Staphylococcus aureus protein A (SpA) is the most popular affinity ligand for 
immunoglobulin G1 (IgG1). However, the molecular basis for the dissociation 
dynamics of SpA-IgG1 complex is unclear. Herein, coarse-grained (CG) molecular 
dynamics (MD) simulations with the Martini force field were used to study the 
dissociation dynamics of the complex. The CG-MD simulations were first verified 
by the agreement in the structural and interactional properties of SpA and human 
IgG1 (hIgG1) in the association process between the CG-MD and all-atom MD at 
different NaCl concentrations. Then, the CG-MD simulation studies focused on the 
molecular insight into the dissociation dynamics of SpA-hIgG1 complex at pH 3.0. 
It is found that there are four steps in the dissociation process of the 
complex. First, there is a slight conformational adjustment of helix II in SpA. 
This is followed by the phenomena that the electrostatic interactions provided 
by the three hot spots (Glu143, Arg146 and Lys154) of helix II of SpA break up, 
leading to the dissociation of helix II from the binding site of hIgG1. 
Subsequently, breakup of the hydrophobic interactions between helix I (Phe132, 
Tyr133 and His137) in SpA and hIgG1 occurs, resulting in the disengagement of 
helix I from its binding site of hIgG1. Finally, the non-specific interactions 
between SpA and hIgG1 decrease slowly till disappearance, leading to the 
complete dissociation of the SpA-hIgG1 complex. This work has revealed that 
CG-MD coupled with the Martini force field is an effective method for studying 
the dissociation dynamics of protein-protein complex.