Redundant coupling Δm loss of starting material severity: moderate

Arg δ-lactam formation upon activation

Activated Fmoc-Arg(Pbf)-OH cyclizes intramolecularly: the unprotected Nδ of the guanidino attacks its own active ester, forming a 6-membered δ-lactam. The Arg is now inert. Drop in coupling efficiency — sometimes diagnosed as 'incomplete Arg coupling'.

Affected residue(s): R

Why it happens (mechanism)

Pbf protects Nω, Nω' but leaves Nδ free with some residual nucleophilicity. After active-ester formation (HATU/HBTU/DIC), if aminolysis is sluggish, Nδ attacks the activated carbonyl from the same molecule → 6-membered lactam ring → no longer reactive. The active ester is consumed; aminolysis fails; the resin gets less Arg than expected.

When it strikes (triggers)

Microwave Arg coupling — major aggravator. Long pre-activation time. Hindered C-terminus on the resin (slow aminolysis). C-terminal Arg in fragment condensation (worst — both lactam loss AND coupling failure stop the synthesis).

How to spot it (MS signature)

Not a +Δm. Symptom: incomplete Arg coupling, residual H-Xxx- on resin (Kaiser test positive). Sometimes a small δ-lactam-Arg byproduct visible by HPLC of the reaction filtrate.

How to prevent it

Short pre-activation (≤2 min). Don't sit on activated Arg.
Avoid microwave coupling for Arg-containing residues, especially C-terminal Arg.
Double-couple Arg, especially on hindered C-terminal residues.
Use Arg(Boc)₂ for fragment condensation where C-terminal Arg is to be activated — fully shielded guanidino blocks the lactam.
Switch from HATU to DIC + HOAt — slower activation, less buildup of unreacted active species.

If it already happened (salvage)

Re-couple with fresh Arg + activator. After 2-3 cycles, capping incomplete sites with acetic anhydride before next residue is standard.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 5, §5.3.