Giovanni Gottarelli

The Self‐Assembly of Lipophilic Guanosine Derivatives in Solution and on Solid Surfaces

The self-assembly of lipophilic deoxyguanosine derivatives 1 and 2 has been studied in solution by NMR spectroscopy and ESI-MS (electrospray ionization mass spectrometry). NMR data show the existence of two types of self-assembled, ribbonlike structures (A and B), which are connected at the guanine moieties through two different H-bonded networks. The first species (A), which is stable in the solid state and characterised by cyclic NH(2)-O(6) and NH(1)-N(7) hydrogen bonds, is detected soon after dissolving the polycrystalline powder in rigorously anhydrous CDCl3. In solution it slowly undergoes a structural transition towards a thermodynamically stable ribbon characterised by NH(1)-O(6) and NH(2)-N(3) cyclic hydrogen bonds (B). On the other hand, at surfaces, self-assembled ribbon nanostructures have been grown from solutions of derivative 1 both on mica and at the graphite-solution interface. They have been investigated by means of tapping mode scanning force microscopy (SFM) and scanning tunnelling microscopy (STM), respectively. SFM revealed dry, micrometer-long nanoribbons with a molecular cross-section. while STM imaging at submolecular resolution indicates a molecular packing of type A, like the one detected in the solid state. This indicates that, upon adsorption at the solid-liquid interface, the guanosine moieties undergo a structural rearrangement from a B-type to an A-type ribbon.

Gel-Like Lyomesophases Formed in Organic Solvents by Self-Assembled Guanine Ribbons

Lipophilic guanosine derivatives are self-assembled into ribbonlike aggregates, both in the crystal state and in solution. The structure of the ribbons has been characterised by single-crystal X-ray diffraction and, in solution, by NMR spectroscopy and ESI-MS. Two different ribbons with different patterns of hydrogen bonds are present in the solid state and in chloroform solutions. The gel-like phases obtained in hexadecane, toluene and chloroform have been investigated by optical microscopy and small-angle X-ray diffraction: the type of phase observed is related to the molecular structure of the compounds and depends dramatically on the solvent. The structures of the phases are discussed, with the presence of the two different ribbons being taken into account.

Induced Cholesteric Mesophases: Origin and Application

Abstract The relationship between the twisting power (β) and the molecular structure of the dopant and neraatic solvent are discussed. High values of β are observed whenever two (or more) planar or quasi-planar moieties, chirally distorted one with respect to the other, are present in the dopant. For high twisting powers, the mechanism of induction seems to be connected to the transfer of chirality from the solute to the solvent via chiral conformations of the latter. Sterochemical and analytical applications are briefly discussed.

Photodetectors fabricated from a self-assembly of a deoxyguanosine derivative

A metal–semiconductor–metal (MSM) photodetector has been fabricated using as the semiconductor, a self-assembled layer of a DNA basis, namely a deoxyguanosine derivative, deposited between two gold electrodes. These were defined lithographically on a SiO2 substrate, separated by a distance of about 120 nm. The resulting self-assembled guanosine crystal has been deposited in such a way to achieve striking semiconducting properties. We show that with these conditions, the I–V characteristics are independent of the crystal orientation. The device shows a high current response (differential resistance at room temperature ranges in MΩ) which is symmetric with respect to bias sign and dependent on the illumination conditions. This behavior can be explained by taking into account the standard MSM theory and its applications as a photodetector.

THE SELF-ASSEMBLY OF A LIPOPHILIC DEOXYGUANOSINE DERIVATIVE AND THE FORMATION OF A LIQUID-CRYSTALLINE PHASE IN HYDROCARBON SOLVENTS

The lipophilic 3′,5′-di-O-decanoyl-2′-deoxyguanosine (1) in CHCl3 undergoes extensive self-assembly, mediated by H-bonding between the guanine bases, to give ribbon-like aggregates. X-Ray investigation of the platelets obtained from CHCl3 reveals a disordered fibre-like structure consisting of stacks of the ribbon-like aggregates. The aggregates are completely different from the columnar structures, based on G-quartets, which are the building blocks of the mesophases formed by deoxyguanosine oligonucleotides in H2O. In pure hydrocarbons or in CHCl3/hydrocarbons, 1 forms a lyotropic liquid-crystalline phase.

Induction of cholesteric mesophases in nematic liquid crystals by some chiral aryl alkyl carbinols : A quantitative investigation

Abstract A series of chiral aryl alkyl carbinols, dissolved in MBBA and other nematic solvents, induce cholesteric structures. The handedness of the induced mesophases and the twisting power of the dopant alcohols are studied by means of CD and Grandjean-Cano microscopic techniques. The investigation points out the possibility of obtaining information on the stereochemistry of the dopant by studying the characteristics of the induced helices. Two different types of H- bonds between alcohols and MBBA are discussed in connection with a possible model of induction.

Induction of cholesteric mesophases in nematic liquid crystals, and correlation of absolute configurations of some chiral oxiranes and thiiranes

Abstract It is shown that the presence of two chirally-distorted aromatic groups is the principal factor determining the twisting power of optically-active trans-stilbene oxide and analogous molecules in nematic liquid crystals. The absolute configuration of a series of chiral oxiranes and thiiranes can be correlated by comparing their twisting powers in MBBA; this liquid crystal method being in some cases complementary to CD spectroscopy.

Hybrid molecular electronic devices based on modified deoxyguanosines

A new type of organic semiconductor, based on self-assembled modified DNA nucleosides (deoxyguanosine lipophilic derivative), is employed to fabricate hybrid molecular devices. Two different modifications of the guanosine are synthesized in order to change the oxidation potential of the active molecular system. The influence of the solvent on the self-assembly process and on the carrier transport is investigated by atomic force microscopy and electrical measurements, respectively. Our results demonstrate that upon appropriate engineering of the molecule and choice of the solvent, self-organization of molecules in ordered structures is achieved, resulting in high conductivity and electronic rectification in solid-state, planar molecular devices. The transport properties depend strongly on the molecular structure and on the arrangement of the molecules into the hybrid devices, thus opening the route to molecular transport engineering in hybrid molecular electronics.

Enantioselective Extraction of Dinitrophenyl Amino Acids Mediated by Lipophilic Deoxyguanosine Derivatives: Chiral Discrimination by Self-Assembly.

The transfer of potassium salts of dinitrophenyl amino acids from water to chloroform by the lipophilic guanosine derivative 1 takes place enantioselectively. Depending on the K(+):1 ratio, G-quartets of 1 self-assemble into octamers (O) or polymers.

Liquid crystals and chirality : application to the determination of absolute configurations and to the conformational analysis of biaryls

Abstract Nematic liquid crystals are extremely sensitive to optically active molecules, which at very low concentration, convert them into cholesteric liquid crystals. It will be shown that conformational factors are extremely important in determining the twisting powers of optically active trans stilbene oxide and biarylmolecules in nematic phases. The results showed that it is possible to correlate the absolute configurations of optically active bridged biaryl compounds and also the conformation (cisoid or transoid) of optically active open-chain biaryls to the sign of their twisting powers in biphenyl nematics, this liquid crystal method being in some cases complementary to CD spectroscopy.