1,721,416 research outputs found
Recensione di C. Vela, M. Marchi (a cura di), "Meraviglie di Gadda".
No full textNote de lecture de C. Vela, M. Marchi (éds), "Meraviglie di Gadda", parue dans OBLIO, 16, 2014, p. 82-84.Note de lecture de C. Vela, M. Marchi (éds), "Meraviglie di Gadda", parue dans OBLIO, 16, 2014, p. 82-84
molecular dynamics simulation of POPC at low hydration near the liquid crystal phase transition
No full textWe report results of a preliminary molecular dynamics study of a 1-palmitoyl-2-oleoylphosphatidyicholine (POPC) bilayer at low hydration (5% in weight). Our results suggest a gel phase structure where the oleic chain is bent recalling the crystalline oleic acid structure. We have also found a discontinuity in the volume/temperature curve which might be related to the gel to liquid crystal phase transition in POP
On the space group of garronite
No full textThe crystal structure of the natural zeolite garronite from Goble, Oregon has been refined using high resolution synchrotron X-ray powder diffraction data. Garronite has the same tetrahedral aluminosilicate framework as gismondine [GIS], and earlier structural models indicated a strong tetragonal pseudosymmetry. Proposed models in the literature were based on the I (4) over bar m2 and 14(1)/a space groups, on account of symmetry lowering from the topological 14(1)/amd space due to partial cation/water molecule order in the zeolitic cavities. Test structure analysis has been performed in all possible space subgroups including monoclinic space groups, and the refinement has been successfully carried out in space group 12/a (C2/c). The resulting monoclinic structure model is to be preferred over the tetragonal ones on the basis of: (1) lower agreement indices of the refinement; (2) a chemically sound framework geometry; and (3) a more satisfactory interpretation of the Ca atoms coordination in the extraframework cages. (C) 1999 International Centre for Diffraction Data. [S0885-7156(98)01104-X]
Introduzione [volume monografico "A cinquanta anni dal primo Centro-sinistra: un bilancio nel contesto internazionale"]
No full textSIMULATION AND MODELING OF THE RHODOBACTER SPHAEROIDES BACTERIAL REACTION CENTER: primary charge separation
No full textThis paper is focused on the molecular dynamics modeling of the primary charge separation in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides. The kinetic parameters for the electron transfer along the active (L) and inactive (M) sides were obtained from a long MD trajectory, 3.4 ns, of the RC in an amphophilic environment made of detergent and water. Both nuclear and electronic polarizations are explicitly included in our calculation. With no postprocessing parameter fit, our modeling computes, for two different charge distributions, the driving forces for the transfer of an excess electron to BL and HL from P*, in good agreement with experiments. The multiexponential kinetics of the primary charge separation is also predicted, consistent with experimentally observed kinetics. The decay of the P* state is composed of four characteristic times due to both the conformational heterogeneity of the protein and the two possible mechanisms, superexchange and sequential. At room temperature, the latter is favored over superexchange with decay rates close to experimental rates. Nevertheless, the proximity between the computed diabatic free-energy surfaces on the L side yields a superexchange electronic coupling matrix element very near its resonance point and, thus, very sensitive to changes in the driving forces. For variations of at most 1.3 kcal mol-1, smaller than the accuracy of our theoretical approach, superexchange might be favored over the two- step mechanism. Finally, our molecular modeling strongly indicates that the position of the diabatic free- energy surfaces for the primary charge separation cannot by itself account for the directionality of the primary charge separation. A strong electrostatic potential around the special pair that favors the polarization of the transferring electron toward regions closer to BL than to BM is found. This polarization could significantly increase the electronic coupling between P* and BL, thus accounting, at least in part, for the directionality of the electron transfer
Simulation and modeling of the rhodobacter sphaeroides bacterial reaction center: structure and interaction
No full textIn this paper, we present the structural and dynamic results of a 3.4 ns molecular dynamics simulation of a reaction center protein immersed in a micelle-like environment formed by a detergent, lauryl dimethyl amino oxide or LDAO, and hydrated by more than 6000 additional water molecules. The whole system, ≈40 000 atoms, was simulated using an all-atom force field with refined potential parameters developed by us to describe the protein cofactors and the detergent molecules. LDAO, inserted at the beginning of the run in a configuration far from equilibrium, rearranged forming a micelle attached to the hydrophobic regions of the protein. The micelle is a stable and dynamic structure over the whole trajectory and prevents the protein and the internal cofactors from contacts with water. Comparing our simulated system with the high-resolution crystallographic structure, the deviations of the backbone atoms are small, less than 1.8 Å after 3.4 ns, and the chromophore arrangement geometry is stable and close to X-ray for the whole simulation. Thus, our system constitutes a realistic model to investigate at an atomic level the role of the protein environment on the charge transfer processes taking place in this complex. Related to the functionality of this RC protein, we observe the isomerization of the tyr M210 which is directly coupled with the primary electron transfer. Concerning the branch functionality of the RC, we observe that the computed minimum distance between the chromophores on the L and M side has a different dynamic behavior and is smaller on average for transitions on the L side. This finding might have some bearing on the electron-transfer asymmetry of the primary charge transfer
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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