Improved methods of native chemical ligation are provided. The methods involve reacting a thioacid (e.g. a peptide thioacid) with an aziridinyl compound (e.g. an aziridinyl peptide) under mild conditions without the use of protecting groups, and without requiring that a cysteine residue be present in the ligation product. Initial coupling of the thioacid and the aziridinyl compound yields a ligation product which contains an aziridinyl ring. Subsequent opening of the aziridinyl ring (e.g. via a nucleophilic attack) produces a linearized and modified ligation product.