Effects of Anions on Nanostructuring of Cationic Amphiphilic Peptides

The effects of addition of a series of stoichiometric salts on the nanostructuring of cationic amphiphilic peptides have been investigated through the combination of atomic force microscopy (AFM), circular dichroism (CD), and turbidity measurements. The results revealed that anions had more pronounced effects than cations in tuning the nanostructures formed from these peptides. Addition of ClO3-, NO3-, and Br- could stabilize the primary nanostructures (nanostacks, nanospheres, or short nanorods) formed by A(9)K and I3K and effectively inhibit their growth into longer nanostructures (nanorods or nanotubes). In contrast, the anions of Cl-, SO42-, HPO42-, PO43-, and C6H5O73- (citrate) favored the axial growth of these peptides to form long intersecting nanofibrils and led to an increase in diameter and surface roughness, as well, clearly enhancing their propensity for nanostructuring. The efficiency of different anions in promoting the growth of peptide nanoaggregates into larger ones could be ordered as ClO3- < NO3- <= Br- < Cl- < SO42- < HPO42- < PO43- < C6HSO73-, broadly consistent with the Hofmeister anion sequence. These observations were well rationalized by considering different aspects of direct interactions of the anions with the peptide molecules.