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Structure and spatial distribution of the spin-labelled lipopeptide trichogin GA IV in a phospholipid membrane studied by pulsed electron-electron double resonance (PELDOR)
No full textThe method of pulsed electron - electron double resonance (PELDOR) is exploited to study intra- and intermolecular dipole - dipole interactions between the spin labels of trichogin GA IV analogues. This lipopeptaibol antibiotic was studied in multilamellar membranes of dipalmitoylphosphatidylcholine frozen to 77 K. For mono-labelled trichogin analogues, the molecules are shown not to form aggregates in the lipid membranes studied. For the double-labelled trichogin analogues, a function of the distance distribution between the spin labels has been obtained. We determined that the distribution function has two main maxima located at distances of 1.25 nm and 1.75 nm. The value of 1.25 nm is close to the distance between labels of a alpha-helical structure. On the other hand, a distance of 1.75 nm corresponds to a mixed 3D-structure in which a 3(10)-helix is combined with a more elongated conformation
Self-aggregation of spin-labeled alamethicin in ePC vesicles studied by pulsed electron-electron double resonance
No full textThe pulsed electron-electron double resonance technique was used to study the dipole-dipole interactions between 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid spin labels located at position 16 of an alamethicin analogue in ePC vesicles that were frozen to 77 K. We show that under these conditions the alamethicin molecules tend to form aggregates over the range of peptide concentrations 3.5 x 10(-3) to 1 x 10(-2) M. The number of molecules in the aggregate is found to be 4.2 +/- 0.2. A spin-label distance distribution function is also obtained with a maximum at a distance of 2.3 nm and a half-height width of 1.3 nm. We envisage that these data will permit us to generate a molecular model of cellular ion channel
Aggregation of spin-labeled alamethicin in low-polarity solutions as studied by PELDOR spectroscopy
No full textAlamethicin is a 20-residue antibiotic peptide. Interest
in studying alamethicin stems from its ability to
form conducting channels in biological membranes,
thus changing their permeability. Although such channels
in artificial and biological membranes have been
well documented (see, e.g., [1, 2] and references
therein), available reliable data are still insufficient to
elucidate the mechanism of action of alamethicin on the
properties of membranes. Therefore, it is of interest to
study the self-aggregation of alamethicin molecules in
media mimicking the membrane surface and interfacial
layer, for example, in polar and nonpolar media.
One approach to studying peptide self-aggregation
in solutions is the spin label method in combination
with CW ESR and pulsed ESR electron–electron double
resonance (PELDOR). In this work, using these
methods, we obtained the first reliable structural data
on alamethicin aggregates in nonpolar media
Solvent effect on the distance distribution between spin labels in aggregated spin labeled trichogin GA IV dimer peptides as studied by pulsed electron-electron double resonance
No full textPulsed electron-electron double resonance (PELDOR) was used to study aggregate formation in frozen glassy solutions of mono- and double-spin labeled trichogin GA IV dimers in a toluene-methanol mixture. The modified method proposed and used for the distance distribution function calculation from the PELDOR data. Distance distribution functions between spin labels in the peptide molecules and their aggregates in solution were determined as a function of solvent composition. Double-labeled peptide molecules in aggregates in solutions with low methanol content display two types of structures, i.e. the alpha-helix with a 2.8 nm distance between labels and the 3(10)-helix with a 3.2 nm distance between labels. As the methanol content of the solvent increases, a part of conformations at 3.2 nm changes. An increase of the methanol content leads to disruption of the aggregates and a change to the peptide conformation as well. In pure methanol peptides fail to form aggregates and a wide distribution of distances between labels centered at 3 nm were observed
PELDOR Conformational Analysis of bis-Labeled Alamethicin Aggregated in Phospholipid Vesicles
No full textAlamethicin (Alm) is a linear peptide antibiotic of great interest for its capability to form self-assembled ion channels in lipid membranes. Here, the pulsed electron electron double resonance technique was used to obtain unique conformational information on the aggregated peptide in the lipid membrane-bound state. Since a specific helical conformation implies a given length to the peptide molecule, a distance r was measured at the nanometer scale via the electron dipole-dipole interaction between two 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid spin labels synthetically incorporated at positions 1 and 16 of this 19-mer peptide. Two data sets were collected (at 77 K): (i) from aggregates of Alm in hydrated egg-yolk phosphocholine (ePC) vesicles (at peptide-to-lipid ratios of 1:200 and 1:75) and (ii) from nonaggregated Alm in pure (nonhydrated) ePC and in solvents of different polarity. The intramolecular distance between the two labels obtained in this manner is in excellent agreement with that calculated on the basis of an almost fully developed a-helical conformation for this peptide and is found to be independent of the molecular aggregated state and the environment polarity as well
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
Supramolecular structure of self-assembling alamethicin analog studied by ESR and PELDOR
No full textThree analogs of alamethicin F50/5, labelled with the TOAC (=‘2,2,6,6-tetramethylpiperidin-1-oxyl-4-amino-4-carboxylic acid’) spin label at positions 1 (Alm1), 8 (Alm8), and 16 (Alm16), resp., were studied by Electron-Spin-Resonance (ESR) and Pulsed Electron–Electron Double-Resonance (PELDOR) techniques in solvents of different polarity to investigate the self-assembly of amphipathic helical peptides in membrane-mimicking environments. In polar solvents, alamethicin forms homogeneous solutions. In the weakly polar chloroform/toluene 1 : 1 mixture, however, this peptide forms aggregates that are detectable at 293 K by ESR in liquid solution, as well as by PELDOR in frozen, glassy solution at 77 K. In liquid solution, free alamethicin molecules and their aggregates show rotational-mobility correlation times τr of 0.87 and 5.9 ns, resp. Based on these values and analysis of dipole–dipole interactions of the TOAC labels in the aggregates, as determined by PELDOR, the average number N of alamethicin molecules in the aggregates is estimated to be less than nine. A distance-distribution function between spin labels in the supramolecular aggregate was obtained. This function exhibits two maxima: a broad one at a distance of 3.0 nm, and a wide one at a distance of ca. 7 nm. A molecular-dynamics (MD)-based model of the aggregate, consisting of two parallel tetramers, each composed of four molecules arranged in a ‘head-to-tail’ fashion, is proposed, accounting for the observed distances and their distribution
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
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