The effect of temperature and salt concentration on the circular dichroism exhibited by unionized derivatives of L‐alanine in aqueous solution

The circular dichroism of Ac–Ala–NHMe, cyclo (–Ala–Ala–), Ac–Ala–OMe, Ac–Ala–Ala–OMe, and Ac–Ala–Ala–Ala–OMe has been measured in water and in aqueous salt solutions as a function of temperature. Only cyclo (–Ala–Ala–) exhibits circular dichroism which is independent of temperature. Each of the linear derivatives of L ‐alanine exhibits a positive circular dichroism in the range 208–218 nm at 15°C in water. Heating reduces the intensity of the positive circular dichroism, and only Ac–Ala–OMe retains positive circular dichroism at 75°C in water. Isothermal addition of salts produces changes in the circular dichroism of linear derivatives of L ‐alanine which resemble those seen on heating. The relative effectiveness of the salts tested, at a concentration of 4 M , is LiCl ⩽ KCl = NaCl < MgCl 2 ⩽ CaCl 2 ⩽ NaClO 4 . The circular dichroism of cyclo (–Ala–Ala–) is also affected by the salts. Extrapolation of the results obtained with Ac–Ala–OMe, Ac–Ala–Ala–OMe, and Ac–Ala–Ala–Ala–OMe to a long polypeptide with a –CH 2 R side chain in the L ‐configuration leads to the conclusion that this polypeptide should exhibit a temperature‐dependent salt‐sensitive positive circular dichroism between 208 and 218 nm when it exists as a statstical coil.