Volume and sound velocity changes accompanying the α‐helix to β‐form and coil to α‐helix transitions in aqueous solution

The volume increment per amino acid residue for the α‐helix to β‐form transition of uncharged poly‐ L ‐lysine in aqueous solution was 3.8 ml in water and 4.3 ml in 0.2 M and 1 M NaBr solutions at 26°C, respectively. The sound velocity of the polymer solution was greater with the β‐helix than with the β‐form, but the difference was less in dilute salt solutions and disappeared in 1 or 2 M NaBr solution. Thus, the β poly‐ L ‐lysine solution was slightly more compressible than the α‐polymer solution, but this difference was diminished with increasing salt concentration. Both the volume change and the change in adiabatic compressibility of the polymer solution suggest that hydrophobic interactions among the lysyl groups in the β‐form reduce the amount of “icebergs” surrounding the polymer molecules as compared with the amount originally present with the α‐helix. The coil‐to‐helix transition of poly‐ L ‐glutamic acid in aqueous solution was also accompanied by a decrease in sound velocity. This can be attributed to the reduction of the water of hydration which is less compressible than free water.