The comparable impacts of Z-Abu and Val on proteolytic stability suggests that the former residue is likely to be preferred to the latter due to its relative ease of synthesis, except in cases where the increased rigidity caused by the elevated A1,3-strain in Val is desired. The inertness of both of these residues to thiols suggests that they can be incorporated into bioactive peptides without fear of degradation by biological nucleophiles. When viewed as a whole, our results provide indications of the promise of medium-sized and bulky AAs as tools for imparting rigidity and proteolytic stability to peptides. Future studies will focus on the impact of AAs on other secondary structures as well as the synthesis of additional types of AAs.