Provided are methods for forming a reactive S-nitroso thioacid (NTA), comprising nitrosation of a thioacid with a nitrosation reagent. Also provided are methods for: acylating a nucleophile including selective acylation with a high degree of selectivity toward amines over hydroxyls; amide or peptide bond formation; forming a dipeptide or polypeptide; and peptide coupling/ligation, comprising use of thioacid and amine starting materials, wherein the reactions are mediated by very reactive S-nitroso thioacid (NTA) intermediates enabling extremely fast reactions under mild conditions, providing for broad applications.