The effect of neutral salts on the conformational transition of poly-alpha-amino acids. I. Potentiometric titration and optical rotatory dispersion studies.

Abstract

AbstractThe effect of salts on the coil‐to‐helix transition of poly‐α‐amino acids was investigated by optical rotatory dispersion and potentiometric titration techniques. Both charge‐dependent and charge‐independent contributions to the free energy were considered.The free energy of formation ΔF° of the uncharged α‐helix from the uncharged random coil for poly‐L‐glutamic acid (PGA) decreases very rapidly in the limit of zero added salt concentration. This effect probably depends on the uncertainty affecting the choice of the extrapolation of the apparent pK for the random coil at low ionic strength. Above 0.1 M salt, where the free energy determination becomes meaningful, the anions and cations investigated do not affect the value of ΔF°, with the exception of Li+. Our data support the point of view that this cation binds to the peptide group.A class of salts produces an increase of the helical content of poly‐L‐ornithine (PO) both at low and high degree of ionization. This effect appears to be anion dependent. It is shown that (1) no change of ΔF° is involved; (2) recent theories of polyelectrolyte solutions cannot account for our results. We suggest that a true site binding of the anions to the charged amino groups occurs. The role of electrostatic binding in determining the conformational stability of proteins in the presence of some anions is stressed, and a general treatment for the electrostatic binding equilibria is outlined.