Figure 1. Binding of H₃O⁺ to the Asp–Arg SF.

Fully optimized B3-LYP/6-31+G(3d,p) structures of (a) ion-free Asp^––Arg⁺ SF, (b) Asp⁰–Arg⁰ SF, (c) initial configurations of the SF-H₃O⁺ complex and (d) final configuration of the SF–H₃O⁺ complex, AspH⁰–H₂O–Arg⁺ with H in grey, C in green, N in blue and O in red. A dashed line denotes a hydrogen bond, which is defined by a donor–acceptor distance ≤3.5 Å and a H–acceptor distance ≤2.5 Å. The reaction between SF and H₃O⁺ is depicted in (e) with free energies given in kcal/mol; ΔG¹ is the binding free energy in the gas phase, whereas ΔG⁴ and ΔG³⁰ are the corresponding free energies in the SF characterized by an effective dielectric constant of 4 and 30, respectively.