SynopsisThe conformational properties of poly-halanine have been examined in aqueous solutions in order to investigate the influence of hydrophobic interactions on the helixrandom coil transition. Since water is a poor solvent for poly-~-alanine, water-soluble copolymers of the type (~,~lysine),-(~-alanine).,-(~,~-lysine),, having 10,160,450, and 1000 alanyl residues, respectively, in the central block, were synthezised. The optical rotatory dispersion of the samples waa investigated in the range 190-500 mp, and the rotation at 231 mp was related to the or-helix content, 8H, of the alanhe section. In saltfree solutions, at neutral pH, the three large polymers show high & values, which are greatly reduced when the temperature is increased from 5 to 8OOC. No helicity was observed for the small (n = 10) polymer. By applying the Lifson-Roig theory, the following parameters were obtained for the transition of a residue from a coil to a helical state: v = 0.012; AH = -190 f 40 cal./mole; A S = -0.55 f 0.12 e.u. Since AH and hs differ from the values expected for a process involving only the formation of a hydrogen bond, and in a manner predicted by theories for the influence of hydrophobic bonding on helix stability, it is concluded that a hydrophobic interaction is also involved. In the presence of salt (0.2M NaCl), or when the camino groups of the lysyl residues are not protonated (pH = 12)) the helical form of the two large polymers (n = 450 and n = 1000) is more stable than in water. Since the electrostatic repulsion between the lysine end blocks is greatly reduced under these conditions, the alanine helical sections fold back on themselves, and this conformation is stabilized by interchain hydrophobic bonds. This structure waa predicted by the theory for the equilibrium between such interacting helices, non-interacting helices, and the random coil.
