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141.
J Wen S W Nelson R B Honzatko H J Fromm J W Petrich 《Photochemistry and photobiology》2001,74(5):679-685
The environment of Trp57, introduced by the mutation of a tyrosine in the dynamic loop of porcine liver fructose-1,6-bisphosphatase (FBPase), was examined using time-resolved fluorescence and directed mutation. The Trp57 enzyme was studied previously by X-ray crystallography and steady-state fluorescence, the latter revealing an unexpected redshift in the wavelength of maximum fluorescence emission for the R-state conformer. The redshift was attributed to the negative charge of Asp127 in contact with the indole side chain of Trp57. Time-resolved fluorescence experiments here reveal an indole side chain less solvent exposed and more rigid in the R-state, than in the T-state of the enzyme, consistent with X-ray crystal structures. Replacement of Asp127 with an asparagine causes a 6 nm blueshift in the wavelength of maximum fluorescence emission for the R-state conformer, with little effect on the emission maximum of the T-state enzyme. The data here support the direct correspondence between X-ray crystal structures of FBPase and conformational states of the enzyme in solution, and provide a clear example of the influence of microenvironment on the fluorescence properties of tryptophan. 相似文献
142.
143.
Brett W. Boote Himashi P. Andaraarachchi Bryan A. Rosales Rafael Blome-Fernández Feng Zhu Malinda D. Reichert Kalyan Santra Jingzhe Li Jacob W. Petrich Javier Vela Emily A. Smith 《Chemphyschem》2019,20(20):2647-2656
Lead halide perovskites possess unique characteristics that are well-suited for optoelectronic and energy capture devices, however, concerns about their long-term stability remain. Limited stability is often linked to the methylammonium cation, and all-inorganic CsPbX3 (X=Cl, Br, I) perovskite nanocrystals have been reported with improved stability. In this work, the photostability and thermal stability properties of CsPbX3 (X=Cl, Br, I) nanocrystals were investigated by means of electron microscopy, X-ray diffraction, thermogravimetric analysis coupled with FTIR (TGA-FTIR), ensemble and single particle spectral characterization. CsPbBr3 was found to be stable under 1-sun illumination for 16 h in ambient conditions, although single crystal luminescence analysis after illumination using a solar simulator indicates that the luminescence states are changing over time. CsPbBr3 was also stable to heating to 250 °C. Large CsPbI3 crystals (34±5 nm) were shown to be the least stable composition under the same conditions as both XRD reflections and Raman bands diminish under irradiation; and with heating the γ (black) phase reverts to the non-luminescent δ phase. Smaller CsPbI3 nanocrystals (14±2 nm) purified by a different washing strategy exhibited improved photostability with no evidence of crystal growth but were still thermally unstable. Both CsPbCl3 and CsPbBr3 show crystal growth under irradiation or heat, likely with a preferential orientation based on XRD patterns. TGA-FTIR revealed nanocrystal mass loss was only from liberation and subsequent degradation of surface ligands. Encapsulation or other protective strategies should be employed for long-term stability of these materials under conditions of high irradiance or temperature. 相似文献
144.
Kalyan Santra Emily A. Smith Xueyu Song Jacob W. Petrich 《Photochemistry and photobiology》2019,95(3):773-779
The measurement of fluorescence lifetimes, especially in small sample volumes, presents the dual challenge of probing a small number of fluorophores and fitting the concomitant sparse data set to the appropriate excited‐state decay function. A common method of analysis, such as the maximum likelihood (ML) technique, assumes a uniform probability distribution of the parameters describing the fluorescence decay function. An improvement is thus suggested by implementing a suitable nonuniform distribution, as is provided by a Bayesian framework, where the distribution of parameters is obtained from both their prior knowledge and the evidence‐based likelihood of an event for a given set of parameters. We have also considered the Dirichlet prior distribution, whose mathematical form enables analytical solutions of the fitting parameters to be rapidly obtained. If Gaussian and exponential prior distributions are judiciously chosen, they reproduce the experimental target lifetime to within 20% with as few as 20 total photon counts for the data set, as does the Dirichlet prior distribution. But because of the analytical solutions afforded by the Dirichlet prior distribution, it is proposed to employ a Dirichlet prior to search parameter space rapidly to provide, if necessary, appropriate parameters for subsequent employment of a Gaussian or exponential prior distribution. 相似文献
145.
146.
Mario Petrich 《Results in Mathematics》2015,67(1-2):153-176
147.
148.
Mario Petrich 《Results in Mathematics》2004,45(3-4):328-348
149.
Mario Petrich 《代数通讯》2017,45(7):2783-2794
Completely regular semigroups S are taken here with the unary operation of inversion within the maximal subgroups of S. As such they form a variety 𝒞? whose lattice of subvarieties is denoted by ?(𝒞?). The relation on ?(𝒞?) which identifies two varieties if they contain the same bands is denoted by B∧. The upper ends of B∧-classes which are neither equal to 𝒞? nor contained in the variety 𝒞𝒮 of completely simple semigroups are generated by two countably infinite ascending chains called canonical varieties. In a previous publication, we constructed the sublattice Σ of ?(𝒞?) generated by 𝒞𝒮 and the first four canonical varieties. Here we extend Σ to the sublattice Ψ of ?(𝒞?) generated by 𝒞𝒮 and the first six canonical varieties. For each of the varieties in Ψ?Σ, we construct the ladder and a basis of its identities. 相似文献
150.
Mario Petrich 《Monatshefte für Mathematik》2000,24(1):329-340
A subsemigroup S of a semigroup Q is an order in Q if for every there exist such that , where a and d are contained in (maximal) subgroups of Q, and and are their inverses in these subgroups. A regular semigroup S is strict if it is a subdirect product of completely (0-)simple semigroups. We construct all orders and involutions in Auinger’s model of a strict regular semigroup. This is used to find necessary and sufficient conditions on an involution on an order S in a strict regular semigroup Q for extendibility to an involution on Q. 相似文献