In contrast, Val-containing peptides 2a and 3a as well as Z-Abu-containing peptides 2c and 3c were inert to cysteamine at temperatures up to 100 °C, at which point the thiol decomposed completely. Thus, neither of these residues is reactive to thiols when embedded within a peptide (i.e., not located at the Cterminus). While further investigation is warranted, these results suggest that both medium-sized and bulky AAs can be incorporated into bioactive peptides without fear of in vivo degradation due to conjugate addition. Interestingly, subjection of -methylene--butyrolactone to cysteamine caused complete consumption of the electrophile within 5 minutes at room temperature. This result shows that Ala-containing peptides 2b and 3b are only modestly reactive to cysteamine by comparison, implying that peptides including this residue might also be useful for some biological applications.