Noncanonical Amino Acids Dictate Peptide Assembly in Living Cells

Emulating the structural features or functions of natural systems has been demonstrated as a state-of-the-art strategy to create artificial functional materials. Inspired by the assembly and bioactivity of proteins, the self-assembly of peptides into nanostructures represents a promising approach for creating biomaterials. Conventional assembled peptide biomaterials are typically formulated in solution and delivered to pathological sites for implementing theranostic objectives. However, this translocation entails a switch from formulation conditions to the physiological environment and raises concerns about material performance. In addition, the precise and efficient accumulation of administered biomaterials at target sites remains a significant challenge, leading to potential biosafety issues associated with off-target effects. These limitations significantly hinder the progress of advanced biomaterials. To address these concerns, the past few years have witnessed the development of in situ assembly of peptides in living systems as a new endeavor for optimizing biomaterial performance benefiting from the advances of stimuli-responsive reactions regulating noncovalent interactions. In situ assembly of peptides refers to the processes of regulating assembly via stimuli-responsive reactions at target sites. Due to the advantages of precisely forming well-defined nanostructures at pathological lesions, in situ-formed assemblies with integrated bioactivity are interesting for the development of next-generation biomedical agents.Despite the great potential of in situ assembly of peptides for developing biomedical agents, this research area still suffers from a limited toolkit for operating peptide assembly under complicated physiological conditions. Considering the advantages of amino acids in being incorporated into peptide backbones and modified with stimuli-responsive units, development of an amino acid toolkit is promising to address this concern. Therefore, our laboratory has been intensively engaged in designing and discovering stimuli-responsive noncanonical amino acids (ncAAs) to expand the toolkit for manipulating peptide assembly under various biological conditions. Thus far, we have synthesized peptides containing ncAAs 4-aminoproline, 2-nitroimidazole alanine, Se-methionine, sulfated tyrosine, and glycosylated serine, which allow us to develop acid-responsive, redox-responsive, and enzyme-responsive assembly systems. Based on these stimuli-responsive ncAAs, we have established complex self-sorting assembly, self-amplified assembly, and dissipative assembly systems in living cells to optimize the bioactivity of peptides. The resulting in situ assembly systems exhibit morphological adaptability to the biological microenvironment, which contributes to overcoming delivery barriers and improvement of targeting accumulation. Therefore, by utilizing the developed toolkit, we have further created supramolecular PROTACs, supramolecular antagonists, and supramolecular probes for cancer treatment and diagnosis to highlight the implications of ncAAs for biomedical usage. In this Account, we summarize our journey of in situ self-assembly of peptides in living cells utilizing stimuli-responsive ncAAs, with an emphasis on the mechanism for regulating peptide assembly and optimizing the bioactivity of peptides. Eventually, we also provide our forward conceiving prospects on the challenges for the further development of in situ assembly of peptides in living systems and the clinical translation of in situ-formulated biomaterials.