Conjugation of Synthetic Peptides with 2,4,6-trinitrobenzenesulfonic acid (TNBS) v1

This protocol describes the quantification of primary amine groups in proteins and peptides using the 2,4,6-trinitrobenzenesulfonic acid (TNBS) assay, and the preparation of trinitrophenylated peptide conjugates (TNP-POPs) for downstream applications. The TNBS assay relies on the reaction between TNBS and free primary amines under mildly alkaline conditions to form a chromogenic trinitrophenyl (TNP) derivative with a characteristic absorbance at 450 nm. Glycine standards are prepared in 0.1 M sodium bicarbonate buffer for calibration, while peptides are dissolved in either sodium bicarbonate buffer or HEPES-buffered saline (HBS) to avoid precipitation. Reaction mixtures consist of the amine-containing sample, sodium bicarbonate (or HBS), and freshly diluted TNBS solution (1.7 mM final), incubated at 37 °C for 2 hours. Absorbance is measured in duplicate on a microplate reader, and primary amine content is calculated from the glycine standard curve or molar extinction coefficient. For TNP-POP conjugation, peptide stocks are reacted with TNBS in HBS with 0.008% Tween 20, without glycine quenching, using stoichiometric ratios to approximate complete TNBS saturation of amine groups. The resulting TNP-conjugated peptides are prepared fresh before clotting and fibrinolysis assays to ensure stability and solubility. This method is adapted from Thermo Scientific and Yi et al. (2008) with modifications for improved absorbance sensitivity and compatibility with peptide-based assays.