In contrast to the canonical phosphoproteomes (P-Ser, P-Thr, P-Tyr), the noncanonial phosphoproteomes include phosphorylated side chains not typically acid-stable and thus often missed in standard phosphopeptide mass spectrometry protocols. In this regard the N-phosphohistidinyl residues, the beta aspartyl-phosphate residues, and the S-phosphocysteinyl residues are all thermodynamically activated, hydrolytically unstable and do not need dedicated phosphatases for their decomposition, in sharp chemical contrast to the canonical P-Ser, P-Thr, and P-Tyr residues. The signal transduction relay systems in bacteria mediate phosphoryl transfers from ATP to histidine residues in transmembrane receiver proteins to aspartate side chain carboxylates in response receiver transcription factors. These cascades of tandem phosphoryl group transfers reflect the thermodynamic activation of the phosphoramidates and acyl phosphates as reactive transferable –PO₃²⁻ species.