Lipophilic G-Quadruplexes: Structural Studies, Post-Assembly Modification, and Covalent Capture

Abstract

New nanostructures and functional materials are built through the self-assembly of guanosine. Both the size and regiochemistry of these noncovalent structures are controlled. Lipophilic G-quadruplexes are further stabilized through covalent capture techniques. These new nanostructures demonstrate the ability to bind cations and transport monovalent cation through phospholipid membranes. Diffusion NMR is demonstrated as a valuable technique in characterizing the size of lipophilic G-quadruplexes. Control over the size of self-assembled G-quadruplexes is demonstrated through modifying the guanosine nucleosides and the cation concentration. The solution structure of [G 8]16 4K+ 4pic- is determined to be a hexadecamer using diffusion NMR. Additionally, G 24 is also shown to form a hexadecamer G-quadruplex, which has an octameric intermediate structure. Two different octamers, a singly and doubly charged octamer, formed by G 29 are elucidated by diffusion NMR. The information gained from the diffusion NMR technique allowed for a better understanding of the self-assembly processes, especially regarding the roles of cation, anion and solvent. The use of a kinetically controlled exchange reaction to effect regioselective modification of a hydrogen-bonded assembly is discussed. The pseudo-regioselective exchange of isotopically labeled G 35-d into [G 8-h]16 4K+ 4pic- is demonstrated. Both the bound anion and cation can control the exchange of ligand into the different layers of a synthetic G-quadruplex. This regioselective exchange process allows for functionalized G-quadruplex structures to be built. Covalent capture of lipophilic G-quadruplex 60 with reactive groups on the periphery generates a unimolecular G-quadruplex 61. This unimolecular G-quadruplex 61 shows exceptional stability in nonpolar and polar solvents, even without the presence of cations. Furthermore, this unimolecular G-quadruplex transports monovalent cation across phospholipid membranes. The design of transmembrane transporters is of particular interest for their potential as new ion sensors, catalysts and anti-microbial agents.