Effects of Hydrophobicity and Anions on Self-assembly of the Peptide EMK16-II

Effects of hydrophobic and electrostatic interactions on the self-assembling process of the ionic-complementary peptide EMK16-II are investigated by atomic force microscopy imaging, circular dichroism spectra, light scattering, and chromatography. It is found that the hydrophobicity of the peptide promotes the aggregation in pure water even at a very low concentration, resulting in a much lower critical aggregation concentration than that of another peptide, EAK16-II. The effect of anions in solution with different valences on electrostatic interactions is also important. Monovalent anions (Cl(-) and Ac(-)) with a proper concentration can facilitate the formation of peptide fibrils, with Cl(-) of smaller size being more effective than Ac(-) of larger size. However, only small amounts of fibrils, but plenty of large amorphous aggregates, are found when the peptide solution is incubated with multivalent anions, such as SO(4)(2-), C(6)H(5)O(7) (3-), and HPO(4)(2-). More importantly, by gel filtration chromatography, the citrate anion, which induces a similar effect on the self-assembling process of EMK16-II as that of SO(4)(2-) and HPO(4)(2-), can interact with two or more positively charged residues of the peptide and reside in the amorphous aggregates. This implies a "salt bridge" effect of multivalent anions on the peptide self-assembling process, which can interpret a previous puzzle why divalent cations inhibit the formation of ordered nanofibrils of the ionic-complementary peptides. Thus, our results clarify the important effects of hydrophobic and electrostatic interactions on the self-assembling process of the ionic-complementary peptides. These are greatly helpful for us to understand the mechanism of peptides' self-assembling process and protein folding and aggregation.