Interestingly, the conformations of the Val and Z-Abu residues in peptides of the same series are quite similar (i.e., 2a and 2c, 3a and 3c). However, the Z-Abu-containing analogs 2c and 3c exhibit higher RMSD values than the corresponding Val-containing analogs 2a and 3a. While both residues apparently prefer similar low-energy conformations, it appears that the additional A1,3 strain inherent in a tetrasubstituted alkene relative to a trisubstituted alkene destabilizes the high-energy conformations of Val to a greater degree than those of Z-Abu. Thus, our data suggest that medium-sized and bulky AAs exert similar effects on the structures of peptides, but that bulky AAs impart more rigidity. The stability of 1 and its analogs to proteolytic degradation was determined by treating the peptides with Pronase, a cocktail of proteases from Streptomyces griseus that aggressively cleaves peptide bonds in a nonspecific fashion. Since 1 is relatively stable to proteolysis due to its two Aib residues, a high concentration of Pronase was required for it to degrade on a reasonable timescale. As a result, the kinetics of peptide degradation were affected by self-proteolysis of the enzymes. Although this prevented us from measuring half-lives, we were able to compare the amount of each peptide that remained after 30 minutes of incubation with Pronase as recorded in Table . Peptides 2b and 3b containing Ala were significantly more labile than Balaram's peptide, as the relative abundance of the former peptides after 30 minutes of exposure to Pronase was ca. 10-30% that of the latter. Thus, it is clear that Aib is more effective than Ala at protecting peptides from proteolysis. Interestingly, the effects of Z-Abu and Val were dependent on their location in the peptide chain. Analogs 2a and 2c possessing a AA at the N-terminus were equally or slightly less stable than 1, respectively. In contrast, variants 3a and 3c in which the AA replaces Aib-3 were more stable than 1, as approximately twice as much of each remained after exposure to Pronase for 30 minutes. While the Val-containing peptides 2a and 3a were slightly more stable than the Z-Abu-containing peptides 2b and 3b, the differences in performance were not dramatic. Although a more detailed and comprehensive study would be required to generate solid conclusions, these data suggest that Z-Abu and Val might confer comparable increases in proteolytic stability upon peptides that contain them. Importantly, our results show that in some cases the stabilizing impact of these two AAs can exceed that of the ,-disubstituted residue Aib. Degradation of peptides was monitored using HPLC.