Aspartimide formation

The most notorious side reaction in Fmoc/tBu SPPS. Asp(OtBu) backbone amide attacks the side-chain ester, forming a 5-membered imide. Each piperidine cycle accumulates more — purity loss is cumulative, not sequence-stage-limited.

Why it happens (mechanism)

The amide NH on the C-terminal side of Asp(OR) deprotonates under base (piperidine, DBU, even DIEA), then nucleophilically attacks the side-chain ester carbonyl, expelling the alkoxide leaving group and forming a 5-membered succinimide ring. The Hα on the imide is much more acidic than on the parent Asp, so racemization (L→D) accompanies imide formation almost stoichiometrically. The imide can then ring-open by water, alcohol, or piperidine — giving α-Asp, iso-Asp, or Asp/iso-Asp piperidide respectively. So one mechanistic event spawns up to 8 detectable impurities (4 isomers × ±18 Da or ±67 Da).

When it strikes (triggers)

Sequence motif Asp(OtBu)-Xxx where Xxx is Gly (worst, Gly's small side chain leaves the backbone amide freely accessible) or Asn(Trt), Ser, Thr (free OH catalyzes), Asp(OtBu) (e.g. -DD- runs), or His (acid-catalyzed at the imidazolyl Nπ). Also: every piperidine cycle adds to the count, so position of Asp from the C-terminus directly predicts severity. Microwave SPPS dramatically aggravates it.

How to spot it (MS signature)

−18.01 Da (anhydro-imide). If the imide is opened by piperidine: +67.07 Da (Asp piperidide, isomers). If hydrolyzed during cleanup: same mass as parent but a-Asp/iso-Asp isomers run as separate peaks on RP-HPLC, so look for shoulders/satellites near the main peak. The D-Asp/iso-Asp pair is invisible to MS1 — only HRMS + chiral / iso-Asp-specific digestion catches it.

How to prevent it

• Use bulky / electron-rich Asp side-chain protection: Fmoc-Asp(OMpe)-OH or Fmoc-Asp(OBno)-OH (β-3-methylpent-3-yl / β-2-phenylisopropyl ester) — these can drop aspartimide from ~30% to <2% in -DG- sequences. OtBu alone is often insufficient.
• Use a backbone-amide-protecting Asp building block: Fmoc-Asp(OtBu)-(Dmb)Gly-OH (or Hmb on the susceptible Xxx). The Dmb/Hmb on the C-terminal-side amide *physically blocks* the nucleophile that initiates the cyclization. This is the most reliable cure for -DG-.
• Add 0.1 M HOBt (or 0.1 M Oxyma) to your 20% piperidine/DMF — buffers the basicity, reduces aspartimide ~3-5×.
• Replace piperidine with piperazine (less basic, slower deblock but much less aspartimide). 2% DBU/2% piperidine works for hardest cases.
• Drop deblock temperature: room temp instead of 50 °C. Avoid microwave on Asp-containing sequences.
• Switch coupling solvent to DMF/DCM (1:1) or pure DCM for the residue C-terminal to Asp — polarity drives the cyclization.
• Cleavage: use cooled TFA (4 °C), shorten cleavage time, avoid storage of the protected peptidyl-resin in basic conditions.

If it already happened (salvage)

• Once formed, aspartimide is hard to revert cleanly. Mild base hydrolysis (pH 9, 37 °C) ring-opens the imide back to Asp + iso-Asp mixture — the iso-Asp is still off-spec. Practical answer is usually to re-synthesize with proper protecting groups, not to rescue the batch.

Source

Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 6, §6.1, pp. 119-152.