π-π stacking interactions mediate molecular recognition between arginine and tryptophan containing peptides derived from human islet polypeptide

Cation-π interactions, often found in protein assemblies, are characterized by favorable electrostatic interactions between an aromatic π-electron surface and a positively charged species. There are evidences that reveal the importance of cation-π interactions between arginine and aromatic residues in protein structure and function. In this paper, the effect of cation-π interactions on the aggregation propensity of peptides derived from human islet polypeptide (hIAPP) was explored using UV resonance Raman and fluorescence spectroscopy. By employing an analog of hIAPP_22–29 in which Phe-23 is replaced with tryptophan (NWGAILSS), we were able to demonstrate an increase in the amyloidogenic propensity of this mutant in the presence of Zn²⁺ that is attributable to cation-π interactions. In contrast, no cation-π interactions were observed when the cationic F23R analog of hIAPP_22–29 (NRGAILSS) was allowed to interact with NWGAILSS. From these observations, it was surmised that in these peptides, the dominant interaction between arginine and tryptophan involves the π-cloud of the guanidino group and the indole ring, not cation-π interactions. The spectroscopic data, supported by density functional theory-based simulation results, suggest that arginine-tryptophan interaction involves π-π stacking where the guanidino group is oriented parallel to the indole ring. These hydrophobic interactions, coupled with the hydrotropic effect of the guanidine functionality of arginine, led to a delay in the aggregation kinetics of NWGAILSS. These unique interactions were further exploited to design a peptide inhibitor of full-length amylin self-assembly.