Before revision:  mostly positively charged patches on the spike, and thereby mask the basic residues of the receptor binding domain and the basic S1/S2 site , which is unique specific to SAR-CoV-2 and is important for furin-mediated spike cleavage and the subsequent formation of its post-fusion conformation . Experiments corroborated the simulation results by showing that heparin binds the basic S1/S2 site of spike, inhibiting cleavage by furin. The simulations show that heparin can act on the hinge region responsible for the motion of the RBD between inactive closed and active open conformations of the spike. In simulations of the closed spike, heparin binds the RBD and the N-terminal domain of two adjacent spike subunits and hinders the opening. In simulations of the open spike, heparin binds similarly but induces stabilization of the hinge region and a change in RBD motion.
After revision:  mostly positively charged patches on the spike, and thereby mask the basic residues of the receptor binding domain and the basic S1/S2 site , which is unique specific to SAR-CoV-2 and is important for furin-mediated spike cleavage and the subsequent formation of its post-fusion conformation . Experiments corroborated the simulation results by showing that heparin binds the basic S1/S2 site .