 is apparently caused by protonation of the counterion and proton acceptor, and that at high pH reflects the pKa of the internal proton donor, Glu94, at the times in the photocycle when this group equilibrates with the bulk. 相似文献
54.
Delannay E Toribio A Boudesocque L Nuzillard JM Zèches-Hanrot M Dardennes E Le Dour G Sapi J Renault JH 《Journal of chromatography. A》2006,1127(1-2):45-51
Nowadays, centrifugal partition chromatography (CPC) separations can be routinely achieved at the laboratory scale. The solvent system selection has been made easy, as generic sets of solvent systems are described in publications and books. This approach, however, generally reduces the scope of optimization strategies for two important parameters: selectivity and sample solubility. This can be very limiting for the preparative separation of structurally similar compounds. Multiple dual-mode (MDM) CPC has been developed to provide an easy-to-use alternative technique to circumvent this problem. A MDM separation consists of a succession of dual-mode runs (i.e. multiple inversion of stationary and mobile phase) that can only be achieved because both chromatographic phases are liquids. This original elution mode is thus a semi-continuous process with a classical sample injection and which only requires a single CPC column. Underlying mechanisms of MDM were studied using a model mixture of acenaphthylene and naphthalene. A mixture of two synthetic pairs of diastereomers was then successfully submitted to MDM CPC, in the framework of the synthesis of biologically active compounds. 相似文献
55.
56.
57.
A Ramsey-Type Result for Convex Sets 总被引:1,自引:0,他引:1
Larman David; Matousek Jiri; Pach Janos; Torocsik Jeno 《Bulletin London Mathematical Society》1994,26(2):132-136
Given a family of n convex compact sets in the plane, one canalways choose n of them which are either pairwise disjoint orpairwise intersecting. On the other hand, there exists a familyof n segments in the plane such that the maximum size of a subfamilywith pairwise disjoint or pairwise intersecting elements innlog2/log5 n0·431. 相似文献
58.
Hawryluk RJ Adler H Alling P Ancher C Anderson H Anderson JL Ashcroft D Barnes CW Barnes G Batha S Bell MG Bell R Bitter M Blanchard W Bretz NL Budny R Bush CE Camp R Caorlin M Cauffman S Chang Z Cheng CZ Collins J Coward G Darrow DS DeLooper J Duong H Dudek L Durst R Efthimion PC Ernst D Fisher R Fonck RJ Fredrickson E Fromm N Fu GY Furth HP Gentile C Gorelenkov N Grek B Grisham LR Hammett G Hanson GR Heidbrink W Herrmann HW Hill KW Hosea J Hsuan H Janos A Jassby DL Jobes FC Johnson DW 《Physical review letters》1994,72(22):3530-3533
59.
Alfred Wunderli Janos Zsindely Hans-Jürgen Hansen Hans Schmid 《Helvetica chimica acta》1973,56(3):989-1011
Pure 10β-(trans-2′-butenyl)-17β-hydroxy-estra-1,4-dien-3-one ( 6 ), 10-(trans-2′-butenyl)-2-oxo-Δ1(9),3(4)-hexahydronaphthalene ( 13 ), trans-2′-butenyl 17β-hydroxy-3-estra-1,3,5-(10)-trienyl ether ( 12 ) and trans-2′-butenyl 5,6,7,8-tetrahydro-2-naphthyl ether ( 14 ) were prepared by direct C- and O-alkylation, respectively, of the corresponding phenols (cf. [3] [10]), namely estra-3, 17β-diol and 5,6,7,8-tetrahydro-2-naphthol. The Claisen rearrangement of the ether 14 (200°, 12 h) yielded 53% 1-(1′-methylallyl)- and 34% 3-(1′-methylallyl)-5,6,7,8-tetrahydro-2-naphthol ( 15 and 16 , respectively), whereas in the thermal (120°, 14 h) and in the acid-catalysed (boron trifluoride in ether, 20°, 20 min) reaction of the corresponding dienone 13 nearly equal amounts of 15 (53–54%) and 16 (46%) were formed by thermal and charge-induced aromatic [3s, 3s]-sigmatropic rearrangements [2]. The steroid dienone 6 , on heating at 120°, was rearranged by [3s, 3s]-sigmatropic processes to form 52% of 2-(1′-methylallyl)- and 48% of 4′-(1′-methylallyl)-3, 17β-dihydroxy-estra-1,3,5,(10)-triene ( 7 and 8 , respectively). The steroid phenols 7 and 8 were carefully separated; subsequent hydrogenation (Raney-Ni in alcohol) and ozonolysis yielded 2-methylbutyric acid ( 9 ): from 7 , S-(+)- 9 , and from 8 , R-(?)- 9 , obtained in 88,5 and 88,0% optical purity (cf. [4a]). This means (cf. scheme 2 and Table 2) that both phenols are formed to the extent of at least 94% via a chair-like activated complex, and of at most 6% via a both-like activated complex (ΔΔG = 2.15 kcal/mol). Similarly, the boron trifluoride-induced rearrangement of 6 (born trifluoride in ether, 0°, 45 min) yielded 7 and 8 , from which S-(+)- 9 and R-(?)- 9 were respectively obtained in 89% and 98% optical purity. For these induced rearrangements this corresponds to at least 94,5 and 99%, respectively, of the chair-like, and to only 5.5 and 1% of the boat-like activated complex (ΔΔG = 1.5–2.5 kcal/mol). These results demonstrate that the activated complexes of both [3s,3s]-sigmatropic processes, i.e. the pure thermal reaction at 120° and the charge-induced reaction occurring at 0°, must be very similar. Thus, the boron trifluoride accelerates the Cope-like reactions 6 → 7 + 8 , but does not influence the geometries of their transition states. The Claisen rearrangement of the steroid ether 12 (200°, 15 h), yielding 7 and 8 , was not influenced by the chiral steroid skeleton, because no optical induction was observed (both phenols, 7 and 8 , yielded on degradation racemic 2-methylbutyric acid (9)). 相似文献
60.
Ludvik Janos 《Annali di Matematica Pura ed Applicata》1977,112(1):273-283
Summary We consider the boundary value problem αz″(x)+m(x)y(x)=0, αy″(x)+p(x)z(x)=0, xε[0, 1], y(0)=y(1)=z(0)=0, where the functions m(x) and p(x) are assumed integrable and positive everywhere in [0, 1]. As the main result we obtain the inequalities
for n=1, 2, ... where δn(m, p) stands for the product
of the first n eigenvalues αi(m, p) of the above system and where δn(m) abbreviates δn(m, m).
Entrata in Redazione il 6 febbraio 1976. 相似文献