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261.
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
262.
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
263.
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
264.
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
265.
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
266.
Loredana Protesescu Joaquín Calbo Kristopher Williams William Tisdale Aron Walsh Mircea Dinc 《Chemical science》2021,12(17):6129
The development of synthetic routes to access stable, ultra-small (i.e. <5 nm) lead halide perovskite (LHP) quantum dots (QDs) is of fundamental and technological interest. The considerable challenges include the high solubility of the ionic LHPs in polar solvents and aggregation to form larger particles. Here, we demonstrate a simple and effective host–guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites. Cr3O(OH)(H2O)2(terephthalate)3 (Cr-MIL-101), made of large mesopore-sized pseudo-spherical cages, allows fast and efficient diffusion of perovskite precursors within its pores, and promotes the formation of stable, ∼3 nm-wide lead bromide perovskite QDs. CsPbBr3, MAPbBr3 (MA+ = methylammonium), and (FA)PbBr3 (FA+ = formamidinium) QDs exhibit significantly blue-shifted emission maxima at 440 nm, 446 nm, and 450 nm, respectively, as expected for strongly confined perovskite QDs. Optical characterization and composite modelling confirm that the APbBr3 (A = Cs, MA, FA) QDs owe their stability within the MIL-101 nanocrystals to both short- and long-range interfacial interactions with the MOF pore walls.We demonstrate a simple and effective host–guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites. 相似文献
267.
Héctor Beltrán Nahum Masó Beatriz Julian Eloisa Cordoncillo Juan B. Carda Purificación Escribano Anthony R. West 《Journal of Sol-Gel Science and Technology》2003,26(1-3):1061-1065
Lead Magnesium Niobate, Pb(Mg1/3Nb2/3)O3 (PMN), is an important relaxor ferroelectric material. A significant problem exists, however, in the preparation of this material: it is very difficult to prepare pure phase, without the presence of a pyrochlore impurity phase which degrades the dielectric properties. Depending on the processing conditions, the amount of pyrochlore phase varies.Considering the ternary diagram PbO-MgO-Nb2O5, different compositions have been prepared by a simple sol-gel method at room temperature using Pb(CH3COO)2, Mg(CH3COO)2, Nb(OC2H5)5 as precursors. After the heat treatment, the samples obtained were analyzed by XRD and EPMA with the purpose of studying the compositions formed.From the analysis of these results, it seems to be that the appearance of pyrochlore can be related to the reactivity of the MgO and/or the presence of other phases of the binary system PbO-Nb2O5. An excess of Pb and Mg is necessary for compositions to be formed near the PMN. The control of the amount of these two elements is very important because an excess of MgO would lead to rich compositions in Mg as a secondary phase. 相似文献
268.
Barberá J Puig L Romero P Serrano JL Sierra T 《Journal of the American Chemical Society》2005,127(1):458-464
The work described here concerns a challenge of general interest in supramolecular chemistry: the achievement of chiral helical organizations with controlled structures. This work provides a strategy to obtain supramolecular polymers in which a chiral helical conformation has been induced by a noncovalent association, that is, through hydrogen bonding. Polycatenar 2,4,6-triarylamino-1,3,5-triazines, which organize into columnar mesophases and are susceptible to H-bonding interactions, were chosen as a starting point to build up the chiral supramolecular structure. The stacking of these mesogens has been forced to wind in a helical way by means of H-bond association with (R)-3-methyladipic acid, within the mesophase. The optically active columnar organization has been studied in depth by optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction, and circular dichroism. Formation of stable complexes between the triazine units and (R)-3-methyladipic acid has also been investigated by means of NMR diffusion-ordered spectroscopy (DOSY) experiments in chloroform. 相似文献
269.
Elena B Lesage A Steuernagel S Böckmann A Emsley L 《Journal of the American Chemical Society》2005,127(49):17296-17302
A refocused INEPT through-bond coherence transfer technique is demonstrated for NMR of rigid organic solids and is shown to provide a valuable building block for the development of NMR correlation experiments in biological solids. The use of efficient proton homonuclear dipolar decoupling in combination with a direct spectral optimization procedure provides minimization of the transverse dephasing of coherences and leads to very efficient through-bond (1)H-(13)C INEPT transfer for crystalline organic compounds. Application of this technique to 2D heteronuclear correlation spectroscopy leads to up to a factor of 3 increase in sensitivity for a carbon-13 enriched sample in comparison to standard through-bond experiments and provides excellent selectivity for one-bond transfer. The method is demonstrated on a microcrystalline sample of the protein Crh (2 x 10.4 kDa). 相似文献
270.
Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy
Concepción López Rosa M. Claramunt M. Ángeles García José Elguero 《Central European Journal of Chemistry》2004,2(4):660-671
Equimolar mixtures of 3,5-dimethylpyrazole (1) with four NH-imidazoles (2–5) have been studied by13C and 15N CPMAS NMR and by DSC. In three cases, the solid mixture behaves as the sum of the individual components [imidazole (2), 2-methylimidazole (3) and 2,4(5)-dimethylimidazole (5)]. In one case [4,5-dimethylimidazole (4)], the mixture corresponds to a new species in which the dynamic behavior of1 no longer exists. 相似文献