Spezifische Ionen‐Effekte am Beispiel eines Oligopeptids: die Rolle zweizähniger Koordination beim Guanidinium‐Kation

Abstract

Ion-protein interactions are important for protein function, yet challenging to rationalize due to the multitude of ion-protein interaction possibilities. To explore specific ion effects on protein binding sites, we investigate the interaction of different salts with the zwitterionic peptide triglycine in solution. Dielectric spectroscopy experiments show that salts affect the peptide's reorientational dynamics, with a more pronounced effect of denaturing cations (Li + , guanidinium Gdm + ) and anions (I -, SCN -) than weakly denaturing ones (K + , Cl -). Notably, we find the effect of Gdm + and Li + to be comparable. Molecular dynamics simulations confirm the enhanced binding of Gdm + and Li + to triglycine, yet with a different binding geometry: While Li + predominantly binds to the C-terminal carboxylate group, bidentate binding to the terminus and the nearest amide is particularly important for Gdm + . This bidentate binding markedly affects peptide conformation. As such, this bidentate binding geometry may help explain the high denaturation activity of Gdm + salts.Specific ion effects, i.e., salts affecting macroscopic properties like surface tension, [1] solubilities, [2,3] interfacial potentials, [4] colloidal stability, [5] and biological activity [5,6] are ubiquitous. Yet, a molecular-level understanding of how ions alter such properties beyond electrostatic effects has not been fully obtained. [7] In particular, understanding how ions affect proteins remains challenging. [6,8,9] Such challenges arise from the structural complexity of proteins and the concomitant broad range of chemically different molecular protein sites. [10] Model systems such as amide-rich molecules have been often used to elucidate ionprotein interaction. [3,[11][12][13][14][15][16][17][18] These amide models, however, lack charged residues (e.g., the C-and N-termini) that are intrinsically sensitive to electrostatic interaction with