Redundant coupling Δm +1 truncation (the lipidated AA fails to incorporate) severity: moderate-high

Sluggish coupling at lipidated Lys (semaglutide / tirzepatide)

GLP-1 class peptides feature a long fatty-acid-modified Lys (e.g., Lys₂₀ in semaglutide bears γ-Glu-C18-diacid). The bulky branched side chain dramatically slows aminolysis kinetics; single coupling cycles routinely give incomplete incorporation → truncations.

Affected residue(s): K (with γGlu-C18-diacid or fatty acid side chain)

Why it happens (mechanism)

The fatty-acid + γ-Glu spacer on Lys-Nε is large and hydrophobic. When this Lys is the next residue to be coupled, both the activated ester (already bulky) and the resin-bound peptide chain (now hydrophobic) suffer reduced solvation. Aminolysis kinetics drop ~10× vs. an ordinary Lys. Single coupling = incomplete; the unreacted Nα will get capped or cause downstream truncations.

When it strikes (triggers)

Pre-loaded Fmoc-Lys(γGlu-C18-diacid) building block on the resin. Especially when adjacent residues are also bulky/aggregating (β-sheet-prone GLP-1 mid-segment). Standard 1× coupling at room temperature with 2-3 eq AA. DMF as solvent.

How to spot it (MS signature)

Truncation at the position of the lipidated Lys (peptide stops at Xxx-1, missing the lipidated residue and everything C-terminal in fragment-coupling strategies).

How to prevent it

Iterative coupling (double or triple coupling) at the lipidated position. Industrial protocols now use 2× coupling with extended time (60-90 min each) routinely.
On-resin lipidation (orthogonal Lys protection like Mtt/Mmt or Alloc, deprotect after Lys is incorporated, then attach the fatty acid in a separate step) — avoids carrying the bulky side chain through the synthesis.
Use DMSO/NMP mixed solvent for coupling at the lipidated step — better solvation of fatty chain.
Higher building-block excess (5-6 eq instead of 3 eq) for the lipidated residue.
Heated coupling (40-50 °C) for this step only — but watch racemization risk.

If it already happened (salvage)

Re-couple with extended time + heat. After 2-3 attempts, capping the unreacted Nα with acetic anhydride is standard before continuing — gives an acetyl-truncation impurity that's easier to separate than a failed coupling-site peptide.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 5, §5.1 (general coupling background); literature: ACS OPRD 2021, Lilly tirzepatide GMP paper, doi:10.1021/acs.oprd.1c00108.