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961.
Lorenzo CaggianoDamiano Castoldi Raphael BeumerPau Bayón Joachim TelserCesare Gennari 《Tetrahedron letters》2003,44(43):7913-7919
During the course of our synthetic studies towards simplified eleuthesides, we have found that p-methoxyphenyl (PMP) protected allylic alcohols are compatible with the RCM reaction and can give better yields than the corresponding free allylic alcohols. 相似文献
962.
We report only the second example of a thermo-responsive 'schizophrenic' diblock copolymer surfactant: unlike the original (meth)acrylamide-based example reported by Laschewsky and co-workers (J. Am. Chem. Soc., 2002, 124, 3787), this new methacrylate-based diblock copolymer is near-monodisperse, readily synthesized in high yield and exhibits a broad temperature range between the two micelle transitions. 相似文献
963.
The reduction of titanocene dithiocyanate in various solvents has been examined using the techniques of polarography, voltammetry, controlled potential electrolysis and cyclic voltammetry. In THF and CH2Cl2, Cp2Ti(NCS)2 undergoes successive one-electron transfer reactions, and by using a combination of electrochemical and EPR techniques it has been possible to confirm the stability of [Cp2Ti(NCS)2]? in these solvents. In DMF, however, an irreversible dimerization follows the first electron transfer. 相似文献
964.
The equilibrium constants, K
2, have been determined for the proton-transfer reactions of 1-phenacylquinolinium ion, PHQ+, with several amines {triethylamine (TEA), N,N,N′,N′-tetramethylethylenediamine (ED), N,N,N′, N′-tetramethylpropanediamine (PD), N,N,N′,N′-tetramethylbutanediamine (BD), and 1,8-bis(dimethylamino-naphthalene (DMAN)} in acetonitrile (AN), AN-tetrahydrofuran (THF)
and AN-ethanol (EtOH) mixtures. The reaction was followed spectrophotometrically using a stopped-flow technique. The K
2 value decreased for DMAN and increased for TEA with increasing vol-% of THF in AN-THF mixtures. The changes in the K
2 value for ED, PD and BD changed in the order: ED, PD and BD from a pattern similar to TEA to a pattern similar to DMAN. The
change in the K
2 value for DMAN with increasing vol-% of THF in AN-THF mixtures was explained by the effect of polarity on the stability of
P−Q+ (the deprotonated product of PHQ+). The effect of THF on the K
2 value is consistent with that of the peak wavelength of the absorption spectrum of P−Q+. The change in the K
2 value for TEA, ED, PD and BD depended on the structures of the protonated bases, one of the products for this reaction. The
effect of EtOH on the K
2 value for DMAN was examined in ternary EtOH-THF-AN mixtures that contain different amounts of EtOH and whose relative permittivities
were adjusted to that of EtOH. The K
2 value increased with increasing vol-% of EtOH because of the stabilization of P−Q+ upon the formation of the hydrogen-bonded complex with EtOH. The absorption spectrum of P−Q+ demonstrated a blue shift as the vol-% of EtOH increased. 相似文献
965.
The total energy of all π-electrons in a conjugated hydrocarbon (within the framework of HMO approximation) is the sum of the absolute value of all
the eigenvalues of its corresponding graph. In this paper, we consider “double hexagonal chains” as benzenoids constructed
by successive fusions of successive naphthalenes along a zig–zag sequence of triples of edges as appear on opposite sides
of each naphthalene unit. It is shown that if the fusions are such as to give a polyaceacene then the total π-electron energy
is the minimum from among all the double hexagonal chains with the same number of naphthalene units.
相似文献
966.
New, sterically demanding 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (2) as NHC precursors have been synthesized and characterized. These salts, in combination with palladium acetate, provided active catalysts for the cross-coupling of aryl chlorides and bromides under mild conditions. The catalytic system was applied to the Heck, Suzuki and benzaldehyde (Kumada) coupling reactions. Catalyst activity was found to be influenced by the presence of a methoxy group on the ring of the p-position of benzyl substituent of the ligand precursor. 相似文献
967.
I. Gascón S. Martín P. Cea M. C. López F. M. Royo 《Journal of solution chemistry》2002,31(11):905-915
Densities and speeds of sound of the binary mixtures 1,3-dioxolane + 1-butanol, 1,3-dioxolane + 2-butanol, 1,4-dioxane + 1-butanol, and 1,4-dioxane + 2-butanol have been measured at 25 and 40°C. The excess molar volumes and excess isentropic compressibility coefficients were calculated from experimental data and fitted to a Redlich–Kister polynomial function. Results were analyzed in terms of molecular interactions and compared with literature data. 相似文献
968.
Windberg E Hudecz F Marquardt A Sebestyén F Kiss A Bösze S Medzihradszky-Schweiger H Przybylski M 《Rapid communications in mass spectrometry : RCM》2002,16(9):834-839
An epitope motif, TX(1)TX(2)T, of mucin-2 glycoprotein was identified by means of a mucin-2-specific monoclonal antibody, mAb 994, raised against a synthetic mucin-derived 15-mer peptide conjugate. For determination of the epitope sequence recognised with highest affinity by mAb 994, a combinatorial approach was applied using the portioning-mixing technique excluding Cys. Antibody binding of libraries was most profound when Gln was at the X(1) position. Analytical characterisation of the TQTX(2)T library was conducted by amino acid analysis and matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) and electrospray ionisation Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometric methods. Control libraries were prepared by mixing 19 individual peptides corresponding to the TQTX(2)T sequence. Thus, mixtures of 6, 10 and 19 pentapeptides were analysed and compared with the combinatorial mixture. MALDI-TOFMS was able to detect only partially the components in the 6- and 10-member mixtures, but failed to characterise a more complex 19-member mixture. In contrast, ESI-FTICRMS resolved all mixtures of higher complexity and provided direct identification at monoisotopic resolution, such as for a peptide library containing 'isobaric' lysine and glutamine (Delta m = 0.0364 Da). The results of this study suggest that ESI-FTICRMS is a powerful tool for characterisation of combinatorial peptide libraries of higher complexity. 相似文献
969.
Maria do Carmo B. Pimentel Eduardo Henrique M. Melo José Luiz Lima Filho William M. Ledingham Nelson Durán 《Applied biochemistry and biotechnology》1997,66(2):185-195
This work is a study of lipase production by a Brazilian strain ofPenicillium citrinum using an inexpensive and simple medium without organic nitrogen sources and of some important industrial properties, including
thermostability in relation to ionic strength. The maximal lipase activity (1585 U/L) was obtained whenPenicillium citrinum was cultured on 0.75% ammonium sulfate complemented with minerals salts instead of yeast extract. Although this activity
was about 55% lower than that produced in medium with yeast extract (2850 U/L), the specific activity (7.8 U/mg proteins)
was higher than that obtained with the yeast extract (4.9 U/mg proteins). The morphology of fungus changed totally, with yeast
extract there are smooth, solid, and spherical pellets whereas on ammonium sulfate there are small “hairy” pellets uniformly
suspended in the medium. The effect of ferrous (Fe++) ions was carried out using medium MA with and without Fe++ ions. Lipase production byPenicillium citrinum in medium MA requires Fe++ ions, the absence of which caused a decreased of about 50% in the specific activity (3.5 U/mg proteins). The utilization
of commercial, locally available oils as carbon sources, such as soybean oil (236 U/L) and corn oil (74 U/L) resulted in lower
activity compared to olive oil, showing that lipase production byPenicillium citrinum is specifically induced by olive oil. Potassium concentration in the medium can effects the production of lipase (1 mM (1585
U/L), 10 mM (1290 U/L), and 30 mM (1238 U/L), 50 mM (195 U/L), and 100 mM (2 U/L). The crude culture filtered was susceptable
to thermal deactivation. It was stable at pH 6.0, but was not stable at the optimum pH (8.0-8.5) at 50 mM. At the low ionic
concentration (1-25 mM) this lipase was stable at low pH (3.5-4.0). The activation energy was 22.4 ±2.2 Kcal. mol 1. 相似文献
970.
Fernaldo Richtia Winnerdy Bla Bakalar Poulomi Das Brahim Heddi Adrien Marchand Frdric Rosu Valrie Gabelica Anh Tun Phan 《Chemical science》2021,12(20):7151
Cations are critical for the folding and assembly of nucleic acids. In G-quadruplex structures, cations can bind between stacked G-tetrads and coordinate with negatively charged guanine carbonyl oxygens. They usually exchange between binding sites and with the bulk in solution with time constants ranging from sub-millisecond to seconds. Here we report the first observation of extremely long-lived K+ and NH4+ ions, with an exchange time constant on the order of an hour, when coordinated at the center of a left-handed G-quadruplex DNA. A single-base mutation, that switched one half of the structure from left- to right-handed conformation resulting in a right–left hybrid G-quadruplex, was shown to remove this long-lived behaviour of the central cation.An extremely long-lived cation has been detected in left-handed G-quadruplexes. 相似文献