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.