4-Guanidinobenzoic acid hydrochloride is primarily used in biochemical research as a potent trypsin inhibitor and reagent for studying enzyme inhibition (particularly serine proteases) and protein interactions. Its guanidinium group enables it to bind strongly to negatively charged molecules, such as the aspartate residue in protease active sites, making it useful in investigating binding mechanisms in biological systems. Additionally, it finds application in the synthesis of peptide derivatives and as a building block in organic chemistry for creating more complex molecules. Its role in protecting proteins from proteolytic degradation (stabilizing) and enhancing solubility in aqueous solutions also makes it valuable in biophysical studies.