Before revision:  The  free energy change in conformation transition of S protein is introduced and we demonstrated that it includes two parts, one from the difference of conformation potential and another from the variation of structural elasticity.
After revision:  The conformational state population is deduced from the free energy change in conformation transition of S protein is introduced and we demonstrated that it includes two parts, one from the difference of conformation potential and another from the variation of structural elasticity.