| Abstract: | Poly-L -histidine (PLH) films at different degrees of protonation were produced mid subjected to infrared spectroscopic investigation (range 4000-650 cm?1). In addition, the N-deuterated film spectra were plotted. The amide II and III bands show that the peptide group is present in the trans form. The amide I and II bands show that at 0% and 50% protonation the PLH occurs as an α-helix and at 100% protonation as a random coil with some ranges in β structure. At 0% and 50% protonation, no hydration water is bound to the backbone. At 0% protonation all NH groups are linked to each other or to water molecules via hydrogen bonds. At 50% protonation NH+?N bonds form between the imidazole rings. These protons are present in continuous energy level distribution. Such bonds with tunneling protons are extremely polarizable and between these bonds may act proton dispersion forces. The Cl? ions are bonded to the NH groups of the imidazole groups. The hydration water is bonded to the Cl?? ions and to the NH groups. At 100% protonation, hydration water is bonded also to the CO groups of the backbone. The NH groups of the backbone, like those of the rings, endeavor especially in the dry state to bond to the Cl? ions. This leads to a strong steric constraint of the random coil. |
