Polyaspartates as Thermoresponsive Enantiodifferentiating Helically Chiral Alignment Media for Anisotropic NMR Spectroscopy

Abstract

Lyotropic liquid crystalline phases of poly-β-phenethylaspartates are presented as new helically chiral enantiodifferentiating alignment media with thermoresponsive properties. In addition to displaying the highest enantiodifferentiation observed for homopolypeptides, the alignment media undergo a temperature induced helix reversal without perturbing the nematic phase. This offers the opportunity to measure residual dipolar couplings (RDCs) in high and low temperature helix conformation (P- and M-helix) in one polymer. Thus different mean orientations of a chiral analyte can be determined within the same sample. Furthermore, we investigated whether the resulting media are diastereomeric and whether we are able to obtain information about the alignment process.

Thermoresponsive Alignment Media in NMR Spectroscopy: Helix Reversal of a Copolyaspartate at Ambient Temperatures

“…Here it should be noted that the helix conformation changes from, for example, P‐ to M‐ helix, but the amino acid configuration remains the same. In our previous paper we demonstrated that polyaspartates before and after helix reversal are diastereomeric alignment media, thus furnishing β ‐angles very different from zero. For the copos this becomes quite evident when comparing RDCs obtained at 300 K with RDCs obtained at 312 K (Figure ).…”

“…Recently, we started to investigate LLC phases of high molecular weight poly‐β‐phenethylaspartates (PPA) as new helically chiral alignment media . In addition to displaying enantiodifferentiating abilities, they possess the intrinsic property to undergo a temperature induced helix reversal without perturbing the nematic phase .…”

Poly‐γ‐p‐Biphenylmethyl‐Glutamate as Enantiodifferentiating Alignment Medium for NMR Spectroscopy with Temperature‐Tunable Properties

“…To evaluate these, we performed a series of experiments with both enantiomers of isopinocampheol [(+)‐ and (−)‐IPC]. We chose IPC as an analyte because both enantiomers are commercially available and it has proven as a reliable test substance for new alignment media . We prepared the anisotropic samples above the critical concentration and verified the stability of the LLC‐phase by 2 H NMR spectra until no line shape change occurred.…”

“…The ability to invert the helix sense in the backbone of the polymer at a certain temperature changes the orientation of the analyte. Therefore, two sets of RDCs as a result of two independent orientations in one sample are obtained …”

Orientation and conformation of two [6]carbohelicenes in stretched polystyrene and a thermoresponsive polyaspartate

“…Recently, Schwab et al . achieved extraordinarily high enantiodifferentiation ( β values of up to 80°) in poly‐ β ‐phenetyl‐ L ‐aspartate (PPLA) and a copolymer made of PPLA and poly‐ β ‐benzyl‐ L ‐aspartate (PBLA) . From the thermoresponsive behavior of the aspartates, they could show that the enantiodifferentiation is imposed by both the helicity of the alignment medium and the chirality of the amino acid.…”