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Multisegmented Hybrid Polymer Based on Oligo-Amino Acids: Synthesis and Secondary Structure in Solution and in the Solid State

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journal contribution
posted on 13.06.2019, 00:00 by Jan Freudenberg, Wolfgang H. Binder
The synthesis of multisegmented polymers containing repetitive oligo-(BnAspn, BnGlun) sequences separated by methylene units and their secondary structure formation in solution and in the solid state are reported. The oligo-amino acids act as dynamic folding elements, introducing conformational changes within these multisegmented polymers depending on their nature, chain length, and temperature in either hexafluoroisopropanol (HFIP) solution or in the solid state. Combining ring-opening polymerization (ROP) of N-carboxyanhydrides with postfunctionalization of the respective N-terminus yields the oligo-(BnAsp3, BnAsp10, BnGlu3, and BnGlu10) sequences bearing terminal vinyl groups on either side of the polymer chain. Subsequent acyclic diene metathesis (ADMET) polymerization and fractionation by preparative gel permeation chromatography generate polymers of a precise degree of polymerization (n = 3, 10; m = 1–136), allowing to investigate chain length-dependent secondary structure formation of the oligo-(BnAspn, BnGlun) sequences. Polymers bearing BnAspn-esters as the repeating element show a transition from α-helices to β-sheets in HFIP solution, as proven by Fourier transform infrared and circular dichroism spectroscopic investigations, whereas BnGlun esters in the solid state stabilize preferably into β-sheets with increasing chain length and temperature. As a consequence, β-sheet conformation is identified as the thermodynamically more stable conformation, as supported by our experiments in the solid state and in solution.