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TRIGGERING AND CD INVESTIGATION OF GUANOSINE SELF-ASSEMBLY
No full textIn this lecture I discuss two different issues that have been recently considered in my research group. The first topic concerns the triggering of (lipophilic) guanosine self-assembly by means of light and modification of the solvent polarity. The second topic consists in the non empirical re-visitation of the Circular Dichroism spectra obtained by G-quadruplexes of different topology.
Triggering of guanosine self-assembly by light and solvent variation. – The chemically driven switching between different supramolecular motifs for lipoG both in solution and at the solid-liquid interface has been already reported.1,2 I will discuss here two examples of triggering the self-assembly by means of two different types of external stimuli: light and solvent polarity variation.
In the former example, the photocontrolled self-assembly of a modified guanosine nucleobase with a photoactive unit at C8 is presented.3 The compound, when in the E configuration at the photoactive unit, in the presence of a measured amount of KI self-assembles into a D4-symmetric complex consisting of two stacked G-quartets. Photoisomerization to the Z isomer determines the decomposition of the octameric complex, which is re-formed when the molecule is reverted to the E form either by thermal or photochemical back isomerization.
As a second example, I report our findings on a lipophilic guanosine derivative armed with a terthiophene unit that undergoes a pronounced variation of its supramolecular organisation by changing the polarity of the solvent.4 In chloroform the guanosine derivative, templated by alkali metal ions, assembles via H-bonding in G-quartet based D4-symmetric octamers. In the more polar (and H-bond competing) acetonitrile different aggregates are observed, where the terthiophene chains are p-p stacked in a helicoidal (left-handed) arrangement. The system can be switched back and forth by subsequent addition of chloroform and acetonitrile.
A non-empirical chromophoric interpretation of CD spectra of DNA G-quadruplex structures. – The number of publications where CD spectroscopy has been used to study G4-DNAs, is extremely high. However, with very few exceptions, these investigations use an empirical interpretation of CD spectra. In this interpretation two basic types of CD spectra have been associated to a single specific difference in the features of the strand folding, i.e. the relative orientation of the strands, “parallel” (all strands have the same 5’ to 3’ orientation) or “antiparallel”. The different spectral features observed for different G-quadruplexes is rationalised in terms of chromophores responsible for the electronic transitions.5 A simplified exciton coupling approach or more refined QM calculations allow to interpret the different CD features in terms of different stacking orientation (head-to-tail, head-to-head, tail-to-tail) between adjacent G-quartets irrespectively of the relative orientation of the strands (parallel/antiparallel).
1) G. P. Spada, S. Lena, S. Masiero, S. Pieraccini, M. Surin, P. Samorì, Adv. Mat. 2008, 20, 2433-2438
2) A. Ciesielski, S. Lena, S. Masiero, G. P. Spada, P. Samorì, Angew. Chem. Int. Ed. 2010, 49, 1963-1966
3) S. Lena, P. Neviani, S. Masiero, S. Pieraccini, G. P. Spada, Angew. Chem. Int. Ed. 2010,49, 3657-3660
4) S. Pieraccini, S. Bonacchi, S. Lena, S. Masiero, M. Montalti, N. Zaccheroni, G. P. Spada, Org. Biomol. Chem. 2010, 8, 774-781;
5) S. Masiero, R. Trotta, S. Pieraccini, S. De Tito, R. Perone, A. Randazzo, G. P. Spada,
Org. Biomol. Chem. 2010, 8, 2683-2692
The liquid crystal - linear dichroism (L.C.-L.D.) of organic molecules by a modulation technique. Part.2. The phenylthio and thiophten chromophores studied by an 'L.D. substitution approach'
No full textNANOSCALE MONITORING OF RESPONSIVE SUPRAMOLECULAR NANOSTRUCTURES AT THE SOLID-LIQUID INTERFACE
No full textThe self-assembly of small molecular modules into non-covalently linked polymeric nanostructures is a subject of continuous interest [1]. In particular, supramolecular structures with a high degree of order can be obtained through the self-association of organic molecules on flat solid surfaces. Such structures can be used as scaffolds to position electrically/optically active groups in pre-determined locations in 2D [2] thereby paving the way towards a wide range of applications, e.g. in electronic and optical devices. Among weak interactions, H-bonding offers high control over the process of molecular self-assembly because it combines reversibility, directionality, specificity and cooperativity. Such a unique character is the basis of sophisticated programs for self-assembly such as those based on the Watson–Crick base pairing which directs the formation of the helical structure of DNA.
While the self-assembly of guanines into G-quartet based architectures on solid surfaces has been studied by Scanning Tunneling Microscopy (STM) under ultrahigh vacuum (UHV) [3], STM explorations at the solid-liquid interface have been primarily carried out on guanosine derivatives [4]. In this lecture we will present a sub-molecularly resolved STM study at the solid-liquid interface of the metal templated reversible assembly/reassembly process of a N9-alkylguanine into highly ordered quartets and ribbons on highly oriented pyrolitic graphite (HOPG) surfaces. The self-assembly of guanine derivatives on HOPG has been studied as neat component[5] and upon sub-sequent addition of cryptand [2.2.2] molecules, potassium picrate and triflic acid to trigger the reversible interconversion between two different highly ordered supramolecular motifs, i.e. H-bonded ribbon and G-quartet based architectures. In-situ STM imaging provided the first direct evidence on the sub-nm scale of a dynamer operating at surfaces. [6]
Finally we will also provide the first example of sub-molecularly resolved vertically oriented switchable chromophores in physisorbed monolayers by monitoring the cis-trans isomerization of a physisorbed azobenzene at surfaces. [7]
The nanoscale visualization of such supramolecular interconversion at the solid-liquid interface opens new avenues towards understanding the mechanism of formation and functioning of complex architectures. Furthermore, the in-situ reversible assembly and re-assembly between two highly ordered supramolecular structures at a given surface represents the first step towards the generation of nanopatterned responsive architectures.
[1] J. M. Lehn, Science, 295, 2002, 2400.
[2] G. P. Spada, S. Lena, S. Masiero, S. Pieraccini, M. Surin, P. Samorì, Adv. Mater., 2008, 20, 2433; A. Ciesielski, L. Piot, P. Samorì, A. Jouaiti, M. W. Hosseini, Adv. Mater., 2009, 21, 1131.
[3] R. Otero, M. Schock, L. M. Molina, E. Laegsgaard, I. Stensgaard, B. Hammer, F. Besenbacher, Angew. Chem. Int. Ed., 2005, 44, 2270.
[4] G. Gottarelli, S. Masiero, E. Mezzina, S. Pieraccini, J. P. Rabe, P. Samorì, G. P. Spada, Chem. Eur. J., 2000, 6, 3242; T. Giorgi, S. Lena, P. Mariani, M. A. Cremonini, S. Masiero, S. Pieraccini, J. P. Rabe, P. Samorì, G. P. Spada, G. Gottarelli, J. Am. Chem. Soc., 2003, 125, 14741; S. Lena, G. Brancolini, G. Gottarelli, P. Mariani, S. Masiero, A. Venturini, V. Palermo, O. Pandoli, S. Pieraccini, P. Samorì, G. P. Spada, Chem. Eur. J., 2007, 13, 3757.
[5] A. Ciesielski, R. Perone, S. Pieraccini, G.P. Spada, P. Samorì, Chem. Commun 2010, 46, 4493.
[6] A. Ciesielski, S. Lena, S. Masiero, G. P. Spada, P. Samorì, Angew. Chem. Int. Ed. 2010, 49, 1963.
[7] D. Bléger, A. Ciesielski, P. Samorì, S. Hecht, 2010 submitted
Chiral doping of nematic phases and its application to the determination of absolute configuration
No full textDoping nematic liquid crystals with nonracemic chiral compounds induces a twisted nematic (cholesteric) phase. The ability of solutes to twist the nematic phase may be related to the overall shape of the chiral dopant and consequently to its absolute configuration. The cholesteric induction is therefore a powerful tool complementary to chiroptical techniques to obtain stereochemical information on chiral molecules
A study of the structure of the lyomesophases formed by the dinucleoside phosphate d(GpG). An approach by X-ray diffraction and optical microscopy
No full text
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
Chiral amplification in a cyanobiphenyl nematic liquid crystal doped with helicene-like derivatives
Full text linkThe addition of a chiral non-racemic dopant to a nematic liquid crystal (LC) has the effect of transferring the molecular chirality to the phase organization and a chiral nematic phase is formed. This molecular chirality amplification in the LC provides a unique possibility for investigating the relationship between molecular structure, intermolecular interactions, and mesoscale organization. It is known that axially chiral or helical-shaped molecules with reduced conformational disorder are good candidates for high helical twisting power derivatives. In particular, biaryl derivatives are known to be efficient chiral inducers in biaryl nematic mesophases. In this paper, we focus on a new series of helicene-like molecules of known absolute configuration. We have integrated cholesteric pitch measurements with geometry optimization by DFT calculations and analysis of the twisting ability by the Surface Chirality model to shed light on the structural features responsible for the analogies and differences exhibited by these derivatives. The investigation of these dopants with well-defined geometry, by virtue of the low conformational freedom, and the substituents variously distributed around the core, allows us to extend our knowledge of the molecular origin of the chirality amplification in liquid crystals and to confirm the simple relationship “molecular P-helicity” → “cholesteric P-handedness” for helical-shaped helicene-like derivatives
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
The self-assembly of dideoxyguanosine (3-3') and (5-5') monophosphates.
No full textThe title compounds show a pronounced cation-directed ability to self-assemble in water and to gives columnar structures similar to four-stranded helices; for compound (5'-->5')-d(GpG), this leads to the formation of cholesteric and hexagonal liquid crystalline phases. Both phases are columnar and the cholesteric phase is left handed. This behaviour is a further confirmation of the tendency of guanine derivatives to self-assemble to give stacked columnar structures whenever not impossible for structural reasons. The CD spectra of the aggregates in isotropic solutions are dominated by a negative exciton couplet centred around 250 nn associated to a left-handed columnar chirality. The shapes of the profiles, in the 220-300-nm region, for (5'-->5')-d(GpG) (in water or in saline solutions) and for (3'-->3')-d(GpG) (in KCI solution) are quasi-mirror images of those of poly(G) and (3'-->5')-d(GpG). The appearance of relatively intense CD signals around 280-300 nm in solution of (3'-->3')-d(GpG) in the presence of NaCl resembles that of (3'-->5')-d(GpG) in the presence of Rb+ or Na+. In the compounds investigated in this work, which present two equivalent ends, one observes the two CD features that have been associated, in the current literature, with the signature of four-stranded parallel and antiparallel structures: hence the origin of these CD bands cannot be found in the polarity of the strands. Self-assembly is favoured by the addition of extra salt and the stabilising effect of K+ is greater than that of Na+ in the case of (3'-->3')-d(GpG), an assembled species could be detected by CD only in the presence of extra salt
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