Triggering of Guanosine Self-Assembly by Light

Heterocycle-modified 2'-Deoxyguanosine Nucleolipid Analogs Stabilize Guanosine Gels and Self-assemble to Form Green Fluorescent Gels

Nucleoside-lipid conjugates are very useful supramolecular building blocks to construct self-assembled architectures suited for biomedical and material applications. Such nucleoside derivatives can be further synthetically manipulated to endow additional functionalities that could augment the assembling process and impart interesting properties. Here, we report the design, synthesis and self-assembling process of multifunctional supramolecular nucleolipid synthons containing an environment-sensitive fluorescent guanine. The amphiphilic synthons are composed of an 8-(2-(benzofuran-2-yl)vinyl)-guanine core and alkyl chains attached to 3'-O and 5'-O-positions of 2'-deoxyguanosine. The 2-(benzofuran-2-yl)vinyl (BFV) moiety attached at the C8 position of the nucleobase adopted a syn conformation about the glycosidic bond, which facilitated the self-assembly process through the formation of a G-tetrad as the basic unit. While 3',5'-diacylated BFV-modified dG analog stabilized the guanosine hydrogel by hampering the crystallization process and imparted fluorescence, BFV-modified dGs containing longer alkyl chains formed a green fluorescent organogel, which transformed into a yellow fluorescent gel in the presence of a complementary non-fluorescent cytidine nucleolipid. The ability of the dG analog containing short alkyl chains to modulate the mechanical property of a gel, and interesting fluorescence properties and self-assembling behavior exhibited by the dG analogs containing long alkyl chains in response to heat and complementary base underscore the potential use of these new supramolecular synthons in material applications.

Development of Minimal Diguanosinyl Motif toward RNA G-Quadruplex-Like Structures in Solution

Among the non-canonical structures of B-DNA, the G-quadruplex is of particular interest because of its well-defined conformation, high stability, and versatility. Herein we report our studies on the development of an amide-linked minimal diguanosinyl motif that forms a G-quadruplex-like structure in solution in the presence of potassium cations; various linear guanosine amino acid dimers were synthesized with linkers of different chain lengths to investigate the optimum flexibility required to form such structures.

Templating and Catalyzing [2+2] Photocycloaddition in Solution Using a Dynamic G-Quadruplex

We describe a templating/covalent capture strategy that enables photochemical formation of 8 cyclobutanes in one noncovalent assembly. This process was characterized by experiment and quantum mechanical/molecular mechanics (ONIOM) calculations. Thus, KI and 16 units of 5'-cinnamate guanosine form a G-quadruplex where C=C π bonds in neighboring G₄ -quartets are separated by 3.3 Å, enabling [2+2] photocycloaddition in solution. This reaction is high-yielding (>90 %), regio- and diastereoselective. Since all components are in dynamic equilibrium this photocycloaddition is catalytic in K⁺ .

Co-existence of Distinct Supramolecular Assemblies in Solution and in the Solid State

The formation of distinct supramolecular assemblies, including a metastable species, is revealed for a lipophilic guanosine (G) derivative in solution and in the solid state. Structurally different G‐quartet‐based assemblies are formed in chloroform depending on the nature of the cation, anion and the salt concentration, as characterized by circular dichroism and time course diffusion‐ordered NMR spectroscopy data. Intriguingly, even the presence of potassium ions that stabilize G‐quartets in chloroform was insufficient to exclusively retain such assemblies in the solid state, leading to the formation of mixed quartet and ribbon‐like assemblies as revealed by fast magic‐angle spinning (MAS) NMR spectroscopy. Distinct N−H⋅⋅⋅N and N−H⋅⋅⋅O intermolecular hydrogen bonding interactions drive quartet and ribbon‐like self‐assembly resulting in markedly different 2D ¹H solid‐state NMR spectra, thus facilitating a direct identification of mixed assemblies. A dissolution NMR experiment confirmed that the quartet and ribbon interconversion is reversible–further demonstrating the changes that occur in the self‐assembly process of a lipophilic nucleoside upon a solid‐state to solution‐state transition and vice versa. A systematic study for complexation with different cations (K⁺, Sr²⁺) and anions (picrate, ethanoate and iodide) emphasizes that the existence of a stable solution or solid‐state structure may not reflect the stability of the same supramolecular entity in another phase.

Photoresponsive Formation of an Intermolecular Minimal G-Quadruplex Motif

The ability of three different bifunctional azobenzene linkers to enable the photoreversible formation of a defined intermolecular two-tetrad G-quadruplex upon UV/Vis irradiation was investigated. Circular dichroism and NMR spectroscopic data showed the formation of G-quadruplexes with K(+)  ions at room temperature in all three cases with the corresponding azobenzene linker in an E conformation. However, only the para-para-substituted azobenzene derivative enables photoswitching between a nonpolymorphic, stacked, tetramolecular G-quadruplex and an unstructured state after E-Z isomerization.

Molecular-glue-triggered DNA assembly to form a robust and photoresponsive nano-network

A robust and photoresponsive DNA network has been designed and constructed from branched DNA and molecular glue. The molecular glue is photoswitchable and can specifically bind to G-G mismatched double-stranded DNA. The assembly process can be reversibly controlled by manipulating the wavelength of light. The approach is flexible, allowing tuning of the size, morphology as well as the cavity of the network by variation of the molar ratio and the isotropic/anisotropic character of the branched building blocks. The assembled architectures are versatile and heat tolerant. These properties should allow the use of the network in further applications.