β-elimination Δm -98 severity: high

β-elimination of phosphopeptide

Phosphate on Ser/Thr is a good leaving group; under base or in the gas phase, β-elimination to dehydroalanine/dehydrobutyrine. -98 Da.

Affected residue(s): pS pT

Why it happens (mechanism)

Hα is acidic (β to phosphate). Base abstracts Hα → enolate → β-elimination of phosphate (HPO₃²⁻, mass 80) + ring-opening of the proton on Cβ-O (further -18). Net: -98 Da. Gas-phase MS/MS of phosphopeptides also shows -98 (and -80) loss as a diagnostic signature.

When it strikes (triggers)

Base treatment of phosphopeptides. Heat. Strong acid > pH 1. Even MS source heating. Phospho-tyrosine is much more stable.

How to spot it (MS signature)

-98 Da is the canonical phospho-elimination signature in MS/MS. Use it diagnostically: presence of -98 in CID confirms phospho-Ser/Thr.

How to prevent it

Avoid heated MS source / aggressive CID for phospho identification — switch to ETD/EThcD.
Use mild deprotection conditions; avoid hot piperidine on phospho-Ser/Thr.
Consider Fmoc-Ser(PO(OBzl)-OH)-OH (benzyl-protected phosphate) — more stable than free phosphate during synthesis.

If it already happened (salvage)

Once eliminated, dehydroalanine is reactive but not phosphate-recoverable. Re-synthesize.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 12, §12.1; appendix-I.