Helix-coil-transition in solid polypeptides

Abstract Poly(L-glutamic acid) (PLGA) was synthesized by living anionic ring-opening polymerization of the NCA monomer, which was obtained by reacting diphosgene with an amino acid derivative. The chemical structures and thermal properties were characterized by 1H-NMR, 13C-NMR, TGA and DSC. XRD powder patterns found to be amorphous for all polymers obtained. The molecular weights could be determined under severe limitations due to low solubility and high aggregation tendency. The secondary structure of the PLGA films was analyzed in the solid state by IR spectroscopy; the order was determined mainly by XRD. Uniform bulk films (1-5 µm) were produced by drop-casting of PLGA solutions in TFA on silica. The XRD film analysis indicated the absence of a long range order or an orientation even if a helical microstructure was confirmed by IR spectroscopy. The coil solvent TFA delivered constantly a helical or a β-sheet structure in the solid state depending on the water content of the solvent which was observed for the first time to exhibit a high influence on the crystallization process for PLGA. Temperature dependent in-situ IR measurements were examined to analyze if a helix-coil transition occurs, but there could be no solvent system determined, which resulted in a disordered coil structure in the solid state. General parameters like solvent systems, evaporation conditions, concentration, substrates etc. were analyzed. New crystallizations were obtained on silica prepared by drop-casting of solutions of PLGA in DMF, DMA, TMU, NMP, and pyridine/water mixtures, respectively. PSCBC in DMF, CDCl3/TFA-d, and PSBC in CDCl3/TFA-d exhibited the same crystalline diffraction patterns like PLGA. The long range order in the X-ray diffraction pattern is proven by extremely sharp crystalline signals, which are not changing the shape or the position of the peak by increasing the temperature up to 160°C. The substrate seems to play a decisive role because the crystalline structures were not obtainable on glass. The crystal structure consists probably of two different layered structures based on the intensity ratios of the two series of crystalline signals in the X-ray diffraction patterns. The source of the layered structure remains unclear and needs further studies to investigate the spatial arrangement of the chains in more detail. The secondary structure was still not changing upon heating even if a highly crystalline diffraction pattern occurs. Concluding that even the newly investigated crystallization did not show a helix-coil transition in the solid state by annealing, the phenomenon known in solution has to be claimed as unachievable in the solid state based on the results of this work. A remaining open question represents the observation that the same crystalline pattern can be reproducibly prepared with exhibiting two different ordered secondary structures (helix and β-sheet). After the investigation that the evaporation time cannot be decisive for the crystal growth, the choice of a strong hydrogen bonding interrupting solvent is most probably the key to support and induce the crystallization process.