1,721,138 research outputs found
The prebiotic role of Liquid Crystal self-assembly of DNA oligomers
No full textMolecular crowding has a crucial role in tuning the hierarchical self-assembly of complex macromolecular structures and in boosting chemical reactions, allowing the proximity of reactants.
It has been recently shown that at high concentration (> 200 mg/ml) short DNA oligomers (4-20 bases) associated in double helices may order into Liquid Crystal (LC) phases despite their nearly globular shape [1]. In these systems the formation of LC is mediated by the end-to-end aggregation of DNA duplexes into columns of chemically distinct but physically continuous duplexes.
LC ordering of DNA oligomers appears to be a robust phenomenon even in crowded molecular mixtures. Indeed LC phases are found in concentrated solution of random sequence DNA oligomers [2], and in systems in which double stranded DNA is mixed with DNA single strands or with poly-(ethylene glycol) (PEG) chains [3]. In these systems the formation of LC domains is associated with phase separations providing a mechanism of self-selection and compartmentalization of DNA in water, otherwise unusual without the presence of vesicle membrane (see cartoons in Fig.). Moreover we found that DNA-PEG phase separation influences the quality and the yield of non-enzymatic ligation of DNA duplexes, catalyzing the formation of longer strands (graph in Fig.). 1) M. Nakata et al.,Science, 318, 1276 (2007).
2) T. Bellini et al., PNAS, 109, 1110 (2012).
3) G. Zanchetta, M. Nakata, M. Buscaglia, T. Bellini, N.A. Clark, PNAS, 105, 1111 (2008)
The electrokinetic behavior of charged non-spherical colloids
No full textThe response of charged colloids to electric fields is determined by combined phenomena occurring first in the electric double layer to then develop into long-range perturbations of ion concentration, local fields, and solvent flows. When particles are non-spherical, the loss of symmetry affects the short- and long-ranged processes modifying their behavior as observed through their electrophoretic mobility, dielectric permittivity, and electro-optical response. Recent measurements and theoretical developments have revealed phenomena characteristic for non-spherical particles, such as the doubling of the relaxations in the dielectric spectra, the appearance of torque-inducing hydrodynamic flows, and the anomalous perpendicular alignment. In this article we discuss in a unifying frame the recent experimental and theoretical progresses about the electrokinetic behavior of charged non-spherical colloids
Differential dynamic microscopy of fluctuating liquid crystals
No full textFluctuations of the director orientation in liquid crystalline samples can reveal precious information about their viscoelasticity. Laser light scattering (LLS) is a well established tool for extracting such information and has been extensively used for a long time [1]. Although these fluctuations can be easily seen in depolarized microscopy with the naked eye, only in a few isolated cases a quantitative study was attempted [2-4]. We present here experimental results obtained with the recently introduced Differential Dynamic Microscopy (DDM) [5,6] on thin layers of nematic liquid crystals (LC). DDM allows to obtain scattering information from the study of microscopy images. We show that depolarized DDM is perfectly suitable to determine the viscoelastic properties of thin layers of nematic LC, providing direct access to the intermediate scattering function at small scattering wavevectors, which are precluded to ordinary LLS. The differential nature of the technique allows also relaxing the strict cleanliness requirements typically needed in LLS experiments. With a single experiment less than 4 s long, all the three viscoelastic ratios can be measured in a LC sample with suitable alignment, thereby demonstrating a very powerful tool for the rapid characterization of LC. Our results, in agreement with literature values, suggest a routine use of microscopes for the determination of the viscoelastic properties of thermotropic and lyotropic LCs in harsh conditions and for the characterization of various optically anisotropic fluids. References
[1] H. F. Gleeson in Handbook of Liquid Crystals, edited by D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess (Wiley-VCH, Halle, 2008), pp. 699-718.
[2] Y. Galerne, I. Poinsot, and D. Schaegis, Appl. Phys. Lett. 71, 222 (1997)
[3] H. Orihara, A. Sakai, and T. Nagaya, Mol. Cryst. Liq. Cryst. 366, 143 (2001)
[4] A. Yethiraj, R. Mukhopadhyay, and J. Bechhoefer Phys. Rev. E 65, 021702 (2002)
[5] R. Cerbino, and V. Trappe, Phys. Rev. Lett. 100, 188102 (2008)
[6] F. Giavazzi, D. Brogioli, V. Trappe, T. Bellini, and R. Cerbino, Phys. Rev. E 80, 031403 (2009
Kinetics of intramolecular contact formation in disordered peptides and unfolded proteins
No full textThe formation of intramolecular contacts between distant residues is, perhaps, the simplest among the elementary processes taking place during the folding of a protein. Quenching of the triplet state of tryptophan by close contact with cysteine after nanosecond laser excitation has been used to measure both equilibrium (reaction-limited) and dynamic (diffusion-limited) contact formation rates in model peptides and engineered proteins. The triplet quenching data and FRET end-to-end distances are analyzed by means of a chain model with realistic backbone conformations and simplified hard-sphere interaction between the residues. The effect of amino acid sequence on the chain swelling is discussed
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
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