Fragmentation / deletion Δm loss of N-terminal residue severity: moderate

FITC-induced N-terminal Edman degradation

FITC-labeled peptides at the N-α (no spacer) undergo Edman-like cleavage under TFA: the FITC thiocarbamoyl + adjacent amide form a thiazolinone that splits off the first residue + FITC.

Affected residue(s): any N-terminus

Why it happens (mechanism)

FITC + Nα-amine → thiocarbamoyl. Under TFA, the thiocarbamoyl S attacks the proximal amide carbonyl → 5-membered thiazolinone intermediate → splits off as fluorescein-thiazolinone (rearranges to thiohydantoin in workup) + the truncated peptide minus the first residue. Identical chemistry to classical Edman sequencing.

When it strikes (triggers)

FITC directly attached to N-α (no spacer) + any TFA exposure. Heated TFA. Long cleavage time.

How to spot it (MS signature)

Truncated peptide (lost first residue + FITC). Fluorescein-thiohydantoin small molecule visible by UV at 488 nm.

How to prevent it

Insert an ε-Ahx (6-aminohexanoic acid) spacer between FITC and the peptide N-terminus. Distance suppresses the 5-membered ring formation.
Cleave the peptide before labeling — label the free peptide in solution at the desired position (e.g., on Lys-NH₂ via FITC-NHS) rather than on-resin.
Cold TFA, short time.

If it already happened (salvage)

Bond is broken; not recoverable. Re-label with spacer.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 1, §1.7.