Transient Photoactivation of Anionophores by Using Redshifted Fast-Relaxing Azobenzenes.

Photo-regulated transmembrane ionophores enable spatial and temporal control over activity, offering promise as targeted therapeutics. Key to such applications is control using bio-compatible visible light. Herein, we report red-shifted azobenzene-derived synthetic anionophores that use amber or red light to trigger (E)-(Z) photoisomerisation and activation of transmembrane chloride transport. We demonstrate that by tuning the thermal half-life of the more active, but thermodynamically unstable, Z isomer to relax on the timescale of minutes, transient activation of ion transport can be achieved by activating with solely with visible light and deactivating by thermal relaxation.

Red-shifted tetra-ortho-halo-azobenzenes for photo-regulated transmembrane anion transport.

Photo-responsive synthetic ion transporters are of interest as tools for studying transmembrane transport processes and have potential applications as targeted therapeutics, due to the possibility of spatiotemporal control and wavelength-dependent function. Here we report the synthesis of novel symmetric and non-symmetric red-shifted tetra-ortho-chloro- and tetra-ortho-fluoro azobenzenes, bearing pendant amine functionality. Functionalisation of the photo-switchable scaffolds with squaramide hydrogen bond donors enabled the preparation of a family of anion receptors, which act as photo-regulated transmembrane chloride transporters in response to green or red light. The subtle effects of chlorine/fluorine substitution, meta/para positioning of the anion receptors, and the use of more flexible linkers are explored. NMR titration experiments on the structurally diverse photo-switchable receptors reveal cooperative binding of chloride in the Z, but not E isomer, by the two squaramide binding sites. These results are supported by molecular dynamics simulations in explicit solvent and model membranes. We show that this intramolecular anion recognition leads to effective switching of transport activity in lipid bilayer membranes, in which optimal Z isomer activity is achieved using a combination of fluorine substitution and para-methylene spacer units.

Urotensin-II peptidomimetic incorporating a non-reducible 1,5-triazole disulfide bond reveals a pseudo-irreversible covalent binding mechanism to the urotensin G-protein coupled receptor.

The urotensin-II receptor (UTR) is a class A GPCR that predominantly binds to the pleiotropic cyclic peptide urotensin-II (U-II). U-II is constrained by a disulfide bridge that induces a ß-turn structure and binds pseudo-irreversibly to UTR and is believed to result in a structural rearrangement of the receptor. However, it is not well understood how U-II binds pseudo-irreversibly and the nature of the reorganization of the receptor that results in G-protein activation. Here we describe a series of U-II peptidomimetics incorporating a non-reducible disulfide bond structural surrogate to investigate the feasibility that native U-II binds to the G protein-coupled receptor through disulfide bond shuffling as a mechanism of covalent interaction. Disubstituted 1,2,3-triazoles were designed with the aid of computational modeling as a non-reducible mimic of the disulfide bridge (Cys5-Cys10) in U-II. Solid phase synthesis using CuAAC or RuAAC as the key macrocyclisation step provided four analogues of U-II(4-11) incorporating either a 1,5-triazole bridge (5, 6) or 1,4-triazole bridge (9, 10). Biological evaluation of compounds 5, 6, 9 and 10 was achieved using in vitro [125I]UII binding and [Ca2+]i assays at recombinant human UTR. Compounds 5 and 6 demonstrated high affinity (KD ∼ 10 nM) for the UTR and were also shown to bind reversibly as predicted and activate the UTR to increase [Ca2+]i. Importantly, our results provide new insight into the mechanism of covalent binding of U-II with the UTR.

Ion carrier modulated MRI contrast.

An ion-responsive MRI contrast agent based on a POPC liposomal scaffold is generated that displays a large amplitude relaxivity switch. Entrapment of MR active Gd-DOTA within cholesterol-doped, i.e., membrane rigidified, liposomes dampens the MR response through diminished water exchange across the lipid bilayer. Relaxivity is re-established by integration of ion carriers in the liposome membrane to mediate solvated ion flux.

Fast relaxing red and near-IR switchable azobenzenes with chalcogen and halogen substituents: periodic trends, tuneable thermal half-lives and chalcogen bonding.

Molecular photoswitches operating in the red to near-IR region with controllable thermal relaxation rates are attractive components for photo-regulating biological processes. Herein, we report the synthesis of red-shifted azobenzenes functionalised with the heavier chalcogens and halogens that meet these requirements for biological application; namely fatigue-resistant photo-switching with red and near IR light and functional handles for further functionalisation for application. We report robust periodic trends for the chalcogen and halogen azobenzene series, and exploit intramolecular chalcogen bonding to tune and redshift the absorption maxima, supported by photo-physical measurements and solid-state structural analysis. Remarkably, the rate of the Z → E thermal isomerisation can be tuned over timescales spanning 107 s by judicious choice of chalcogen and halogen substituents.

Photo-switchable anion binding and catalysis with a visible light responsive halogen bonding receptor.

Photo-switchable receptors allow for photo-control over guest binding and release with spatial and temporal precision. Here we report the first halogen bonding photo-switchable anion receptors in which chloride binding may be reversibly modulated by irradiation with red and blue light, with over a 50-fold enhancement in chloride binding affinity observed for the Z isomer. We demonstrate that this switchable binding enables unprecedented photo-controlled catalysis of XB-mediated halide abstractions and a Mukaiyama Aldol reaction.

Supramolecular chemistry in lipid bilayer membranes.

Lipid bilayer membranes form compartments requisite for life. Interfacing supramolecular systems, including receptors, catalysts, signal transducers and ion transporters, enables the function of the membrane to be controlled in artificial and living cellular compartments. In this perspective, we take stock of the current state of the art of this rapidly expanding field, and discuss prospects for the future in both fundamental science and applications in biology and medicine.

Reversible photo-control over transmembrane anion transport using visible-light responsive supramolecular carriers.

Ion transport across lipid bilayer membranes in biology is controlled by membrane proteins, which in turn are regulated in response to chemical-, physical- and photo-stimuli. The design of synthetic supramolecular ion transporters able to be precisely controlled by external signals, in particular bio-compatible wavelengths of visible light, is key for achieving spatio-temporal control over function. Here we report two-colour responsive molecular photo-switches that act as supramolecular transmembrane anion carriers. Reversible switching of the photo-switch within the lipid bilayer membrane is achieved using biocompatible visible wavelengths of light, such that temporal control over transmembrane anion transport is achieved through alternating irradiation with red and blue light.