We investigate the possibility of using the protease thermolysin to drive the synthesis and gelation of ionic-complementary peptides from nongelling precursors. In this system, short peptide fragments are continuously interconverted to form a dynamic peptide library, which eventually favors synthesis of peptides that are thermodynamically stabilized by molecular self-assembly. Thermolysin was added at a fixed concentration (0.3 mg mL(-1)) to solutions (0-300 mg mL(-1)) of the short tetrapeptide FEFK. Initially, the protease partially hydrolyzed the tetrapeptide into dipeptides in all samples. Subsequently, longer peptide sequences were found to form through reverse-hydrolysis. The stability of the different sequences was found to be dependent on their self-assembling properties. The sequences that self-assembled into antiparallel beta-sheet rich fibers became the stable products for the reverse hydrolysis reaction, while the others formed were unstable and disappeared with increasing incubation time. Ultimately, the main product of the system was octapeptide, which suggests that it represents the thermodynamically favored product of this dynamic library. Its concentration dictated the gelation behavior of the sample, and gels with moduli up to 25 kPa where obtained depending on the initial concentration of tetrapeptide.