Hydantoin formation
Five-membered urea-imide ring formed when N-acyl dipeptide cyclizes between α-amine of residue 2 and the urea carbonyl from a urea-derived activator (HBTU, HCTU, etc.). Looks similar to DKP but the ring contains an extra nitrogen.
- +26 — +26 Da hydantoin (urea-bridged variant)
Why it happens (mechanism)
If a urea-type activator (HBTU/HATU/HCTU/COMU) reacts with the C-terminal carboxyl, it can form an O-acylisourea or a guanidinium adduct. If the next α-amine is exposed before fast aminolysis, it cyclizes onto the urea carbonyl forming hydantoin (when the amine is on Gly especially). Acidic environment (DKP-like geometry) helps.
When it strikes (triggers)
Use of strong urea-type couplers (HATU, COMU) with Gly or Sar at position 2. Long activation times. Excess base (DIEA > 2 eq). High temperature.
How to spot it (MS signature)
-17 Da (loss of NH₃ from the dipeptide → hydantoin). Watch for it specifically at -X-Gly- dipeptide motifs after coupling.
How to prevent it
- Use carbodiimide (DIC) + Oxyma instead of guanidinium activators when -X-G- is at risk.
- Limit DIEA stoichiometry to <2 eq.
- Pre-form the active ester (DIC/HOBt) before adding the amine partner — minimizes O-acylisourea lifetime.
- Couple at room temperature, not elevated.
If it already happened (salvage)
- Hydantoin ring is stable. Cannot be reverted. Avoid by upstream protocol fix.
Source
Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 6, §6.4.