The design, synthesis and photopharmacological applications of novel azoarene photoswitches
Creators: name: Thawani, Aditya Raymond
Publisher: name: Imperial College London
Year: 2021

description: Photoswitchable molecules possess physical and chemical properties that can be modulated in a reversible, light dependant manner. Azobenzenes are quintessential photoswitches renowned for their ease of synthesis, high fatigue resistance, high quantum yields, as well as, large changes in end-to-end distance and polarity upon reversible E-Z photoisomerisation. These properties have been harnessed in a myriad of possible applications including catalysis, energy storage, data storage, real-time information transfer, MRI contrast agents and chemical sensing. In recent years, a novel class of azoheteroarenes have been developed showcasing improved photoswitching characteristics compared to the azobenzenes. The azopyrazoles, in particular, are bistable photoswitches with certain members capable of near-complete, bi-directional photoswitching. The spatial and temporal precision with which light can be applied to control the dynamic properties of azoarene photoswitches have engendered several reports of their photopharmacological applications. In this sense, the therapeutic activity of a photoswitchable molecule can be coupled to light of a specific wavelength. Possible benefits include reduced side-effects, as well as, precise control of specific biological pathways. The TRPA1 ion channel exhibits higher channel conductance than current state-of-the-art Channelrhodopsin ion channels commonly utilised in optogenetic experiments. However, TRPA1 is not light addressable and existing TRPA1 agonists show no or partial light dependant activity and, thus, activate the channel with limited spatial and temporal control. Through the use of a zebrafish behavioural screening assay, an azobenzene agonist for the zebrafish Trpa1b ion channel was identified. Following synthetic modification of this azobenzene, an azopyrazole ligand was identified with improved photoswitching characteristics and capable of inducing an equally robust, light dependant response in the zebrafish behavioural assay. Light dependant activation and deactivation of the zebrafish Trpa1b ion channel was induced by the reversible E-Z photoisomerisation of these compounds. This was confirmed through electrophysiology experiments in mammalian cells. The utility of this system was demonstrated in zebrafish larvae exogenously expressing the Trpa1b ion channel in their cardiomyocytes. Normal cardiac rhythms could be interrupted and restored, at will, using two wavelengths of light. Hydrogen bonding effects were found to play a key role in explaining the thermal isomerisation kinetics of these compounds and have been fully characterised through experimental and computational analysis. A high performance, photoswitchable, azopyrazole crosslinker has been synthesised and used to staple truncated peptides from the BH3 domain of Bak. Circular dichroism spectroscopy was used to demonstrate that photocontrol could be exerted over the respective peptides’ secondary structure upon irradiation. Using peptides with different Cys spacings enabled light dependant activation or deactivation of their binding events with Bcl-xL. This represents a useful strategy for interfering with protein-protein interactions in a reversible manner. Ortho-OMe, ortho-F and ortho-Cl azobenzenes display red-shifted absorption spectra. This observation motivated the synthesis and structure-property characterisation of a range of ortho-substituted azopyrazoles with the aim of obtaining high performance azopyrazole photoswitches with red-shifted absorbances. Whilst these novel azopyrazoles were found to not have red-shifted absorbances, they did display significant increases in the thermal stabilities of their Z isomers. One compound possessed a Z isomer thermal half-life of ~46 years. This represents the most thermally stable azoarene photoswitch reported to date in the literature. Diazocine is a cyclic azobenzene photoswitch with a thermodynamically stable Z isomer. Computational analysis indicated that synthetic modification of the all-carbon bridge in diazocine might enable access to a three-state, chiral diazocine photoswitch. The reported work details the synthetic efforts undertakenn in pursuit of this molecule.