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排序方式: 共有87条查询结果,搜索用时 31 毫秒
1.
R. Meij D.J. Stufkens K. Vrieze W. Van Gerresheim C.H. Stam 《Journal of organometallic chemistry》1979,164(3):353-370
Each of the compounds [MCl(Pr3)2(ArylNSO)] (M = RhI, IrI; R = i-Pr, Cy: Aryl = C6H5, 4-MeC6H4, 4-ClC6H4, 2,4,6-Me3C6H2 appears to exist as two isomers both in the solid state and in solution. The molecular and single crystal structure of one of the isomers of [RhCl(P-i-Pr)3)2(4-Me6H4NSO)] shows that the N-sulfinylaniline ligand is in the cis-configuration and coordinated to the rhodium atom via the sulfur-atom. The ligand lies in a plane which includes the rhodium atom and is in agreement with the Rh-S distance of 2.10 Å. IR results of the compounds (solid and solutions), 21P NMR data and 15N NMR of a 15N labelled compound, which yielded a 103Rh15N coupling constant of 15.5 Hz, show that in the second isomer the N-sulfinylaniline ligand is probably bonded to the metal atom via the π-NS bond.The ratio of the metal-π-NS bonded isomer and the metal-S bonded isomer decreases in the order Aryl = 4-ClC6H4 > C6H5 > 4-MeC6H4; R = i-Pr > Cy and M = Rh > Ir. The interconversion of the two isomers is intramolecular and becomes observable on the 31P NMR time scale at about 40° C for M = Rh.In the case of [Ir(P-i-Pr3)2(4-MeC6H4NSO)], cyclometallation of the sul- finylaniline is observed via the ortho-carbon atom, whereas cyclometallation via P-i-Pr3 is observed when the ortho-positions are blocked by methyl groups, e.g. when L = 2,4,6-Me3C6H2NSO. 相似文献
2.
C. H. Stam El Sayed El Ashry Yeldez El Kilany H. C. Van DerPlas 《Journal of heterocyclic chemistry》1980,17(3):617-619
X-ray crystallographic data show that the product obtained in the reaction of 4-(2-acetoxy-ethylidene)-4-hydroxy-2,3-dioxobutyro-1,4-lactone-2-(p-bromophenylhydrazone) with methyl-hydrazine is the bicyclic compound 2,6-dimethyl-3,4-dioxo-2,3,4,6,7,8-hexahydropyridazino-[4,3-c]pyridazine 4-(p-bromophenylhydrazone) ( 10 ) and not as originally suggested 1-methyl-3-(1-methylpyrazolin-3-yl)-4,5-pyrazoledione 4-(p-bromophenylhydrazone) ( 8 ). 相似文献
3.
Engel MF Visser AJ van Mierlo CP 《Langmuir : the ACS journal of surfaces and colloids》2004,20(13):5530-5538
Detailed knowledge of the adsorption-induced conformational changes of proteins is essential to understand the process of protein adsorption. However, not much information about these conformational changes is available. Here, the adsorption of calcium-depleted (APO)- and calcium-containing (HOLO)-bovine alpha-lactalbumin (BLA) on suspended solid polystyrene nanospheres and their subsequent displacement by a surfactant are studied by NMR spectroscopy. To our knowledge, this is the first time that adsorption of proteins on solid nanospheres, with both components present in the NMR sample, is studied by this method. High-quality one-dimensional and two-dimensional 1H NMR spectra of nonadsorbed APO- and HOLO-BLA in the presence of BLA- and/or surfactant-covered solid polystyrene nanospheres in suspension are obtained using standard NMR procedures. BLA and surfactant molecules that are adsorbed on the polystyrene nanospheres give rise to extremely broadened proton resonances. This can be exploited to determine the amount of adsorbed protein and of adsorbed surfactant in a system containing protein, nanospheres, and surfactant, without disturbing the equilibrium of the system. Two-dimensional 1H NMR spectroscopy shows that the chemical shifts of the backbone amide protons of HOLO-BLA after its adsorption and subsequent displacement from polystyrene nanospheres by the surfactant 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS) are identical to those of native HOLO-BLA. The adsorption-induced unfolding of BLA to a molten globule state on polystyrene nanospheres is thus fully reversible at the residue level upon CHAPS-induced displacement of BLA. The latter is the now fulfilled essential requirement that enables the future indirect study, at the residue level, of the conformational characteristics of BLA adsorbed on polystyrene nanospheres by hydrogen/deuterium exchange and NMR spectroscopy. The results presented show that NMR spectroscopy is clearly feasible to study the adsorption of BLA on suspended polystyrene nanospheres. This technique should be applicable to the study of the adsorption of other proteins on other surfaces as well. 相似文献
4.
Christopher H. Evans Morgan Partyka Jan Van Stam 《Journal of inclusion phenomena and macrocyclic chemistry》2000,38(1-4):381-396
Naphthalene forms 1 : 1 complexes with -cyclodextrin (-CD)in water. The binding constant is 377 ± 35 M-1. Addition of linear or branched alcohols causes a reduction in the apparent strength of naphthalene binding (Kapp) compared to the value in the absence of additives. For example, 1% 1-pentanol reduces Kapp to 184 ± 31 M-1. Branching does not alter Kapp much for a given number of carbon atoms, e.g., it is 113 ± 9 M-1for 2-pentanol and 116 ± 8 M-1for 3-pentanol. The exception to this is tert-butanol for which Kapp is 577 ± 40 M-1. The variation in Kapp as a function of [1-pentanol] yields values for the individual equilibrium constants contributing to Kapp. This reveals that a ternary complex forms involving naphthalene, the CD and 1-pentanol. The constant for formation of the ternary complex is 99 ± 29 M-2. NaI quenching of naphthalene fluorescence indicates that the CD cavity partially protects the naphthalene excited state fromthis water phase quencher. Interestingly, the Stern–Volmer constant is lower in the presence of 1-pentanol than in its absence, although there should be more unbound (and therefore more NaI accessible) naphthalene in the former system than in the latter. These apparently contradictory results are discussed in terms of ternary complex formation. 相似文献
5.
The photophysics and photochemistry of alpha-terthiophene (alphaT), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product alpha-hexathiophene (alphaH). By varying the ratio of nonionic-to-anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson-Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of alphaT is formed, from which the alphaT radical cation can be formed by absorption of a second photon. The radical cation can form alphaH if it encounters another alphaT radical cation. Under the experimental conditions used, this implies that the alphaH formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product alphaH, however, does not follow the same trend. A maximum yield of alphaH is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson-Boltzmann simulation results and by comparing charge-density changes as a function of molar fraction with the changes in counterion activity. The alphaH yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical. 相似文献
6.
7.
Petra A W Van Den Berg Koert Grever Arie Van Hoek Willem J H Van Berkel Antonie J W G Visser 《Journal of Chemical Sciences》2007,119(2):123-133
Conformational heterogeneity of the FAD cofactor in p-hydroxybenzoate hydroxylase (PHBH) was investigated with time-resolved polarized flavin fluorescence. For binary enzyme/substrate
(analogue) complexes of wild-type PHBH and Tyr222 mutants, crystallographic studies have revealed two distinct flavin conformations;
the ‘in’ conformation with the isoalloxazine ring located in the active site, and the ‘out’ conformation with the isoalloxazine
ring disposed towards the protein surface. Fluorescence-lifetime analysis of these complexes revealed similar lifetime distributions
for the ‘in’ and ‘out’ conformations. The reason for this is twofold. First, the active site of PHBH contains various potential
fluorescence-quenching sites close to the flavin. Fluorescence analysis of uncomplexed PHBH Y222V and Y222A showed that Tyr222
is responsible for picosecond fluorescence quenching free enzyme. In addition, other potential quenching sites, including
a tryptophan and two tyrosines involved in substrate binding, are located nearby. Since the shortest distance between these
quenching sites and the isoalloxazine ring differs only little on average, these aromatic residues are likely to contribute
to fluorescence quenching. Second, the effect of flavin conformation on the fluorescence lifetime distribution is blurred
by binding of the aromatic substrates: saturation with aromatic substrates induces highly efficient fluorescence quenching.
The flavin conformation is therefore only reflected in the small relative contributions of the longer lifetimes. 相似文献
8.
Antonie J. W. G. Visser Jan L. De Wit Franz Müller Herman J. C. Berendsen 《Helvetica chimica acta》1982,65(8):2422-2430
The water relaxation rates of several flavoproteins in the semiquinone state have been investigated by the spin echo technique. The results indicate a rather unspecific interaction between water and the protein-bound flavosemiquinones. An average interaction distance of 0.3-0.5 nm has been estimated. From the temperature dependence of the rate constants the free energy of activation for proton exchange is calculated to be about 17 kJ/mol. The rate of proton exchange is around 1011 s?1 for the flavosemiquinones investigated are accessible to water regardless of their ionic state. The large difference in relaxation rates of water protons between D - and L - amino-acid oxidases is noticeable. Oxynitrilase exhibits the highest whereas Azotobacter vinelandii flavodoxin shows the lowest water relaxation rate of the flavoproteins studied. The results are discussed in relation to the visible-light absorption properties of the flavoproteins. 相似文献
9.
Effects of Refractive Index and Viscosity on Fluorescence and Anisotropy Decays of Enhanced Cyan and Yellow Fluorescent Proteins 总被引:1,自引:1,他引:0
The fluorescence lifetime strongly depends on the immediate environment of the fluorophore. Time-resolved fluorescence measurements of the enhanced forms of ECFP and EYFP in water–glycerol mixtures were performed to quantify the effects of the refractive index and viscosity on the fluorescence lifetimes of these proteins. The experimental data show for ECFP and EYFP two fluorescence lifetime components: one short lifetime of about 1 ns and a longer lifetime of about 3.7 ns of ECFP and for EYFP 3.4. The fluorescence of ECFP is very heterogeneous, which can be explained by the presence of two populations: a conformation (67% present) where the fluorophore is less quenched than in the other conformation (33% present). The fluorescence decay of EYFP is much more homogeneous and the amplitude of the short fluorescence lifetime is about 5%. The fluorescence anisotropy decays show that the rotational correlation time of both proteins scales with increasing viscosity of the solvent similarly as shown earlier for GFP. The rotational correlation times are identical for ECFP and EYFP, which can be expected since both proteins have the same shape and size. The only difference observed is the slightly lower initial anisotropy for ECFP as compared to the one of EYFP. 相似文献
10.
A. J. Stam 《Studies in Applied Mathematics》1987,77(3):183-193
We study polynomials of binomial type that have an exponential generating function of the form { 1 ? f(u)?x}. They have a close connection with renewal sequences. The asymptotic behavior as n ? ∞ is studied. 相似文献