Investigation of the effect of cation -pi and quadrupolar interactions in alpha helical peptides.

Abstract

Model peptides were used to study interactions between electrostatic quadrupoles associated with aromatic residues and electron-poor aromatic ligands, and with cationic side chains. The intention is to evaluate these interactions as tools for peptide and supramolecular design, as well as to provide knowledge useful towards a general understanding of non-covalent interactions in peptide and protein folding. Complexes between aromatic and fluoroaromatic rings are known to be stabilized by electrostatic interaction between the positively-charged centers and negatively-charged edges of fluoroaromatic rings and the negatively-charged centers and positively-charged edges of aromatic rings. A monomeric alpha-helical peptide containing two tryptophan side chains with appropriate spacing and orientation to form a stacked arrangement with an interspersed electron-poor aromatic molecule was designed and synthesized. However, this peptide did not bind C₆F₆ within the limits of detection, implying the interactions was less than 2 kcal/mol. Other electron-poor aromatic ligands with greater solubility also did not show binding, and future directions for this research will likely involve development of a more sensitive peptide system. The effect of cation-pi interactions on stabilization of alpha helices was measured in a helical coiled coil peptide. Homotrimeric and cross-linked homodimeric parallel helical bundles incorporating cation-aromatic residue pairs in place of intrahelical glutamate-lysine salt bridges were designed and synthesized. Energetics of peptide folding were determined by thermal melts, and by chemical denaturation with urea and guanidinium chloride, monitored by circular dichroism measurements. Evidence for helix-stabilizing arginine-phenylalanine interactions as strong as 2.3 kcal/mole was found. However, no significant stabilization was found for arginine-tryptophan and arginine-tyrosine residue pairs. A relatively large <italic>m</italic> value of folding energy destabilization per molar denaturant for the arginine-phenylalanine residues suggests a hydrophobic packing mechanism for this stabilization, but does not rule out electrostatic cation-pi interaction.