Redundant coupling Δm +1 residue severity: moderate

Redundant amino-acid coupling via premature Fmoc loss

Trace amine (residual piperidine from previous deblock, or DIEA in Fmoc-AA-OH stock) cleaves a small fraction of Fmoc on the incoming Fmoc-AA-OH; the deblocked AA also couples → +1 residue insertion.

Affected residue(s): any

Why it happens (mechanism)

Fmoc-AA-OH solution + trace amine → Fmoc cleaved → H-AA-OH (free amine) + activator → H-AA-AA-coupled. Now the resin has -AA-AA- where only -AA- was wanted. +1 residue insertion.

When it strikes (triggers)

Insufficient washing after piperidine deblock. Old Fmoc-AA stocks contaminated by basic amines. Long activation times. Some Fmoc-AA-OH are inherently labile (esp. -Gly, -Ile).

How to spot it (MS signature)

+1 residue (specific Δm depends on which residue duplicated; e.g., +57 for Gly insertion, +97 for Pro, etc.).

How to prevent it

Wash resin thoroughly (≥6× DMF, ≥3× DCM) after piperidine deblock; DMF should be free of residual base.
Pre-form active ester for hard-to-couple residues; don't store activated Fmoc-AA solutions.
Use Fmoc-AA from a fresh, dry batch.
For Gly-Gly couplings or Pro-Pro: monitor by Kaiser test — multiple insertion shows up as deeper blue.

If it already happened (salvage)

Insertion is permanent; truncation and re-synthesis usually required.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 10, §10.2.