Photoswitchable Peptides as Molecular Tools to Encode Structural Order and Disorder in Intracellular Assemblies

Abstract Understanding how self‐assembled structure formation affects cells remains a central challenge in supramolecular chemistry. However, chemical tools that allow access to both ordered and disordered intracellular assemblies from a single molecular scaffold are rare due to design complexity. Here, we present a photoswitchable isotripeptide incorporating an arylazopyrazole (AAP) unit, which undergoes intracellular cleavage to yield a self‐assembling monomer. Upon photoisomerization, the planar trans ‐isomer forms β ‐sheet‐rich nanofibers with strong aromatic interactions, while the non‐planar cis ‐isomer assembles into disordered, random‐coil aggregates lacking aromatic contribution. The structural dynamics of the assemblies are demonstrated by repeated photoswitching between the two states in buffered conditions. Notably, A549 cancer cell viability correlates with the isomer‐dependent assembly behavior and critical aggregation concentrations (CACs): the trans ‐isomer, with higher aggregation propensity, exhibits greater cytotoxicity. This photoswitchable peptide system thus provides a powerful platform with fast, reversible and robust switching kinetics, long isomer half‐lives, and high photostability to probe the intracellular consequences of supramolecular order and disorder using a single molecular scaffold.