Polypeptides. VIII. Molecular configurations of poly‐L‐glutamic acid in water‐dioxane solution

The configuration of a poly‐L‐glutamic acid sample of 34,000 molecular weight has been studied in 0.2 M NaCl‐dioxane (2:1) as a function of pH and temperature by intrinsic viscosity, optical rotation, optical rotatory dispersion and infrared spectroscopy. The intrinsic viscosity and specific rotation fall sharply in the pH range of 5.4–6.4 reaching values that remain constant at higher pH. The indications that these changes correspond to a reversible helix‐random coil transition are substantiated by the demonstration that the optical rotatory dispersion shifts from the type characteristic of helices to the normal type as the pH is increased through the same region. Further evidence of the transition is found in infrared spectroscopic studies in solutions in which D₂O has replaced H₂O. The temperature dependence of the specific rotation and infrared spectra shows that within the pH region of 5.4 to 6.4 the equilibrium is shifted toward the coiled configuration by an increase in temperature. The analogy of this behavior with protein denaturation is obvious. It is noted that the transition from the helix to the coil does not begin until about 40% of the residues have become charged. The implications of the ability of the helical configuration to withstand this degree of electrostatic repulsion within its side groups is discussed in relation to the configuration of proteins.