1,720,961 research outputs found
Effetti della chiralità su librerie covalenti dinamiche di peptidi
No full textI peptidi sono una classe di biomolecole in grado di attrarre molto interesse in diversi campi scientifici, grazie alla loro biocompatibilità e alla possibilità di assemblare in superstrutture in acqua. In particolare, i ciclopeptidi composti da α-amino acidi sono stati intensivamente studiati per la loro abilità di assemblare in nanotubi supramolecolari, i quali sono tenuti assieme da interazioni di legame idrogeno. L’intensiva ricerca in questa area ha elucidato le regole del design del loro auto-assemblaggio in termini di stereoconfigurazione e composizione amino-acidica. Tuttavia, uno studio estensivo di questa classe di composti richiede uno sforzo sintetico non banale. La chimica dinamica covalente offre quindi un’alternativa interessante alla classica sintesi consentendo di produrre ciclopeptidi da mattoncini lineari molto più semplici da sintetizzare. In questo modo, i reagenti si combinano in una libreria dinamica covalente. Inoltre, la formazione di legami covalenti sotto equilibrio termodinamico in concomitanza all’autoassemblaggio possono essere usati come possibile strada per scoprire nuove suprastrutture, grazie all’effetto di autoreplicazione.
Un set di stereoisomeri tripeptidici Cys-Phe-Cys sono stati sintetizzati per studiare la loro abilità di reagire covalentemente in acqua alcalina attraverso lo scambio tiolo/disolfuro. Lo stereoisomerismo ha influito sulla chimica dinamica portando alla formazione di diversi oligomeri ciclici e ha permesso di fare luce sulle differenze chiave tra la libreria omochirale e quelle eterochirali. Gli oligomeri ciclici sono stati purificati con la cromatografia ad alta prestazione a fase inversa e caratterizzati da RMN e diffrazione a raggi X. Una particolare stereoconfigurazione ha permesso alla libreria di auto assemblarsi in un idrogel composto da un solo macrociclo all’equilibrio termodinamico. L’analisi del materiale supramolecolare da reologia, dicroismo circolare e microscopia a trasmissione elettronica ha rivelato la presenza di una struttura anisotropica allungata compatibile con la presenza di nanotubi, i quali il diametro potrebbe corrispondere alla taglia del mattoncino macrociclico. Questo composto ha conformazioni simili a diversi pH ma è in grado di auto assemblarsi solo quando si trova in forma zwitterionica.
Riassumendo, questo studio dimostra come una scelta appropriata della stereoconfigurazione degli amino acidi su semplici tripeptidi può essere strategica nel progettare diverse strutture cicliche e in casi specifici, ottenere librerie in grado di convergere verso un gel composto da un unico prodotto. Questa ricerca pone quindi le basi per accedere facilmente ad una nuova classe di ciclopeptidi auto assemblanti che possono trovare varie applicazioni nella scienza di materiali e oltre.Peptides are a class of biomolecules that has attracted a lot of interest in different scientific fields, thanks to their biocompatibility and potential to self-assemble into supramolecular structures in water. In particular, cyclopeptides composed of α-amino acids have been intensively studied for their ability to stack into supramolecular nanotubes, which are held together by intermolecular hydrogen bonding. The intense research efforts in this area have unravelled the design rules for self-assembly in terms of stereoconfiguration, and amino-acidic composition. However, an extensive investigation on this class of compounds requires a synthetic effort that is not trivial. Dynamic covalent chemistry (DCC) thus offers an appealing alternative to classical chemical synthesis, to yield cyclopeptides from linear building blocks that are simpler to synthesise. In this way, the reagents can combine into a dynamic covalent library (DCL). Furthermore, the formation of covalent bonds under thermodynamic equilibrium can be used together with the self-assembly process, as a possible route to discover new suprastructures, thanks to the self-templating effect.
A set of Cys-Phe-Cys tripeptide stereoisomers was synthesised to study their ability to undergo DCC in alkaline water, through thiol/disulfide exchange. The stereoisomerism impacted the DCC leading to the formation of diverse cyclic oligomers, and it allowed to shed light on key differences between the homochiral, and the heterochiral DCLs. Cyclic oligomers were purified by reverse-phase HPLC, and characterised by NMR and single-crystal XRD. A particular stereoconfiguration enabled the DCL to self-assemble into a hydrogel composed of only one macrocycle at the thermodynamic equilibrium. The analysis on the supramolecular material by rheology, circular dichroism, and transmission electron microscopy revealed the presence of an anisotropic and elongated supramolecular structure, compatible with the presence of nanotubes whose diameter could correspond to the size of its macrocyclic building block. This compound had a similar conformation in solution at different pH values, but it was able to self-assemble into hydrogels only in the zwitterionic form.
Overall, this study demonstrated how an appropriate choice of amino acid stereoconfiguration on simple tripeptides can be a strategic design tool towards diverse cyclic structures and, in specific cases, to yield DCLs that eventually converge into a main gelator product. This research work thus laid the basis to easily access a new class of self-assembling cyclopeptides that could find various applications in materials science, and beyond
Unrevealing the Nitrogen Elusive Chirality of 3‐Sulfanyl and 3‐Sulfinyl N‐Tosyl Isoindolinones by ECD Spectra: An Experimental and Theoretical Investigation
No full textThe nitrogen–hybridization/pyramidalization of two solvated N-tosylisoindolinone derivatives having chiral residues in adjacent (I) or adjacent and distal (II) position has been investigated by a theoretical-computational procedure based on Molecular Dynamics simulations and Quantum–Chemical calculations. After validation of our methodology in providing a reliable repertory of conformations by modeling the electronic circular dichroism (EDC) spectra, the electronic features associated with N-pyramidalization were further characterized through Natural Bond Order (NBO) analysis. Comparing against the N-geometry observed in crystal structures as a reference, our findings reveal that the presence of neighbouring chiral centers induces a more pronounced N-pyramidalization in solution than in the solid state, both in I and II. Furthermore, NBO analysis confirms that the N-lactam mostly retains the sp2 character but exhibits slight configurational distortion (ξI=13°; ξII=21°), which significantly influences the chiroptical activities observed in ECD spectra of I and II. This substantiates the N-lactams as configurationally stable chiral centers
Single-atom substitution enables supramolecular diversity from dipeptide building blocks
Full text linkDipeptides are popular building blocks for supramolecular gels that do not persist in the environment and may find various applications. In this work, we show that a simple substitution on the aromatic side-chain of phenylalanine with either fluorine or iodine enables supramolecular diversity upon self-assembly at neutral pH, leading to hydrogels or crystals. Each building block is characterized by H-1- and C-13-NMR spectroscopy, LC-MS, circular dichroism, and molecular models. The supramolecular behaviour is monitored with a variety of techniques, including circular dichroism, oscillatory rheology, transmission electron microscopy, attenuated total reflectance Fourier-transformed infrared spectroscopy, visible Raman spectroscopy, synchrotron-radiation single-crystal X-ray diffraction and UV Resonance Raman spectroscopy, allowing key differences to be pinpointed amongst the halogenated analogues
Self-Assembly and Gelation Study of Dipeptide Isomers with Norvaline and Phenylalanine
Full text linkDipeptides have emerged as attractive building blocks for supramolecular materials thanks to their low-cost, inherent biocompatibility, ease of preparation, and environmental friendliness as they do not persist in the environment. In particular, hydrophobic amino acids are ideal candidates for self-assembly in polar and green solvents, as a certain level of hydrophobicity is required to favor their aggregation and reduce the peptide solubility. In this work, we analyzed the ability to self-assemble and the gel of dipeptides based on the amino acids norvaline (Nva) and phenylalanine (Phe), studying all their combinations and not yielding to enantiomers, which display the same physicochemical properties, and hence the same self-assembly behavior in achiral environments as those studied herein. A single-crystal X-ray diffraction of all the compounds revealed fine details over their molecular packing and non-covalent interactions
Self-assembly of heterochiral, aliphatic dipeptides with Leu
Full text linkThis work describes the self-assembly behavior of heterochiral, aliphatic dipeptides, L-Leu-D-Xaa (Xaa=Ala, Val, Ile, Leu), in green solvents such as acetonitrile (MeCN)and buffered water at neutral pH. Interestingly, water plays a structuring role because at 1% v/v, it enables dipeptide self-assembly in MeCN to yield organogels, which then undergo transition towards crystals. Other organic solvents and oils were tested for gelation, and metastable gels were formed in tetrahydrofuran, although at high peptide concentration (80 mM). Single-crystal X-ray diffraction revealed the dipeptides' supramolecular packing modes in amphipathic layers, as opposed to water channels reported for the homochiral Leu–Leu, or hydrophobic columns reported for homochiral Leu–Val and Leu–Ile
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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