Cys β-elimination → dehydroalanine + thiocysteine

Cys disulfide under base eliminates to give dehydroalanine + persulfide (thiocysteine). -34 Da on the disulfide-bridged peptide.

Why it happens (mechanism)

Hα on Cys is acidic (β to S-S). Base abstracts; β-elimination expels -S-S-CH₂-... as thiolate, leaving dehydroalanine. The persulfide can further oxidize to S-sulfocysteine.

When it strikes (triggers)

Basic buffers (pH > 8, esp. > 9) on disulfide-containing peptides. Hot conditions. Reductive scavengers (TCEP at high pH) can also facilitate.

How to spot it (MS signature)

-34 Da. Often paired with +1 from re-protonated dehydroalanine. The persulfide partner also detectable as a separate fragment.

How to prevent it

• Keep disulfide peptides at pH ≤ 7 in any handling.
• Use TCEP at ≤pH 6.5 for selective reduction.
• Avoid prolonged storage at room temp in any aqueous buffer.

If it already happened (salvage)

• Dehydroalanine is highly electrophilic; can re-cyclize with nearby thiols to give a different (wrong) disulfide pattern. Best to prevent.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 9, §9.1; 13.2; appendix-I.