Abstract: The present disclosure relates to a selective and efficient method to connect a polypeptide containing at least one lysine residue to a cargo moiety, where one step of the method involves reaction of a semicarbazide group with an ortho-acylphenyl boronic acid, forming a cyclic diazaborine ring fused to phenyl. Conditions for formation of the cyclic diazaborine are sufficiently mild for the method to be used in the presence of sensitive biomolecules such as polynucleotides. A substituent group on the phenyl can be used to link the cyclic diazaborine to a cargo moiety such as a polynucleotide, bead, or reactive group, providing a polypeptide—cargo moiety conjugate that is useful for various purposes, such as to analyze, identify, track, locate, detect, or immobilize the polypeptide. Also provided are polypeptide—cargo moiety conjugates, wherein the polypeptide and cargo moiety are connected via a linker that comprises a cyclic diazaborine.

Abstract: Provided herein are methods for modifying an arginine residue to provide a reactive handle suitable for attaching the arginine residue to a label, linker, or other target molecule. The methods utilize an Arnold Salt to convert the guanidine group of the arginine residue into a formylpyrimidine, and are efficient and very selective. Provided herein are mild reaction conditions for this arginine modification, making it suitable for use with complex biomolecules like proteins. The methods can include one or more additional steps to attach the modified arginine to a linker for further manipulation or for connecting the arginine residue to a target compound. Provided methods are especially useful for covalently linking an arginine-containing peptide to a target molecule such as a polynucleotide.