Noncanonical Amino Acids for Hypoxia-Responsive Peptide Self-Assembly and Fluorescence.

Design of endogenous stimuli-responsive amino acids allows for precisely modulating proteins or peptides under a biological microenvironment and thereby regulating their performance. Herein we report a noncanonical amino acid 2-nitroimidazol-1-yl alanine and explore its functions in creation of the nitroreductase (NTR)-responsive peptide-based supramolecular probes for efficient hypoxia imaging. On the basis of the reduction potential of the nitroimidazole unit, the amino acid was synthesized via the Mitsunobu reaction between 2-nitroimidazole and a serine derivate. We elucidated the relationship between the NTR-responsiveness of the amino acid and the structural feature of peptides involving a series of peptides. This eventually facilitates development of aromatic peptides undergoing NTR-responsive self-assembly by rationally optimizing the sequences. Due to the intrinsic role of 2-nitroimidazole in the fluorescence quench, we created a morphology-transformable supramolecular probe for imaging hypoxic tumor cells based on NTR reduction. We found that the resulting supramolecular probes penetrated into solid tumors, thus allowing for efficient fluorescence imaging of tumor cells in hypoxic regions. Our findings demonstrate development of a readily synthesized and versatile amino acid with exemplified properties in creating fluorescent peptide nanostructures responsive to a biological microenvironment, thus providing a powerful toolkit for synthetic biology and development of novel biomaterials.