From light-stimulated anion receptors to transmembrane carriers and pumps

Project Information

LIGHTPORT

Grant agreement ID: 802830

DOI

10.3030/802830

Project closed

EC signature date 22 October 2018

Start date 1 March 2019

End date 31 August 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 499 461,00

EU contribution

€ 1 499 461,00

Coordinated by

UNIVERSITEIT LEIDEN
Netherlands

Periodic Reporting for period 4 - LIGHTPORT (From light-stimulated anion receptors to transmembrane carriers and pumps)

Reporting period: 2023-09-01 to 2024-08-31

Summary of the context and overall objectives of the project

The transport of anions across the cell membrane, which is mediated by transport proteins, is essential to many important biological processes. Dysregulation of this transport has been associated to various diseases and therefore, chemists endeavor to develop artificial receptors that mimic the function of natural transporters. Despite much progress over the last decade, the current artificial systems are mostly static, while proteins are able to change their activity dynamically in response to stimuli in the environment. To integrate such stimuli-controlled behavior in synthetic systems is a key contemporary challenge. In view of this, the goal of the proposed research program is to develop anion receptors in which the binding properties can be effectively modulated by light and to apply these receptors as transmembrane carriers and pumps, in order to regulate passive transport (i.e. down a concentration gradient) and to induce active transport (i.e. against a concentration gradient).
This interdisciplinary program is divided into three work packages: WP1 aims at the development of structurally rigid and visible-light-actuated photoswitches and their use as platforms for constructing anion receptors; WP2 deals with the development of mechanically interlocked structures as photoswitchable anionic hosts; WP3 is directed at utilizing these receptors for light-gated transport and light-driven pumping of anions across phospholipid bilayers, whereas also an alternative dual-responsive anion channel will be prepared. Eventually, it is expected that this work will open a new route toward light-based localized pharmacological treatment, e.g. via light-triggered cancer or bacterial cell death. Furthermore, active transport systems, that are able to build up and maintain concentration gradients across membranes, could provide a completely new view on how to convert and store light (solar) energy.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A push-pull substituted stiff-stilbene derivative that can be isomerized by visible instead of UV light was prepared. Further, an aryl-oxindole motif was used to achieve double bond isomerization by visible light. These achievements address the issue of using damaging UV light for E/Z carbon-carbon double bond isomerization. Subsequently, different photoswitchable anion receptors based on the stiff-stilbene motif were prepared additionally, either using a molecular tweezer or bridged macrocycle approach. Most notably, the incorporation of a stiff-stilbene bridge to calix[4]pyrrole afforded an 8000-fold affinity difference between photoaddressable isomers, which is two orders of magnitude larger than the highest affinity changes that had been reported thus far. In a later stage, different derivatives of this bridged calix[4]pyrrole with varying linkers and linker lengths were synthesized, providing insight into the origin of the large affinity difference as well as the optimal strap length. Also, a modified design with a dithienylethene instead of stiff-stilbene photoswitch was developed, in which besides binding affinity, selectivity could be controlled.
In addition to the tweezer-type and macrocyclic receptors, [2]rotaxane and [2]catenane receptors were synthesized, which showed a significant affinity change upon isomerization. The same photoswitchable macrocycles could also be used to control pseudorotaxane (de)threading.
The stiff-stilbene based tweezer-type receptors were successfully used to control passive transmembrane transport by light. Interestingly, we observed in our studies that beside binding affinity, other factors such as membrane incorporation and mobility are at least the same important towards controlling bilayer anion transport. We took this to our advantage in modified transporter designs, e.g. based on azobenzene. Unfortunately, the macrocyclic receptors turned out to be inactive or very poorly active in transmembrane transport assays.

The results of this project were disseminated at various national and international conferences, as well as visits to other universities. The dithienylethene-bridged calix[4]pyrrole was highlighted in C2W magazine and chosen as "Molecule of the Year" in 2024 by its readers.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Visible-light switching of stiff-stilbene is an important achievement and moreover, unprecedented acid-catalyzed thermal isomerization was found for the push-pull derivative. The 8000-fold affinity difference found for the stiff-stilbene bridged calix[4]pyrrole is a major improvement compared to the most successful previous photoswitchable host-guest systems, which typically had affinity differences in the range of 10-to-100-fold. Further, control of anion selectivity had not been achieved yet. The [2]rotaxane and [2]catenane receptors are the first mechanically interlocked structures in which anion binding properties are controlled by light, which is of interest owing to the strong and highly selective binding reported for these kind of structures. Apart from regulating transmembrane transport by light using the tweezer-type receptors, we showed control of membrane potential for the first time, which takes the field a step closer to potential application as physiological tools, as well as active transport. Most importantly, the project originally aimed at creating receptors with very large anion affinity differences between their photoaddressable isomers, in which we succeeded. However, during the course of the project, we learned that other factors than binding affinity majorly influence transport activity. This aspect has been studied and used to our advantage. It is of major importance to the future development of light-switchable transport systems and the progress of this research field.

Anion binding and release by a photoswitchable receptor