Application of Fmoc-His(Boc)-OH in Fmoc-based SPPS

Introduction

Enantiomeric purity greatly affects a peptide’s biological activity; therefore, avoiding increased amounts of D-isomer is important. Histidine is particularly prone to epimerization during activation of the coupling process in solid phase peptide synthesis (SPPS). Histidine’s susceptibility to epimerization is an intramolecular side reaction attributed to the lone pair electrons on the imidazole Nπ, which are in close proximity to the acidic alpha-carbon hydrogen. When the amino acid is activated, the lone pair electrons are sufficiently basic enough for deprotonation thus forming an achiral ester enolate. At this point, there is not a thermodynamically preferred pathway for conversion to the L- or D-isomer. The likelihood of epimerization increases when the reaction site is aggregated and histidine remains in an activated state for longer periods.