Silvia Pieraccini

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.

The Self-Assembly of a Lipophilic Guanosine Nucleoside into Polymeric Columnar Aggregates: The Nucleoside Structure Contains Sufficient Information To Drive the Process towards a Strikingly Regular Polymer

Lipophilic guanosine derivatives act as self-assembled ionophores. In the presence of alkali metal ions in organic solvents, these G derivatives can form tubular polymeric structures. The molecular aggregates formed by 3,5-didecanoyl-2-deoxyguanosine (1) have been characterised by SANS and NMR spectroscopy. The polymer is structured as a pile of stacked G quartets held together by the alkali metal ions that occupy the columns central channel. The deoxyribose moieties, with their alkyl substituents, surround the stacked G quartets, and the nucleosides long-chain alkyl tails are in intimate contact with the organic solvent. In this polymeric structure, there is an amazing regularity in the rotamers around the glycosidic bond within each G quartet and in the repeat sequence of the G quartets along the columns. In hydrocarbon solvents, these columnar aggregates form lyomesophases of the cholesteric and hexagonal types.

Identifying guanosine self assembly at natural isotopic abundance by high-resolution 1H and 13C solid-state NMR spectroscopy.

By means of the (1)H chemical shifts and the proton-proton proximities as identified in (1)H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)(2) and dG(C3)(2) derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by (15)N refocused INADEQUATE spectra in a previous study of (15)N-labeled derivatives (Pham, T. N.; et al. J. Am. Chem. Soc.2005, 127, 16018). The NH (1)H chemical shift is observed to be higher (13-15 ppm) for ribbon-like self-assembly as compared to 10-11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH(2)(1)H chemical shifts is also inverted, with the NH(2) proton closest in space to the NH proton having a higher or lower (1)H chemical shift than that of the other NH(2) proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)(2) derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, (14)N-(1)H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH(2) chemical shifts for the dG(C3)(2) derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face.

A new lyotropic liquid crystalline phase formed in hydrocarbon solvents by a deoxyguanosine derivative through extensive hydrogen bonding

Lipophilic deoxyguanosine 1 in pure hydrocarbons or in mixed CHCl3-hydrocarbon solvents forms a new lyotropic liquid crystalline phase characterized by a two dimensional square packing. The structural elements of the phase are ribbon-like assembled species which are completely different from the columnar structures, based on G-quartets, which are the building blocks of the mesophases formed by deoxyguanosine oligonucleotides in water.

A new axially-chiral photochemical switch

Axially chiral bis(azo) derivative 1 undergoes photochemical isomerisation, which can be seen with circular dichroism and pitch measurements of the induced cholesteric phases.

Columnar lyomesophases formed in hydrocarbon solvents by chiral lipophilic guanosine-alkali metal complexes

The lipophilic guanosine derivative 1 acts as a self-assembled ionophore and, in the presence of alkali metal ions, forms chiral polymeric structures in organic solvents. These polymeric columnar aggregates are comprised of G-quartets held together by alkali metal ions which are located inside the tubular structure; the quartets are surrounded by hydrocarbon chains. In hydrocarbon solvents, these columnar aggregates form lyomesophases of the cholesteric and hexagonal type. Copyright 2000 Wiley-Liss, Inc.

Guanosine derivatives: Self-assembly and lyotropic liquid crystal formation

The self-assembly and lyotropic mesomorphism of guanosine derivatives are reviewed. Natural and synthetic guanosine nucleotides self-assemble into columnar aggregates based on G-quartets; at the appropriate concentrations these aggregates form, in water, lyotropic mesophases of the cholesteric and hexagonal type. Lipophilic derivatives undergo different types of self-assembly: in the presence of alkali metal ions they form, in organic solvents, columnar structures and lyomesophases similar to those observed for the hydrophilic derivatives; in the absence of ions they form instead ribbon-like aggregates which give rise to new types of lyotropic phases. The ribbon-like aggregates have interesting electrical properties.

Effective cholesteric liquid crystal inducers based on axially chiral alleno-acetylenes

Axially chiral alleno-acetylenes, modified with lateral aryl groups via short and convenient synthetic pathways, are presented as efficient chiral dopants for producing cholesteric liquid crystals.

In-peptide synthesis of di-oxazolidinone and dehydroamino acid–oxazolidinone motifs as β-turn inducers

Small and easy-to-do mimetics of β-turns are of great interest to interfere with protein-protein recognition events mediated by β-turn recognition motifs. We propose a straightforward procedure for constraining the conformation of tetrapeptides lacking a pre-formed scaffold. According to the stereochemistry array, N-Ts tetrapeptides including Thr or PhSer (phenylserine) at the positions 2 or 3 gave rise in a single step to the sequences Oxd(2)-Oxd(3) or ΔAbu(2)-Oxd(3) (Oxd, oxazolidin-2-one; ΔAbu, 2,3-dehydro-2-aminobutyric). These pseudo-Pro residues displayed highly constrained ϕ, ψ, and χ dihedral angles, and induced clear β-turns or inverse turns of type I or II, as determined by extensive spectroscopic and computational analyses.

Small-angle X-ray scattering study of self-assembling lipophilic guanines in organic solvents: G-quadruplex formation and cation effects in cyclohexane.

Lipophilic guanilic derivatives (lipoGs) dissolved in organic solvents can undergo different self-assembly pathways based on different H-bonded motifs, e.g., the cyclic discrete G-quartet, which forms in the presence of alkali-metal ions, and the infinite tape-like G-ribbon observed in the absence of ions. Using in-solution small-angle X-ray scattering, we analyzed a series of lipoGs dissolved in cyclohexane in the presence of different salts. The formation of G-quartet based supramolecular aggregates has been confirmed, evidencing the coexistence equilibrium of octamers and noncovalent molecular nanowires (the so-called G-quadruplexes). By global fitting the scattering data, the concentration of the two kinds of particles as well as the nanowire length have been derived as a function of temperature for the different compounds and salts. The thermodynamic parameters show that the self-assembly aggregation process is enthalpy driven, while the observed enthalpy-entropy compensation suggests that similar stacking interactions control the self-assembly of the different compounds. However, the strength of the stacking interactions, and then the nanowire stability, depends on the nature of templating cations and on their capacity to fill the central cavity of quadruplexes, with the order Sr(+) < Na(+) ≲ K(+).