Red-Shifting Azobenzene Photoswitches for in Vivo Use

Abstract

Recently, there has been a great deal of interest in using the photoisomerization of azobenzene compounds to control specific biological targets in vivo. These azo compounds can be used as research tools or, in principle, could act as optically controlled drugs. Such "photopharmaceuticals" offer the prospect of targeted drug action and an unprecedented degree of temporal control. A key feature of azo compounds designed to photoswitch in vivo is the wavelength of light required to cause the photoisomerization. To pass through tissue such as the human hand, wavelengths in the red, far-red, or ideally near infrared region are required. This Account describes our attempts to produce such azo compounds. Introducing electron-donating or push/pull substituents at the para positions delocalizes the azobenzene chromophore and leads to long wavelength absorption but usually also lowers the thermal barrier to interconversion of the isomers. Fast thermal relaxation means it is difficult to produce a large steady state fraction of the cis isomer. Thus, specifically activating or inhibiting a biological process with the cis isomer would require an impractically bright light source. We have found that introducing substituents at all four ortho positions leads to azo compounds with a number of unusual properties that are useful for in vivo photoswitching. When the para substituents are amide groups, these tetra-ortho substituted azo compounds show unusually slow thermal relaxation rates and enhanced separation of n-π* transitions of cis and trans isomers compared to analogues without ortho substituents. When para positions are substituted with amino groups, ortho methoxy groups greatly stabilize the azonium form of the compounds, in which the azo group is protonated. Azonium ions absorb strongly in the red region of the spectrum and can reach into the near-IR. These azonium ions can exhibit robust cis-trans isomerization in aqueous solutions at neutral pH. By varying the nature of ortho substituents, together with the number and nature of meta and para substituents, long wavelength switching, stability to photobleaching, stability to hydrolysis, and stability to reduction by thiols can all be crafted into a photoswitch. Some of these newly developed photoswitches can be used in whole blood and show promise for effective use in vivo. It is hoped they can be combined with appropriate bioactive targets to realize the potential of photopharmacology.

Solvent Effects on the Actinic Step of Donor–Acceptor Stenhouse Adduct Photoswitching

“…[7][8][9][10][11][12] Applications differ markedly in the environment the photoswitch is exposed to,b ei td ifferent solvents, [13,14] matrices or surfaces,a nd understanding how ag iven photoswitch behaves in various environments is crucial for its success in any applications. Donor-acceptor Stenhouse adducts (DASAs,F igure 1a) were introduced in 2014 [17,18] and feature important advantages as compared to traditional photoswitches,i ncluding visible light responsiveness [11,19,20] and negative photochromism. [21] Moreover,t heir modular architecture [22] allows for afine-tuning of properties.…”

Optogenetic approaches for dissecting neuromodulation and GPCR signaling in neural circuits

“…Reversible activation of ligands is achieved through the use of proteins containing photolabile domains, also known as photoswitches. The most common photoswtich used is azobenzene, originally sensitive to UV light; red shifted versions have developed to allow for use in vivo [67,68*] (Figure 2c). A photoswitchable small molecule mu opioid agonist fentanyl has also been developed as well using this approach [69].…”

Light‐Gated Rotation in a Molecular Motor Functionalized with a Dithienylethene Switch

“…Moreover, it can be easily tuned in terms of wavelength and intensity and it can be applied with high spatiotemporal control. These advantages have stimulated the application of light‐driven molecular switches14 and motors in functional materials8, 15, 16, 17, 18, 19, 20 and biological systems 21, 22, 23, 24. However, potentially harmful UV light is typically used for their operation while, for practical applications, the use of visible light is often desired 25, 26, 27, 28, 29, 30, 31…”