Reversible Photocontrol of DNA Melting by Visible-Light-Responsive F4-Coordinated Azobenzene Compounds

Photochromic Nucleosides and Oligonucleotides

Photochromism is a reversible phenomenon wherein a material undergoes a change in color upon exposure to light. In organic photochromes, this effect often results from light-induced isomerization reactions, leading to alterations in either the spatial orientation or electronic properties of the photochrome. The incorporation of photochromic moieties into biomolecules, such as proteins or nucleic acids, has become a prevalent approach to render these biomolecules responsive to light stimuli. Utilizing light as a trigger for the manipulation of biomolecular structure and function offers numerous advantages compared to other stimuli, such as chemical or electrical treatments, due to its non-invasive nature. Consequently, light proves particularly advantageous in cellular and tissue applications. In this review, we emphasize recent advancements in the field of photochromic nucleosides and oligonucleotides. We provide an overview of the design principles of different classes of photochromes, synthetic strategies, critical analytical challenges, as well as structure-property relationships. The applications of photochromic nucleic acid derivatives encompass diverse domains, ranging from the precise photoregulation of gene expression to the controlled modulation of the three-dimensional structures of oligonucleotides and the development of DNA-based fluorescence modulators. Moreover, we present a future perspective on potential modifications and applications.

Site-selected thionated benzothioxanthene chromophores as heavy-atom-free small-molecule photosensitizers for photodynamic therapy

Photodynamic therapy is a clinically approved anticancer modality that employs a light-activated agent (photosensitizer) to generate cytotoxic reactive oxygen species (ROS). There is therefore a growing interest for developing innovative photosensitizing agents with enhanced phototherapeutic performances. Herein, we report on a rational design synthetic procedure that converts the ultrabright benzothioxanthene imide (BTI) dye into three heavy-atom-free thionated compounds featuring close-to-unit singlet oxygen quantum yields. In contrast to the BTI, these thionated analogs display an almost fully quenched fluorescence emission, in agreement with the formation of highly populated triplet states. Indeed, the sequential thionation on the BTI scaffold induces torsion of its skeleton reducing the singlet-triplet energy gaps and enhancing the spin-orbit coupling. These potential PSs show potent cancer-cell ablation under light irradiation while remaining non-toxic under dark condition owing to a photo-cytotoxic mechanism that we believe simultaneously involves singlet oxygen and superoxide species, which could be both characterized in vitro. Our study demonstrates that this simple site-selected thionated platform is an effective strategy to convert conventional carbonyl-containing fluorophores into phototherapeutic agents for anticancer PDT.

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors has been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc . The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.

Light-Induced Modulation of Chiral Functions in G-Quadruplex-Photochrome Systems

The design of artificially engineered chiral structures has received much attention, but the implementation of dynamic functions to modulate the chiroptical response of the systems is less explored. Here, we present a light-responsive G-quadruplex (G4)-based assembly in which chirality enrichment is induced, tuned, and fueled by molecular switches. In particular, the mirror-image dependence on photoactivated azo molecules, undergoing trans-to-cis isomerization, shows chiral recognition effects on the inherent flexibility and conformational diversity of DNA G4s having distinct handedness (right- and left-handed). Through a detailed experimental and computational analysis, we bring compelling evidence on the binding mode of the photochromes on G4s, and we rationalize the origin of the chirality effect that is associated with the complexation event.

Two-photon absorption and two-photon-induced isomerization of azobenzene compounds

The process of two-photon-induced isomerization occurring in various organic molecules, among which azobenzene derivatives hold a prominent position, offers a wide range of functionalities, which can be used in both material and life sciences. This review provides a comprehensive description of nonlinear optical (NLO) properties of azobenzene (AB) derivatives whose geometries can be switched through two-photon absorption (TPA). Employing the nonlinear excitation process allows for deeper penetration of light into the tissues and provides opportunities to regulate biological systems in a non-invasive manner. At the same time, the tight focus of the beam needed to induce nonlinear absorption helps to improve the spatial resolution of the photoinduced structures. Since near-infrared (NIR) wavelengths are employed, the lower photon energies compared to usual one-photon excitation (typically, the azobenzene geometry change from trans to cis form requires the use of UV photons) cause less damage to the biological samples. Herein, we present an overview of the strategies for optimizing azobenzene-based photoswitches for efficient two-photon excitation (TPE) and the potential applications of two-photon-induced isomerization of azobenzenes in biological systems: control of ion flow in ion channels or control of drug release, as well as in materials science, to fabricate data storage media, optical filters, diffraction elements etc., based on phenomena like photoinduced anisotropy, mass transport and phase transition. The extant challenges in the field of two-photon switchable azomolecules are discussed.

Photoinduced binding of malachite green copolymer to parallel G-quadruplex DNA

Designing ligands that selectively target G-quadruplex DNAs has gained attention due to their possible roles in regulation of gene expression and as anti-cancer agents. In this article, we report irradiation-induced ligand binding to G-quadruplex DNAs which offers a novel approach to targeting specific G-quadruplexes. Photoinduced binding to G-quadruplex DNAs was observed for copolymers of poly(vinyl alcohol) carrying a malachite green moiety (PVAMG). This molecule has an aromatic ring with cationic charge, which after irradiation becomes a binding site for G-quadruplex DNA. PVAMGs acted as neutral polymers with no binding affinity under dark conditions. The photoinduced binding was revealed by fluorescence spectroscopy, NMR spectroscopy, UV melting curve, and DNA polymerase stop assay. PVAMGs showed preference to parallel G-quadruplex structures over mixed parallel/antiparallel structures. PVAMGs were found to be noncytotoxic under both dark and irradiated conditions up to a concentration of 20 μM.