Synthesis and characterization of poly-l-leucine initialized and immobilized by rehydrated hydrotalcite: understanding stability and the nature of interaction

Abstract

PLLs were synthesized by the ring-opening polycondensation (ROP) method using α-L-leucine N-carboxyanhydride (NCA) and initialized by triethylamine (Et₃N), water or rehydrated hydrotalcite (HT_rus). The role of temperature, different initiators and water in ROP was further investigated. In general, the initiators used in the polymerization reaction lead to PLL alpha-helical chains containing 5–40 monomers with NCA endgroups via a monomer-activated mechanism. However, the water has a twofold effect on ROP, as both a nucleophile and a base, which involves competition between two different types of initiating mechanisms (nucleophilic attack or deprotonation of the NCA monomer) in the polymerization reaction. This competition provides as a main product NCA endgroups with an alpha-helical structure and leads to the formation of the PLL cyclic-chains and beta-sheet structures which reduce the polymer Mw and the PD of the polypeptide. Furthermore, the water can hydrolyze the NCA endgroups resulting in PLL alpha-helical chains that contain living groups as the main product. On the other hand, the HT_rus presents a double role: as both an initiator and a support. The polymers synthesized in the presence of HT_rus presented a HT-carboxylate endgroup. The PLLs immobilized in HT_rus through an anion-exchange method performed for just 30 minutes presented the PLL immobilized in the interlayer space of the HT_rus. The PLL chains of the immobilized counterpart are stabilized by H-bonding with the M–OH of the HT structure. All the polypeptides and biohybrid materials synthesized have been characterized using different techniques (EA, ICP, XRD, Raman, MALDI-TOF, ESI-TOF, FT-IR at increasing temperatures, TG/DT analyses and TEM).