K vs. Na Effects on the Self-Assembly of Guanosine 5'-Monophosphate: A Solution SAXS Structural Study+.

The hierarchical process of guanosine (G) self-assembly, leading in aqueous solution and in the presence of metal cations to the formation of G-quadruplexes, represents an intriguing topic both for the biological correlation with telomerase activity and for the nano-technological applications, as demonstrated by the current measured in a quadruplex wire 100 nm long. Similar to G-rich DNA sequences and G-oligonucleotides, the guanosine 5'-monophosphate (GMP) self-aggregates in water to form quadruplexes. However, due to the absence of a covalent axial backbone, this system can be very useful to understand the chemical-physical conditions that govern the guanosine supramolecular aggregation. We have then investigated by in-solution Synchrotron Small Angle X-ray Scattering technique the role of different cations in promoting the quadruplex formation as a function of concentration and temperature. Results show how potassium, with its peculiar biological traits, favours the G-quadruplex elongation process in respect to other cations (Na + , but also NH 4 + and Li + ), determining the longest particles in solution. Moreover, the formation and the elongation of G-quadruplexes have been demonstrated to be controlled by both GMP concentration and excess cation content, even if they specifically contribute to these processes in different ways. The occurrence of condensed liquid crystalline phases was also detected, proving that excess cations play also unspecific effects on the effective charges on the G-quadruplex surface.

Metallo-responsive self-assembly of lipophilic guanines in hydrocarbon solvents: a systematic SAXS structural characterization.

Lipophilic guanines (LipoGs) in aprotic solvents undergo different self-assembly processes based on different H-bonded motifs. Cylindrical nanotubes made by π-π stacked guanine tetramers (G-quadruplexes) and flat, tape-like aggregates (G-ribbons) have been observed depending on the presence of alkali metal ions. To obtain information on the structural properties and stability of these LipoG aggregates, Small-Angle X-ray Scattering (SAXS) experiments have been performed in dodecane, both in the presence and in the absence of potassium ions. As a result, the occurrence of the two different metallo-responsive architectures (nanoribbons or columnar nanotubes) was confirmed and we reported here for the first time a systematic study on the dependence of the aggregate properties on composition, temperature and molecular unit structure. Even if dodecane was selected to favour LipoG solubility, a strong tendency to self-organize into ordered lyotropic phases was indeed detected.

Pressure effects on α-synuclein amyloid fibrils: An experimental investigation on their dissociation and reversible nature.

α-synuclein amyloid fibrils are found in surviving neurons of Parkinson's disease affected patients, but the role they play in the disease development is still under debate. A growing number of evidences points to soluble oligomers as the major cytotoxic species, while insoluble fibrillar aggregates could even play a protection role. In this work, we investigate α-synuclein fibrils dissociation induced at high pressure by means of Small Angle X-ray Scattering and Fourier Transform Infrared Spectroscopy. Fibrils were produced from wild type α-synuclein and two familial mutants, A30P and A53T. Our results enlighten the different reversible nature of α-synuclein fibrils fragmentation at high pressure and suggest water excluded volumes presence in the fibrils core. Wild type and A30P species stabilized at high pressure are highly amyloidogenic and quickly re-associate into fibrils upon decompression, while A53T species shows a partial reversibility of the process likely due to the presence of an intermediate oligomeric state stabilized at high pressure. The amyloid fibrils dissociation process is here suggested to be associated to a negative activation volume, supporting the notion that α-synuclein fibrils are in a high-volume and high-compressibility state and hinting at the presence of a hydration-mediated activated state from which dissociation occurs.

A lipophilic "fully-anti" dodecamer from a (5'S)-5',8-cyclo-2'-deoxyguanosine.

The self-assembly of a lipophilic derivative of (5'S)-5',8-cyclo-2'-deoxyguanosine, a mutagenic product formed by hydroxyl radical attack against DNA, has been investigated. This derivative forms, with high fidelity, a dodecameric complex composed of three stacked G-quartets in the presence of strontium picrate. This is the first example of a fully-anti lipophilic G-quadruplex.

How soft are biological helices? A measure of axial and lateral force constants in folate quadruplexes by high-pressure X-ray diffraction.

Alkaline folates self-associate in aqueous solutions to form columnar lyotropic phases. Such phases are made by quadruplexes, which are supramolecular helicoidal structures formed by a stacked array of folate tetramers. High-pressure synchrotron X-ray diffraction is used to analyze alkaline folate quadruplex stability and energetics. Diffraction data show that both inter-helical lateral and tetramer stacking distances decrease as a function of pressure. Lateral and axial quadruplex compressibilities and force constants have been derived and strong correlation between the strength of tetramer stacking and pressure effects demonstrated. In particular, quadruplex rigidity increases by changing Na+ to K+ and by adding excess KCl, as a consequence of increased stacking interactions and quadruplex elongation.