A thorough experimental and computational study of derivatives of (3-sec-butyl-2,3-dihydroisoquinolin-4-ylidene)acetic acid was performed. Some of these compounds are calpain inhibitors and could be useful as therapeutic agents, since this enzyme is a Ca(2+)-dependent cysteine protease involved in a wide variety of metabolic and physiological processes, whose over-activation is associated to several pathological conditions. To gain a better understanding of the structure-activity relationships, a structural analysis was carried out with (1)H and (13)C NMR spectroscopy and DFT calculations together with the X-ray diffraction data of three compounds. The solid state structures showed that the crystal packing as well as the intermolecular interactions depend on the substituent nature of the COOR group. Also, the reactivity of the exocyclic double bond was theoretically evaluated, finding that the more reactive compound is the most potent inhibitor of calpain (IC(50) = 25 nM).