Fragmentation / deletion Δm fragmentation severity: high

N-terminal H-His-Pro-Xaa autodegradation

Peptides starting with H-His-Pro-... (e.g., TRH, GnRH analogs) self-cleave spontaneously: the imidazolyl side chain of His catalyzes intramolecular aminolysis at the Pro-Xaa amide. Result: cyclo(His-Pro) DKP + truncated C-terminal fragment.

Affected residue(s): H

Neighbour(s) that trigger it: P at position 2

Why it happens (mechanism)

His Nα is a nucleophile. The imidazolyl side chain deprotonates it. The cis-amide-favoring His-Pro pair pre-organizes a 6-membered transition state for the Nα to attack the Pro-Xaa amide carbonyl. Result: cyclic His-Pro-DKP detaches; the rest of the peptide is left starting from Xaa-.

When it strikes (triggers)

N-terminal His-Pro + any Xaa at position 3. Aqueous storage, neutral pH, room temperature — all *normal* conditions cause it. TRH (pyroGlu-His-Pro-NH₂) is naturally pyro-Glu-protected against this; analogs that re-expose H-His-... are vulnerable.

How to spot it (MS signature)

Two peaks: cyclo(His-Pro) DKP (~234 Da) and the truncated peptide starting from Xaa. Mass conservation when summed. Often diagnosed only after 'why does my product purity drop on storage?'

How to prevent it

Store at low pH (≤4) — protonation of imidazolyl slows the catalysis dramatically.
Keep cold (≤-20 °C) and dry (lyophilized).
If feasible, replace H-His- with pyroGlu- at the N-terminus to mimic TRH's natural protection — completely shuts down the autodegradation.
If H-His- is required for activity, design a stable pro-form and convert post-formulation.

If it already happened (salvage)

The bond is broken; not recoverable. Mitigation by storage conditions only.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 1, §1.5.