The proteolytic stabilities of 1 and its analogs were evaluated by exposure to Pronase. Ala-containing analogs 2b and 3b were degraded rapidly, demonstrating that this small AA does not protect peptides from proteolysis. Z-Abu and Val increased proteolytic stability to similar degrees, with the most pronounced effect (ca. twice as stable as 1) occurring when the AA replaced Aib-3. Intriguingly, peptides containing Z-Abu and Val were completely unreactive to conjugate addition by the potent nucleophile cysteamine at temperatures up to 100 °C, whereas Ala-containing peptides reacted at room temperature.