Influence of Nε-Protecting Groups on the Protease-Catalyzed Oligomerization of l-Lysine Methyl Ester

The direct oligomerization of l-lys-OMe by bromelain catalysis gave oligo(l-lys) with DPavg ∼ 3.6 and dispersity ∼ 1.1. Since higher chain length oligo(l-lys) with lower dispersity values and one reactive amine for selective conjugation would be beneficial, we explored protease-catalyzed oligomerization of Ne-protected l-lys monomers where Ne-groups included tert-butoxycarbonyl (Boc) or benzyloxycarbonyl (Z) groups. By using Ne-protected l-lys monomers, oligopeptide side-chains are hydrophobic-neutral which should dramatically alter enzyme kinetic and binding constants relative to nonprotected l-lys. Schechter and Berger’s conceptual model guided our choice of papain as the protease catalyst. Papain-catalyzed oligomerization of Ne-Boc-l-Lys-OMe gave products with DPavg values that were pH dependent and varied from 4.7 ± 0.2 to 7.5 ± 0.1. Similarly, oligo(Ne-Z-l-Lys) synthesis was pH dependent, and DPavg values varied from 4.3 ± 0.2 to 5 ± 0.2. Oligo(Ne-Boc/Z-l-Lys) that precipitates from reaction media ha...