Specific ion effects on the solution conformation of poly‐L‐proline

Abstract

AbstractThe effects of various electrolytes on the conformation of poly‐L‐proline II in aqueous and nonaqueous solution have been investigated by optical rotatory techniques. It is shown that these agents induce a linear decrease in the corrected specific levorotaton of poly‐L‐proline with increasing salt concentration, with a molar effectiveness which varies from one salt to another. The salt‐induced rotatory changes may be divided into anion and cation components, and it is shown that the major specific affectors are the anions, which increase in effectiveness in reducing the corrected specific levorotation in the following sequence: Cl− < NO3− < Br− < I− < ClO4− < SCN−. The inorganic monovalent cations tested (Li+, Na+, K+) are all equally effective in decreasing the specific levorotation. Ca++ has a marginally greater effect per mole than the inorganic monovalent cations, while the effectiveness of the ganidinium cation is appreciably less. The tetraalkylammonium cations decrease the specific levorotation more effectively than the inorganic monovalent cations, with the molar effectiveness increasing linearly with total content of methyl plus methylene groups. A similar linear increase with increasing methyl plus methylene content is shown by the aliphatic alcohols, though the effect per mole of CH2 or CH3 group is appreciably smaller than that shown by the tetraaklylammonium cations. Salts dissolved in essentially anhydron for mamide are also appreciably effective. Selected viscosity experiments have also been carried out to show that the observed effects on specific levorotation have a structural as well as an optical basis. These results are interpreted in terms of a model which involves binding of anions at the imide nitrogen, and cations at the carbonyl oxygen. It is proposed that this binding induces an increase in the double‐bond character; of the peptide bond (and thus a shortening of the bond) which is roughly proportional in the polarizability of the bound anion and that this increase is potentiated by cations which decrease the total dielectric constant (e.g., the tetraalkylammonium series), and reduced by cations presenting competitive local anion binding sites (e.g., the guanidinium ion). We propose further that this shortening of the peptide bond is accompanied by a lengthening of the adjacent bond, thus reducing the steric restraints to rotation about this bond (increasing the accessible range of the angle ψ) sufficiently to induce a progressive non‐cooperative collapse of the poly‐L‐proline II structure. Several lines of evidence are presented to support this interpretation. The various neutral salts are also shown to induce a time‐dependent precipitation or “salting‐out” of poly‐L‐proline from solution. In order of decreasing molar effectiveness as salting‐out agents in this system, the various ions may be ranked: SO4− > Ac− > Cl− > Br− > SCN− > I− > ClO4−; and K+ ≃ Na+ > Li+ > Ca++. These rankings follow the usual Hofmeister or lyotropic series, and are quite different from hose which apply to the effects on solution conformation of poly‐L‐proline.