Nmr studies of mixtures of poly‐L‐lysine hydrobromide with water

Abstract 1 H, 2 H, 13 C, and 81 Br nmr measurements of mixtures of poly‐ L ‐lysine hydrobromide with water have been carried out over a range of temperatures and water contents. When n (number of molecules of water per residue) ∼13 at room temperature, a transition occurs from a gel to a liquid phase. The liquid phase contains polymer molecules that are flexible, but contain more intramolecular structure than the same molecules in trifluoracetic acid solution. The gel phase contains junction zones of hexagonally packed α‐helices, linked by flexible regions of polypeptide chain. The α‐helical residues impart to their associated water molecules a slight anisotropy of motion, which is dectable by 2 H nmr. These residues bind up to about seven molecules of water each; the other six required to complete the gel–liquid transition space out the polymer molecules, allowing increased segmental motion of the residues in the flexible regions. This increased motion reduces the energy of the flexible regions and thus increases the proportion of residues in them (increasing the temperature has the same effect); the transition occurs when insufficient residues remain in the α‐helical junction zones.