Toward a Mechanistic Understanding of Ionic Self-Complementary Peptide Self-Assembly: Role of Water Molecules and Ions

Ionic self-complementary peptides are considered an important class of self-assembling peptides. In particular, RADARADARADARADA (RADA(4)) is wellknown to form a relatively regular nanofiber structure that has been primarily studied in terms of its physicochemical properties, as related to its biomedical applications. However, the molecular level interactions that are involved in promoting the self-assembly of this peptide into nanofibers have not been fully elucidated. Herein, a thermodynamic analysis of the influences of peptide chemistry upon self-assembly is discussed for RADA, RADA(4)-K-5, and RADA(4)-S-5. The regular nanofiber structure of the assembled peptides makes it a good candidate for isothermal titration calorimetry (ITC) studies for determining the propensity for self-assembly, the critical assembly concentration (CAC), and the role hydration and ion content play in the assembly of these peptides. First, solutions containing only RADA(4)-K-5 did not self-assemble; illustrating even slight alterations in the asymmetric terminal amino acid chemistry affects assembly. The CAC of the remaining self-assembling peptides was between similar to 0.1 and similar to 0.15 mM. Interestingly, we found that self-assembly was entropically driven with hydrophobic forces being the main driving force for RADA(4) and hydrogen bonding for RADA(4)-S-5. The role of water molecules and counterions in self-assembly was also highlighted: assembly of RADA(4) led to desolvation of interfacial surfaces, whereas the net number of water molecules in the assembled complex increased upon RADA(4)-S-5 self-assembly. Moreover, it was found that counterions did not seem to contribute significantly to self-assembly: a result in contrast to current concepts regarding the role of electrostatic interactions in self-assembly of RADA(4)-like peptides. A molecular level understanding of peptide self-assembly will allow for further engineering of peptides for a vast array of biomedical applications.