Perchlorate-induced formation of the alpha-helical structure of mastoparan

Mastoparan, a basic tetradecapeptide from wasp venom, has been considered to be unfolded under aqueous conditions. On the basis of the far-UV circular dichroism spectrum, we found that sodium perchlorate at molar concentrations stabilizes an alpha-helical structure of mastoparan. To understand the mechanism of the perchlorate-induced stabilization of the alpha-helical structure, we synthesized a dimeric form of mastoparan derivative, which was linked at the C terminal by a disulfide bond. The linkage decreased the concentration of perchlorate required to stabilize the alpha-helical structure by 30-fold. With the dimeric mastoparan derivative, we measured the effects of several salts such as sodium trichloroacetate, sodium trifluoroacetate, and sodium chloride. The concentration of salts required to induce the conformational transition varied and the order of effectiveness of different salts was consistent with the electroselectivity series of anions toward anion-exchange resins, indicating that the anion binding to the positively charged amino groups is responsible for the transition. These results suggest that the salt-induced formation of the alpha-helical state of mastoparan can be explained by a mechanism similar to that proposed for the salt-induced conformational transition of melittin.