Ion mobility and mass spectrometry techniques are used to investigate the stabilities of different conformations of bradykinin (BK, Arg$^1$-Pro$^2$-Pro$^3$-Gly$^4$-Phe$^5$-Ser$^6$-Pro$^7$-Phe$^8$-Arg$^9$). At elevated solution temperatures, we observe a slow protonation reaction, i.e., [BK+2H]$^{2+}$+H$^+$ → [BK+3H]3+, that is regulated by trans → cis isomerization of Arg$^1$-Pro$^2$, resulting in the Arg$^1$- cis-Pro$^2$- cis-Pro$^3$-Gly$^4$-Phe$^5$-Ser$^6$- cis-Pro$^7$-Phe$^8$-Arg$^9$ (all- cis) configuration. Once formed, the all- cis [BK+3H]$^{3+}$ spontaneously cleaves the bond between Pro$^2$-Pro$^3$ with perfect specificity, a bond that is biologically resistant to cleavage by any human enzyme. Temperature-dependent kinetics studies reveal details about the intrinsic peptide processing mechanism. We propose that nonenzymatic cleavage at Pro$^2$-Pro$^3$ occurs through multiple intermediates and is regulated by trans → cis isomerization of Arg$^1$-Pro$^2$. From this mechanism, we can extract transition state thermochemistry: Δ G$^‡$ = 94.8 ± 0.2 kJ·mol$^{-1}$, Δ H$^‡$ = 79.8 ± 0.2 kJ·mol$^{-1}$, and Δ S$^‡$ = -50.4 ± 1.7 J·mol$^{-1}$·K$^{-1}$ for the trans → cis protonation event; and, Δ G$^‡$ = 94.1 ± 9.2 kJ·mol$^{-1}$, Δ H$^‡$ = 107.3 ± 9.2 kJ·mol$^{-1}$, and Δ S$^‡$ = 44.4 ± 5.1 J·mol$^{-1}$·K$^{-1}$ for bond cleavage. Biological resistance to the most favored intrinsic processing pathway prevents formation of Pro$^{3}$-Gly$^{4}$-Phe$^{5}$-Ser$^{6}$- cis-Pro$^{7}$-Phe$^{8}$-Arg$^{9}$ that is approximately an order of magnitude more antigenic than BK.