Relative Strength of Cation-π vs Salt-Bridge Interactions:  The G_tα(340−350) Peptide/Rhodopsin System

Abstract

Interactions between cationic and aromatic side chains of amino acid residues, the so-called cation-pi interaction, are thought to contribute to the overall stability of the folded structure of peptides and proteins. The transferred NOE NMR structure of the G(t)alpha(340-350) peptide bound to photoactivated rhodopsin (R*) geometrically suggests a cation-pi interaction stabilizing the structure between the epsilon-amine of Lys341 and the aromatic ring of the C-terminal residue, Phe350. This interaction has been explored by varying substituents on the phenyl ring to alter the electron density of the aromatic ring of Phe350 and observing the impact on binding of the peptide to R*. The results suggest that while a cation-pi interaction geometrically exists in the G(t)alpha(340-350) peptide when bound to R*, its energetic contribution to the stability of the receptor-bound structure is relatively insignificant, as it was not observed experimentally. The presence of an adjacent and competing salt-bridge interaction between the epsilon-amine of Lys341 and the C-terminal carboxylate of Phe350 effectively shields the charge of the ammonium group. Experimental data supporting a significant cation-pi interaction can be regained through a series of Phe350 analogues where the C-terminal carboxyl has been converted to the neutral carboxamide, thus eliminating the shielding salt-bridge. TrNOE NMR experiments confirmed the existence of the cation-pi interaction in the carboxamide analogues. Various literature estimates of the strength of cation-pi interactions, including some that estimate strengths in excess of salt-bridges, are compromised by omission of the relevant anion in the calculations.

Kation‐π‐Wechselwirkungen im aktiven Zentrum von Faktor Xa: drastische Verstärkung durch stufenweise N‐Alkylierung von Ammoniumionen

“…Im Fall der primären und sekundären Ammoniumionen müssen bei der Einlagerung in den aromatischen Kasten drei ((AE )-16) respektive zwei N + H-Reste ((AE )-17) desolvatisiert werden, was nicht durch den Gewinn an Kation-p-Wechselwirkung kompensiert wird. Die viel stärkere Bindung des tertiären Ammoniumions (AE )-18 ist in Einklang mit den Ergebnissen unserer vorherigen PDB-Recherche: [7] Während die drei Alkylreste günstige C-H···p-Wechselwirkungen im aromati- [16] Lys-und Trp-Reste interagieren bevorzugt über C-H···p-Wechselwirkungen zwischen ihren Seitenketten. Solche Wechselwirkungen werden durch den elektronenziehenden Effekt des terminalen primären Ammoniumions verstärkt, das sich höchstwahrscheinlich von der Indoloberfläche abwendet, um von einer besseren Solvatisierung zu profitieren.…”

Tauroursodeoxycholic acid binds to the G-protein site on light activated rhodopsin

“…The 3D intracellular (IC) loop model of R* resulted from a previous study (Taylor et al, 2007) in which experimental TrNOE structures of Gt α (340–350) and its analogs (Kisselev et al, 1998; Anderson et al, 2006a, 2006b) were docked onto the IC loops. These docked structures revealed a common binding mode with similar residue-residue interactions that were potentially important for complex formation between R* and Gt α (340–350).…”

CHAPTER 1. New Applications of High‐Resolution NMR in Drug Discovery and Development

“…The technique has been widely used for defining the binding of a large range of ligand/protein systems, for instance the interaction of blood group antigens with virus like particles, 55 peptide inhibitors with beta-amyloid fibrils, 56 for demonstrating conformational selection, 57,58 for detecting cation-p interactions, among many others. 59 …”