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71.
Stefano Cipolla Carmine Di Fiore Fabio Durastante Paolo Zellini 《Numerical Linear Algebra with Applications》2019,26(2)
In this work, given a positive definite matrix A, we introduce a class of matrices related to A, which is obtained by suitably combining projections of its powers onto algebras of matrices simultaneously diagonalized by a unitary transform. After a detailed theoretical study of some spectral properties of the matrices of this class, which suggests their use as regularizing preconditioners, we prove that such matrices can be cheaply computed when the matrix A has a Toeplitz structure. We provide numerical evidence of the advantages coming from the employment of the proposed preconditioners when used in regularizing procedures. 相似文献
72.
73.
A. Schenck P. Birrer F. N. Gygax B. Hitti E. Lippelt H. Maletta M. Weber 《Hyperfine Interactions》1991,63(1-4):227-233
Transverse field studies (0.3 T) in HoBa2Cu3O7, DyBa2Cu3O7 and ErBa2Cu3O7 (x=6.40. 6.45, 6.53, 7) were undertaken with the aim of detecting a possible 4f-moment inducedμ
+ Knight shift which could elucidate the nature of the magnetic coupling among the 4f-moments and a possible interference with
superconductivity. AboveT
c
no shift could be detected in any of the systems except in HoBa2Cu3O7 where a split signal appears involving a positive and a negative shift. Unexpected shifts may be present belowT
c
. 相似文献
74.
Giulia Bonente Francesca Passarini Stefano Cazzaniga Carmine Mancone Maria Cristina Buia Marco Tripodi Roberto Bassi Stefano Caffarri 《Photochemistry and photobiology》2008,84(6):1359-1370
To avoid photodamage, photosynthetic organisms have developed mechanisms to evade or dissipate excess energy. Lumen overacidification caused by light‐induced electron transport triggers quenching of excited chlorophylls and dissipation of excess energy into heat. In higher plants participation of the PsbS protein as the sensor of low lumenal pH was clearly demonstrated. Although light‐dependent energy quenching is a property of all photosynthetic organisms, large differences in amplitude and kinetics can be observed thus raising the question whether a single common mechanism is in action. We performed a detailed study of PsbS expression/accumulation in Chlamydomonas reinhardtii and investigated its accumulation in other algae and plants. We showed that PsbS cannot be detected in Chlamydomonas under a wide range of growth conditions. Overexpression of the endogenous psbs gene showed that the corresponding protein could not be addressed to the thylakoid membranes. Survey of different unicellular green algae showed no accumulation of anti‐PsbS reactive proteins differently from multicellular species. Nevertheless, some unicellular species exhibit high energy quenching activity, suggesting that a PsbS‐independent mechanism is activated. By correlating growth habitat and PsbS accumulation in different species, we suggest that during the evolution the light environment has been a determinant factor for the conservation/loss of the PsbS function. 相似文献
75.
Bertini I Calderone V Fragai M Giachetti A Loconte M Luchinat C Maletta M Nativi C Yeo KJ 《Journal of the American Chemical Society》2007,129(9):2466-2475
By solving high-resolution crystal structures of a large number (14 in this case) of adducts of matrix metalloproteinase 12 (MMP12) with strong, nanomolar, inhibitors all derived from a single ligand scaffold, it is shown that the energetics of the ligand-protein interactions can be accounted for directly from the structures to a level of detail that allows us to rationalize for the differential binding affinity between pairs of closely related ligands. In each case, variations in binding affinities can be traced back to slight improvements or worsening of specific interactions with the protein of one or more ligand atoms. Isothermal calorimetry measurements show that the binding of this class of MMP inhibitors is largely enthalpy driven, but a favorable entropic contribution is always present. The binding enthalpy of acetohydroxamic acid (AHA), the prototype zinc-binding group in MMP drug discovery, has been also accurately measured. In principle, this research permits the planning of either improved inhibitors, or inhibitors with improved selectivity for one or another MMP. The present analysis is applicable to any drug target for which structural information on adducts with a series of homologous ligands can be obtained, while structural information obtained from in silico docking is probably not accurate enough for this type of study. 相似文献
76.
Two calix[6]arene directional wheels can be ordered in the right stereosequence by their through-the-annulus threading with a rationally designed bis(benzylalkylammonium) axle. These stereoisomeric pseudo[3]rotaxanes can be considered as a minimal "informational system" because the "written information" on the thread is transferred to a specific sequence stereoisomer. 相似文献
77.
The first examples of rotaxanes based on calixarenes threaded by dialkylammonium ions, which also represent the first examples of calixarene-based molecular shuttles, are reported. The base/acid treatment demonstrated that these systems act as molecular shuttles, which move between three sites on the axle. When small OMe groups are appended at the calix[6]arene lower rim an unprecedented inversion of its shuttling direction is observed, which occurs through a cone-to-cone inversion of the macrocycle. 相似文献
78.
We herein described the synthesis of several 3-benzyl-2,5-diarylselenophene derivatives in moderate to good yields using (Z)-benzylselenoenynes as starting material in carbocyclization reactions. The reactions were carried out under mild conditions using only t-BuOK as base, in the complete absence of transition metals or additives. The cyclized 3-benzyl-2,5-diarylselenophenes obtained in the current protocol appear highly promising and attractive intermediates for the synthesis of polysubstituted selenophenes. For instance, 3-benzyl-2,5-diphenylselenophene was treated with Br(2) provided the corresponding 3-benzyl-4-bromo-2,5-diphenylselenophene in high yield. 4-Bromoselenophene derivative was applied as substrate in the palladium catalyzed cross-coupling reactions with boronic acids to give the Suzuki type products in excellent yields. 相似文献
79.
Erra L Tedesco C Immediata I Gregoli L Gaeta C Merlini M Meneghini C Brunelli M Fitch AN Neri P 《Langmuir : the ACS journal of surfaces and colloids》2012,28(22):8511-8517
The inclusion properties of a calixarene-based porous material have been studied to investigate the adsorption and the desorption of carbon tetrachloride, chloroform, and water in the zeolite-like structure. Uptake and release processes have been studied both by time-resolved powder X-ray diffraction and by thermogravimetric analysis to obtain structural and kinetic information. The selected guests are able to enter the structure with an increase in the host cell volume and with time-dependent diffusivity coefficients. Chloroform molecules act as a permanent porosity switch promoting a phase transition to non-porous triclinic form. 相似文献
80.
A Abbotto C Coluccini E Dell'orto N Manfredi V Trifiletti MM Salamone R Ruffo M Acciarri A Colombo C Dragonetti S Ordanini D Roberto A Valore 《Dalton transactions (Cambridge, England : 2003)》2012,41(38):11731-11738
The first examples of thiocyanate-free thiophene-substituted Ru(ii) cyclometalated complexes, based on thiophene-derived 2-(2,4-difluorophenyl)pyridine ligands, are presented and investigated as photosensitizers in DSCs. Upon thiophene substitution the complexes presented enhanced optical properties compared to the reference dye with no thiophene substitution. DSCs based on the dithienyl-derived dye showed power conversion efficiencies up to 5.7%, more than twice that containing the complex without the thiophene substitution. 相似文献