Mechanism of the screw-sense reversal of tightly hydrogen-bonded α-helical network triggered by the side-chain conformation

Treating anisotropic artefacts in circular dichroism spectroscopy enables investigation of lyotropic liquid crystalline polyaspartate solutions

Circular dichroism (CD) spectroscopy is commonly used for investigation of the secondary structure of biomolecular compounds as well as polymers in isotropic solution. In anisotropic solution, the usage of the apparent CD is prone to misinterpretations due to artefacts from contributions of e.g. linear dichroism (LD). Herein, a method for the complete cancelation of anisotropic artefacts in the apparent CD is developed and its validity proven. The approach is further used for investigation of the conformation and the lyotropic liquid crystalline (LLC) structure of a copolyaspartate. For this system, a temperature-dependent change of the polymer's helical conformation (helix reversal) is known. Furthermore, a rotation of the aligned polymer helices inside a magnetic field (helix realignment) is independently present, occurring at a lower temperature compared to the helix reversal. In the current study, the helix reversal is confirmed and found to be accompanied by a change of the LLC structure. A cholesteric structure is detected and revealed to change its sense (cholesteric reversal) at the temperature at which the helix realigns in the magnetic field. The determination of the cholesteric sense is enabled by measuring the induced CD of an achiral dye, dissolved in the anisotropic polymer solution. Investigation of the anisotropic polymer solution is, thus, only made possible by cancellation of the aforementioned anisotropic artefacts. This allows the observation of changes of the liquid crystal structure from right-handed cholesteric, through left-handed cholesteric, to nematic with increasing temperature.

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

Poly(aspartic acid esters) are known to form either right-or left-handed α-helices depending on the ester group in the side chain, on solvent and/or on temperature. Polyphenethyl-l-aspartates (PPLA) exhibit a helix reversal from the right- to the left-handed form with increasing temperature. We have recently reported the application of polyphenethylaspartates as helically chiral alignment media. The thermoresponsivity observed for these polymers offers the possibility to measure different orientations of analytes before and after helix reversal of the alignment medium at 373 K. Herein we present a synthesized copolymer of phenethyl- and benzylaspartate as a new alignment medium undergoing this helix reversal at 303-313 K. Thus, the measurement of residual dipolar couplings (RDC) before and after the helix reversal is allowed for at ambient temperatures. A complete sign change of all ¹ H-¹³ C RDCs was observed, which is close to the highest possible difference in NMR spectra.