Late-Stage Peptide Nitroarylation: Stereoelectronic Tuning and Molecular Recognition

Abstract

Aromatic amino acids lie at the center of protein folding and interactions. For the three aromatic amino acids commonly found in proteins–phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp)–their large size and hydrophobic character, as well as electrostatic interactions involving their π systems, provide substantial energetic driving forces for folding and interactions. Beyond Phe, Tyr, and Trp, an increasing number of non-canonical aromatic amino acid residues have been explored in both rational and screening approaches for new peptide-based drugs and materials. These efforts are critically dependent on relatively expensive amino acid building blocks, highlighting a need for new approaches to rapidly generate peptide libraries featuring non-canonical aromatic amino acids starting from less expensive building blocks. Here we describe a new approach for site-specific introduction of stereoelectronically diverse aromatic amino acid side-chains into peptides. The key step–nitroarylation of short, nucleophilic side-chains with o-fluoronitroarenes–efficiently produced dozens of nitroarylated peptides from just two peptide intermediates. We demonstrate broad compatibility with standard solid-phase peptide synthesis reagents, including in the context of clinically relevant peptides. Nitroaryl side-chains display significant near UV and/or visible absorption for specific detection, and peptides bearing these side-chains were efficiently recognized and hydrolyzed as protease substrates. Chemoselective nitro reduction on fully deprotected peptides further expands the accessible chemical space to electron-rich aryl side-chains. In providing parallel access to dozens of stereoelectronically diverse aromatic amino acids from shared peptide intermediates, this approach permits rapid peptide library synthesis for structure-function studies, including peptide drug and materials discovery.