Photoswitchable Ligands

Photoswitchable ligands are photo-sensitive compounds that can allow the optical regulation of neuronal ion channels and receptors in response to different wavelengths of light. These photoswitches enable a high degree of spatial and temporal control over neuronal signaling, which is usually not possible with conventional pharmacological agents. Furthermore they offer the advantage of being reversible, meaning they can toggle between their cis and trans conformations, as well as being effective without the need for genetic manipulation or exogenous gene expression.

Photoswitchable AAQ

Figure 1: Schematic showing the change in conformation of AAQ (Cat. No. 5462) following exposure to different wavelengths of light. AAQ is a K_v channel blocker that switches conformation from trans to cis at 380 nm and cis to trans at 500 nm.

Photoswitchable ligands are formed by the attachment of a photo-isomerizable group to the desired ligand. A commonly used photoswitch is azobenzene, as it is easily conjugated to ligands and is photostable. Absorption of photons by the photoswitch triggers a change in ligand conformation. Different wavelengths of light can alter the ligand's conformation between its cis and trans forms determining its biological activity. The first generation of photoswitchable ligands typically adopt a cis conformation in response to near UV light (360-395 nm), while relaxing into their trans form at longer wavelengths of light (e.g. 500 nm) and in the dark. The transition to a cis conformation in response to light is rapid (milliseconds) and lasts for minutes before relaxing into its trans state, ensuring a sustained biological response. The photostable nature of the photoswitch also enables multiple trans to cis transitions without fatigue.

Photoswitchable ligands developed to date include QAQ, AC 4 and AAQ. QAQ is a K_v, Ca_v and Na_v channel blocker that has been shown to cross the cell membrane through open TRPV1 channels, and exhibit light-sensitive analgesic effects in vivo, while AC 4 is a TRPV1 channel blocker and could therefore also have applications as an analgesic. AAQ is a K_v channel blocker that has been shown to restore visual responsiveness in blind mice in response to UV light. These photoswitchable ligands offer neuroscience researchers valuable tools for the manipulation of neuronal circuits, especially for research into photoreceptor degeneration disorders and chronic pain.