Pbf protecting-group residue on Arg

Incomplete Pbf removal from Arg(Pbf) — most common 'partial deprotection' artifact in modern Fmoc synthesis. +252 Da per residual Pbf.

Why it happens (mechanism)

Pbf is normally cleaved cleanly by TFA, but at TFA concentrations <90%, in cocktails with strong nucleophilic scavengers, or with insufficient time, ~5-15% can remain. Each unremoved Pbf is +252 on the affected Arg.

When it strikes (triggers)

TFA <90%. Cleavage time <2 h on long peptides with multiple Arg. Cold cleavage. Excessive scavenger amount that quenches the cation.

How to spot it (MS signature)

+252.08 Da per Arg with Pbf left. Multiple Arg → +252, +504, +756, etc. Often a peak ladder.

How to prevent it

• Standard cleavage cocktail: TFA/TIS/H₂O/EDT = 92.5/2.5/2.5/2.5 for ≥2 h at RT. Pbf comes off completely.
• For Arg-rich peptides, extend cleavage to 3 h.
• Monitor by analytical HPLC pre-prep purification — cluster of Pbf-residue peaks at +252n is diagnostic.
• If problematic, switch to Fmoc-Arg(Mtr) which may behave differently (caveat: Mtr cleavage often needs hot conditions, brings other issues).

If it already happened (salvage)

• Re-treat with TFA (92.5%) for 2 h at RT; repeat once. If Pbf persists, the problem is likely buried in a hydrophobic core — try TFA/TFMSA (5%) cocktail with caution.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 1, Appendix I.