Induction of Chirality in β‑Turn Mimetic Polymer Conjugates via Postpolymerization “Click” Coupling
journal contributionposted on 23.03.2017, 19:18 by Stefanie Deike, Wolfgang H. Binder
Chiral and achiral β-turn mimetic polymer conjugates were synthesized, and their chiroptical properties were investigated. Chiral helical poly(n-hexyl isocyanate)s (PHICs) with alkyne functional end groups (4.0–5.4 kDa) were synthesized via coordination polymerization using either a chiral or an achiral alkyne-functional organotitanium catalyst. The obtained helical polymers were coupled with a chiral β-turn mimetic structure using the copper-catalyzed azide/alkyne cycloaddition (CuAAC) reaction, in turn allowing to tune the distance between the PHIC-helix and the chiral center from 7 to 14 Å. The successful linkage of the PHIC-helix to the β-turn mimetic was confirmed by size exclusion chromatography (SEC), high performance liquid chromatography (HPLC), 1H NMR spectroscopy, and MALDI-ToF mass spectrometry, proving the attachment of either one or two PHIC-helices to the central β-turn. Circular dichroism (CD) spectra confirmed chirality induction from the chiral β-turn mimetic structure to the polymer backbone of the achiral PHIC over five chemical bonds via postpolymerization modification, and revealed a distance-dependent effect for the transmission of stereochemical information. In contrast, a linkage effect leading to reduced chirality was observed when an additional linker unit was incorporated in-between the β-turn mimetic structure and the polymer backbone. The reported data do show that transfer of chirality is possible after polymerization over a relatively short distance of atoms.