Chirality Control of Screw-Sense in Aib-Polymers: Synthesis and Helicity of Amino Acid Functionalized Polymers.

Abstract

2-Aminoisobutyric acid (Aib) is an essential amino acid, leading to the formation of peptAibols as microbiologically active peptides and proteins. We here report on the ring-opening polymerization (ROP) of Aib-NCA (N-carboxy-anhydrides), enabling to prepare distinct Aib-polymers up to molecular weights of 1400 g/mol with precise end-group control. ROP of Aib-NCA was accomplished via various amine initiators in frozen and liquid solvent systems, in all cases revealing the desired structures as detected by MALDI-TOF-MS and 1H NMR. We can prove living polymerization behavior until the limit of solubility via inline-IR spectroscopy in both solution and solid state polymerization, characterizing the final polymers via MALDI-TOF-analysis. The attachment of chiral (d or l)-amino acids onto the polymer' headgroup allows to systematically investigate the helical screw-sense of poly(Aib)n, resulting in chiral induction to form either left (M)- or right (P)-handed screw-senses, depending on the chirality of the attached amino acid. The approach is extended toward a switchable, chiral azo-headgroup, able to change chirality of the attached poly(Aib)n via a light-induced trigger, revealing a thermally stable cis-isomer when polymer units are attached. In contrast to many other amino acids easily polymerized into poly(amino-acids) via ring-opening polymerization (ROP) of their respective N-carboxyanhydrides (NCAs), the ROP of Aib-NCA (N-carboxy-anhydrides) here is novel and opens a general approach toward chiral, end group-modified helices with a defined screw-sense.