Figure 6.

Summary of binding energies of pH-dependent pairwise interactions of His with aromatic and basic residues in solvent. (A) Interactions between His⁰ and aromatic residues (His⁰, Phe, Tyr, and Trp). The energetics of interactions between His⁰ and aromatic residues are compared with interactions between non-His aromatic residues. The interactions are categorized into three groups (based on geometric parameters): stacked (purple), CH–π (red), and H-bonding (gray). Since H-bonding interactions are possible only for some of the pairs and because of their different energetic contribution, they are shown separately. (B) Interactions between His⁰ and each of the basic residues Lys and Arg as well as between His⁺ and each of the aromatic residues His⁰, Phe, Tyr, and Trp. The energetics of cation–π interactions in which His serves as either the π-system (in cation–His⁰ interactions) or the cation in (His⁺–π interactions) is compared with the interactions of Lys and Arg with the aromatic residues Phe, Tyr, and Trp. The interactions are categorized into three groups (based on their geometric parameters): cation–π (blue), CH–π (red), and H-bonding (gray).