Salt-dependent conformational diversity of alternating poly (Glu-Leu).

Alternating poly(Glu-Leu) was synthesized by the condensation of the corresponding dipeptide p-nitrophenyl ester at high concentration. It exhibits a random coil structure in pure water at neutral pH. Addition of monovalent cations, such as NH4+ to a final 0.1 M solution, induces a transition to a water soluble beta-structure. The salt effect is quite selective since no transition was observed with Li+, Na+ or Cs+ ions. Addition of 0.5 equiv of calcium, cobalt or manganese chlorides per glutamyl residue induces similar coil to beta-sheet transitions. No polymer precipitation was observed at these very low salt concentrations. Addition of 0.5 equiv. of Cu2+ or 0.15 equiv. of Fe3- induces a coil to alpha-helix transition. Molecular modeling has been used to understand tentatively the main factors controlling the different conformations observed with the various metal ions.