1,721,069 research outputs found
Collective dynamics in phospholipid bilayers investigated by inelastic neutron scattering: Exploring the dynamics of biological membranes with neutrons
No full textWe present the first inelastic neutron scattering study of the short wavelength dynamics in a phospholipid bilayer. We show that inelastic neutron scattering using a triple-axis spectrometer at the high flux reactor of the ILL yields the necessary resolution and signal to determine the dynamics of model membranes. The results can quantitatively be compared to recent Molecular Dynamics simulations. Reflectivity, in-plane correlations and the corresponding dynamics can be measured simultaneously to gain a maximum amount of information. With this method, dispersion relations can be measured with a high energy resolution. Structure and dynamics in phospholipid bilayers, and the relation between them, can be studied on a molecular length scale
Thermal fluctuations of oriented lipid membranes by nonspecular neutron reflectometry
No full textWe present a novel method to measure and to analyze lipid bilayer fluctuations based on time-of-flight nonspecular neutron reflectivity (TOF-NSNR). To validate this approach, we compare the results obtained on the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) to NSNR data collected in the conventional monochromatic mode. For the DMPC model system in its fluid L-alpha phase, we determine the fluctuation spectrum of an averaged bilayer in an oriented multilamellar stack and compare the result to the predictions of the linearized smectic free energy functional. In particular, we show how the smectic penetration depth Lambda can be determined from the measurements. Furthermore, significant changes in the fluctuation spectrum of DMPC are observed upon the interaction with the antimicrobial peptide magainin 2 and cannot be explained by the smectic model
Collective dynamics of lipid membranes studied by inelastic neutron scattering
No full textWe have studied the collective short wavelength dynamics in deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayers by inelastic neutron scattering. The corresponding dispersion relation (h) over bar omega(Q) is presented for the gel and the fluid phase of this model system. The temperature dependence of the inelastic excitations indicates a phase coexistence between the two phases over a broad range and leads to a different assignment of excitations from that reported in a preceding inelastic x-ray scattering study [Phys. Rev. Lett. 86, 740 (2001)]. As a consequence, we find that the minimum in the dispersion relation is actually deeper in the gel than in the fluid phase. Finally, we can clearly identify an additional nondispersive (optical) mode predicted by molecular dynamics simulations [Phys. Rev. Lett. 87, 238101 (2001)]
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Mimicking the inner mitochondrial membrane with curved supported lipid bilayers: A neutron reflectometry study
No full textCardiolipin (CL) is a phospholipid with an unusual molecular structure exhibiting four acyl chains bound to a polar headgroup. CL is found in curved regions within biological membranes, such as the poles of cylindrically shaped bacteria and the cristae in mitochondria. Like bacterial cells, mitochondria are characterised by two cell membranes. CL is exclusively found in the inner membrane of mitochondria (IMM), and it is vital for the proper functioning of this organelle. Together with CL, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the main lipid components of the IMM. Interestingly, most of the CL in human tissues exhibits C18 acyl
chains and alterations of its molecular structure are related to severe diseases. Here we investigated supported lipid bilayers (SLBs) composed of PC, PE and CL as model systems mimicking the IMM. In our experiments we used tetra-oleyl cardiolipin (TOCL), which exhibits four C18 acyl chains and is therefore close to the CL normally present in the IMM, and tetra-myristoyl cardiolipin (TMCL), which exhibits considerably shorter acyl chains, i.e. C14. All samples were investigated both in the presence and in the absence of Ca2+ ions. The structure of the
produced samples was characterised by neutron reflectometry (NR). Our data indicate that all samples with TMCL were organised as a regular SLB with a small impact of TMCL on the bilayer structure. On the other hand, at a TOCL concentration above 10% mol and upon injection of Ca2+, we observed a large structural rearrangement of the initially formed SLB compatible with the formation of curved bilayer regions that protrude towards the bulk solvent. To the best of our knowledge, this is the first example where specular NR and off-specular scattering revealed buckled SLBs. This experimental evidence indicates the crucial role of CL acyl chain composition in favouring the proper folding of the IMM
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