No abstract

There has been enormous interest in both naturally occurring and synthetic cyclic peptides as scaffolds that preorganize a given amino acid sequence into a rigid conformation. Such molecules have been employed as nanomaterials, imaging agents, and therapeutics. Unfortunately, the laboratory synthesis of cyclic peptides directly from linear precursors is afflicted by several thermodynamic and kinetic challenges, resulting in low chemical yields and poor chemo- and stereoselectivities. Here we report that amphoteric amino aldehydes can be used for efficient syntheses of cyclic peptides in high yields and selectivities starting from alpha-amino acids or linear peptides. The cyclizations effectively operate at unusually high molar concentrations (0.2 M), while side processes such as epimerization and cyclodimerization are not observed. The products are equipped with sites that allow for a highly specific, late-stage structural modification. The overall efficiency of the macrocyclization is due to the coexistence of nucleophilic and electrophilic reaction centers in amphoteric amino aldehydes.

Chemical biology research often requires precise covalent attachment of labels to the native proteins. Such methods are sought after to probe, design, and regulate the properties of proteins. At present, this demand is largely unmet due to the lack of empowering chemical technology. Here, we report a chemical platform that enables site-selective labeling of native proteins. Initially, a reversible intermolecular reaction places the "chemical linchpins" globally on all the accessible Lys residues. These linchpins have the capability to drive site-selective covalent labeling of proteins. The linchpin detaches within physiological conditions and capacitates the late-stage installation of various tags. The chemical platform is modular, and the reagent design regulates the site of modification. The linchpin is a multitasking group and facilitates purification of the labeled protein eliminating the requirement of additional chromatography tag. The methodology allows the labeling of a single protein in a mixture of proteins. The precise modification of an accessible residue in protein ensures that their structure remains unaltered. The enzymatic activity of myoglobin, cytochrome C, aldolase, and lysozyme C remains conserved after labeling. Also, the cellular uptake of modified insulin and its downstream signaling process remain unperturbed. The linchpin directed modification (LDM) provides a convenient route for the conjugation of a fluorophore and drug to a Fab and monoclonal antibody. It delivers trastuzumabdoxorubicin and trastuzumab-emtansine conjugates with selective antiproliferative activity toward Her-2 positive SKBR-3 breast cancer cells.

Labeling of native proteins invites interest from diverse segments of science. However, there remains the significant unmet challenge in precise labeling at a single site of a protein. Here, we report the site-specific labeling of natural or easy-to-engineer N-terminus Gly in proteins with remarkable efficiency and selectivity. The method generates a latent nucleophile from N-terminus imine that reacts with an aldehyde to deliver an aminoalcohol under physiological conditions. It differentiates N-Gly as a unique target amongst other proteinogenic amino acids. The method allows single-site labeling of proteins in isolated form and extends to lysed cells. It administers an orthogonal aldehyde group primed for late-stage tagging with an affinity tag, 19 F NMR probe, and a fluorophore. A user-friendly protocol delivers analytically pure tagged proteins. The mild reaction conditions do not alter the structure and function of the protein. The cellular uptake of fluorophore-tagged insulin and its ability to activate the insulin-receptor mediated signaling remains unperturbed.