Buffer-Specific Interactions of Imidazolium with Elastin-Like Polypeptides

Buffers are commonly added to protein solutions to stabilize their pH and are typically assumed to not influence any other property of the solution. A series of observations, however, indicate buffer-specific effects on protein stability, suggesting interactions of buffers with proteins. We performed molecular dynamics simulations at constant pH to study these interactions between the biological buffer imidazolium and different elastin-like polypeptides. Our analyses reveal an aggregation of neutral imidazolium (IMI°) in the solvation shell of the peptide and hydrogen bonding to its backbone, which is similar for all studied peptides. We furthermore observe an accumulation of positively charged imidazolium (IMI⁺) around negatively charged peptides and its depletion around positively charged peptides, mainly explained by electrostatic effects. Our insights indicate that the interactions of imidazolium with peptides are dominated by these electrostatic interactions, suggesting that the effect of imidazolium on the stability of proteins is highly pH-dependent. The transfer free energy of the peptide from water to an imidazolium solution at pH 6.95 is found to be negative, indicating a more favorable solvation in the presence of imidazolium. The free energy differences are similar to those observed for a transfer to the strongly denaturing osmolyte urea, suggesting a similar effect of imidazolium on protein stability.