Figure 1. Spatial orientation and Cα–Cα distances of salt bridges on protein surfaces.

(A) Statitical analysis of angles (θ₁, θ₂) of 10,556 salt bridges on the surfaces of 6,493 X-ray protein structures. Two angles of a salt bridge (i–j), ∠Cβ₁Cα₁Cα₂ (θ₁) and ∠Cβ₂Cα₂Cα₁ (θ₂), were used to describe the charge group interaction based on the relative orientation of the two residues’ Cα–Cβ vectors as indicated. All of the angles are in the range of 0° to 180° (θ₁ and θ₂ color in black and gray). The length of radius are corresponding to Cα–Cα distance (Å). (B) The scatter plot shows the angles (θ₁, θ₂) of the salt bridges at a backbone distance >7 Å, which are restrained within 0° to 110°.