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
Thin films of fumaramide [2]rotaxane, a mechanically interlocked molecule composed of a macrocycle and a thread in a "bead and thread" configuration, were prepared by vapor deposition on both Ag(111) and Au(111) substrates. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy were used to characterize monolayer and bulklike multilayer films. XPS determination of the relative amounts of carbon, nitrogen, and oxygen indicates that the molecule adsorbs intact. On both metal surfaces, molecules in the first adsorbed layer show an additional component in the C 1s XPS line attributed to chemisorption via amide groups. Molecular-dynamics simulation indicates that the molecule orients two of its eight phenyl rings, one from the macrocycle and one from the thread, in a parallel bonding geometry with respect to the metal surfaces, leaving three amide groups very close to the substrate. In the case of fumaramide [2]rotaxane adsorption on Au(111), the presence of certain out-of-plane phenyl ring and Au-O vibrational modes points to such bonding and a preferential molecular orientation. The theoretical and experimental results imply that the three-dimensional intermolecular configuration permits chemisorption at low coverage to be driven by interactions between the three amide functions of fumaramide [2]rotaxane and the Ag(111) or Au(111) surface. 相似文献
A new class of efficient catalysts was developed for the asymmetric transfer hydrogenation of unsymmetrical ketones. A series of chiral N,S-chelates (6-22) was synthesized to serve as ligands in the iridium(I)-catalyzed reduction of ketones. Both formic acid and 2-propanol proved to be suitable as hydrogen donors. Sulfoxidation of an (R)-cysteine-based aminosulfide provided a diastereomeric ligand family containing a chiral sulfur atom. The two chiral centers of these ligands showed a clear effect of chiral cooperativity. In addition, aminosulfides containing two asymmetric carbon atoms in the backbone were synthesized. Both the sulfoxide-containing beta-amino alcohols and the aminosulfides derived from 1,2-disubstituted amino alcohols gave rise to high reaction rates and moderate to excellent enantioselectivities in the reduction of various ketones. The enantioselective outcome of the reaction was favorably affected by selecting the most appropriate hydrogen donor. Enantioselectivities of up to 97% were reached in the reduction of aryl-alkyl ketones. 相似文献
We prove an analog of the base change functor of Λ-trees in the setting of generalized affine buildings. The proof is mainly
based on local and global combinatorics of the associated spherical buildings. As an application we obtain that the class
of generalized affine buildings is closed under taking ultracones and asymptotic cones. Other applications involve a complex
of groups decompositions and fixed point theorems for certain classes of generalized affine buildings. 相似文献
The paper describes in detail how to evaluate interlaboratory studies for the measurement of authorised and non-permitted veterinary drugs. Examples of different kinds of evaluation will be presented including the results of two interlaboratory studies for anthelmintics in bovine milk and muscle. As compared to an interlaboratory study 2008, a clear development regarding the proficiency of the laboratories of the European Residue Control System could be noted in 2009. As a result of this development, the percentage of false-negative and false-positive results had decreased considerably. The described improvements in the analysis of the included anthelmintics were also reflected in the overall assessment. While in 2008 ten (38.5?%) out of 26 NRLs fulfilled the proficiency criteria of the interlaboratory study, this number had increased to fourteen (53.8?%) by 2009. In some cases, the correct quantification needed to be improved. 相似文献
Acrylamide and acrolein are two short-chained hazardous compounds with neurotoxic, carcinogenic, and mutagenic effects. The aim of this paper is to describe a fast and simple procedure for simultaneous determination of both acrylamide and acrolein under standard conditions, suggest a suitable calibration protocol for custom analysis, and demonstrate its applicability to the analysis of gaseous products from, e.g., cigarettes, cigars, or electronic cigarettes. A gas chromatography–mass spectrometry (GC–MS) method was developed to quantify acrylamide and acrolein in smoke vapor from electronic cigarettes, tobacco cigarettes, and cigars. Nonionic and highly polar molecules with a low boiling point and molecular mass need a suitable derivatization method to achieve appropriate retention and selectivity on commonly used relatively nonpolar stationary phases and to enhance the molecular mass for easy MS detection. The derivatization of acrylamide and acrolein was carried out by a bromination method with elemental bromine. The dibromo derivatives were extracted into an organic solvent and following a dehydrobromination procedure the samples were injected into the GC–MS system. Important experimental parameters were varied, after which the bromination time was defined as 30 min, and the injector temperature and the starting temperature of gradient were set at 280 and 50 °C respectively. Acrolein was found in all tested samples, while acrylamide was detected only in smoke from normal tobacco. Possible mechanisms for the formation of these unsaturated compounds in the samples are discussed. After its validation the newly developed method was successfully and reliably applied to the analysis of both compounds. This short method provides an easy way to determine acrylamide and acrolein in gaseous samples.
The alditol acetate method is a common procedure for sugar analysis, also applied to determine the substituent distribution
in monomer units of polysaccharide ethers like methyl cellulose by gas liquid chromatography. Consisting of several preparation
and work-up steps this procedure is both time consuming and prone to side reactions that promote discrimination of single
constituents, especially when no peralkylation step is performed prior to hydrolysis. As a consequence results scatter in
dependence on individual treatment and conditions. In the context of this work these critical points were overcome by strict
but simplified work-up procedures and using acid instead of alkaline catalyzed acetylation. Under the acidic conditions the
tedious removal of borate is no longer necessary and a reduced time requirement was achieved as well as good reproducibility.
Comparison with independent reference methods excluded a systematic error of the method and confirmed the results obtained.
Without peralkylation, i.e. in the presence of free hydroxyl groups, another fast modification of the method using DMSO as
solvent, no removal of borate, and 1-methylimidazole as catalyst for acetylation was found to produce a systematic error. 相似文献
A confidence interval for the probability of detection across laboratories (LPOD) for qualitative methods, described in the AOAC INTERNATIONAL guidelines for validation of microbiological methods for food and environmental surfaces, is considered. It is demonstrated that under certain conditions, the observed confidence of this confidence interval can be rather low, so that statistical minimum requirements are not fulfilled. A new profile likelihood confidence interval based on a latent random laboratory effect approach is proposed. Observed confidence levels for this confidence interval demonstrate its applicability already for a small number of laboratories. 相似文献