Fmoc-(Fmoc-Hmb)amino acids were introduced in the 1990s to improve synthetic yields of difficult peptides, such as amyloid β, that form aggregates during synthesis. It had been noted that tertiary amide bonds in a peptide sequence prevent interchain hydrogen bonding. The hydroxymethoxybenzyl group was selected as a temporary protecting group that could be removed when the peptide was cleaved from the resin.
Coupling to N-substituted amino acids is difficult but the hydroxy group in the 2-position of the benzyl group assists to make coupling easier. The activated amino acid derivative acylates the hydroxyl group, then an O→N shift takes place to form the native peptide bond. Unfortunately, activated Hmb-amino acids may sometimes form cyclic lactones during coupling which reduces the yield of desired peptide. To avoid this side reaction, the 2,4-dimethoxybenzyl group may be utilized.
(Dmb)Gly cannot form cyclic lactones, but peptide coupling to it is difficult. Instead, the (Dmb)Gly is incorporated into peptide sequences through dipeptides corresponding to glycine and the preceding amino acid. Utilizing Dmb-peptides results in improved reaction rates, higher yields and fewer impurities.
Hmb- and Dmb-protected derivatives are most effective when they are incorporated in or at the start of a hydrophobic sequence. For maximum effect, they should also be at least six residues away from a proline, pseudoproline, or N-methyl amino acid.
The Asp(OtBu)-(Dmb)Gly dipeptide is also useful in preventing the formation of side products arising from aspartimide formation during peptide synthesis. The Dmb group blocks the formation of the aspartimide intermediate. The use of Fmoc-Asp(OtBu)-(Dmb)Gly-OH is becoming the standard method to incorporate Asp-Gly in peptide synthesis.
AAPPTec offers many competitively priced Hmb- or Dmb-protected derivatives for peptide synthesis.
Some recent references utilizing Hmb- or Dmb-protected derivatives: