Dynamic cross-correlation map (DCCM)

Cross-correlation was a 3D matrix representation that could display time-related information between protein residues [27]. In biomolecules, the relative movement of Cα atoms helped to identify the binding area. DCCM was used to detect the time-dependent motions of all Cα atom pairs, and was calculated according to the following formula: [28, 29]

In the formula, i and j indicated the i-th and j-th atoms, respectively. The Δi and Δj indicated the displacement vector corresponding to i-th and j-th atoms. And the <.......> indicated the ensemble average. The cross-correlation coefficient C_ij varied from -1 to 1 [30, 31]. A positive value of C_ij indicated that the motion of atoms was interrelated and in the same direction, and a negative value of C_ij indicated that the residues were inversely correlated [32].