C04 • Structural Control of Peptides by Boron Photoswitches

In this project we aim to develop conjugates of peptides with boron-containing photoswitches derived from cyclic diazobenzenes (cABs) and other photoresponsive small molecules utilizing the reversible formation of B–N native bonds. The irradiation of the latter components with appropriate wavelengths will toggle between their contracted and elongated forms (Z- and E-configuration of a switch). These changes will be translated into the folding-unfolding state of peptides forming α-helices so that the site-specifically integrated switches will allow for the structural control of the helical conformation and cell permeability as prerequisite for controlling cellular uptake (unfolding = no cellular uptake; folded = cellular uptake).
The transition to the energetically less stable form will be accompanied by the increase in steric strain and modification of the electronic structure, leading to a cleavage of a B–N bond in a rationally designed boron-containing molecule. Due to the strong perturbation of electron density around the boron atom, the ring opening will induce pronounced changes in the optical properties of the system, which will offer an additional possibility to monitor the photoisomerization process by a plethora of methods. The B–N bond will be then regenerated upon photoisomerization to the energetically more stable form. These B–N photoswitches will be stapled to cell penetrating peptides (CPP) using site-specific chemistries. Exposure of light will be used to induce structural changes of the peptide, leading to an induced stabilized α-helical conformation, a prerequisite for cellular uptake. To enhance helix stability, protease resistance and cell permeability, we will additionally explore the effect of chiral stapling. To this end, we will introduce boron-containing chiral photoswitches or chiral moieties together with boron-containing photoswitches into the bridges of the targeted peptides. Finally, we plan to explore wavelength-selective independent control of different segments of peptides using orthogonal photoswitches.
We will examine the structural mechanism of our B-N photoswitches in response to light, their structural changes as in-bridge structures within CPPs, and finally their structural performances to fine-tune the conformational properties of peptides by circular dichroism (CD), NMR spectroscopy using an optical fiber-coupled light-emitting diode setup to irradiate a sample inside the NMR spectrometer. Selected B-N stapled peptides will be explored for their impact in biological systems, e.g. to induce cell permeability and transport of cargo molecules across the cellular membrane by helical conformation in response to light.