Toward an improved understanding of the dissociation mechanism of gas phase protein complexes

Understanding the dissociation mechanism of multimeric protein complex ions is important for deciphering gas phase dissociation experiments. The dissociation of cytochrome c' dimer ions in the gas phase was investigated in the present study by constrained molecular dynamics simulations. The center of mass (COM) distance between two monomers was selected as the constrained coordinate. The number of intermolecular hydrogen bonds, smallest distance of intermolecular residuals, value of dipole moments, root-mean-square deviations, and potential energy components of the force field as a function of COM distance were examined for different charge partitionings of the +10 total charge state. These data were rationalized with free energy profiles to produce a qualitative description of the dissociation process. When charges are symmetrically distributed between the monomers in the dimer, dissociation occurs at a well-defined distance with only small structural changes in the monomers. There is an elastic type of stretching that initially resists the separation of the monomers but after dissociation the monomers recoil slightly from this and relax. For asymmetrically distributed charges, the dissociation event is not nearly as well-defined because the more highly charged monomer unfolds before dissociation occurs. It is found in almost all cases, a charged N-terminus tethers this unfolding monomer to its dimer partner by binding in a nonspecific manner. This helps encourage monomer unfolding in the dissociation pathway. It is also shown that while the intermolecular Coulomb repulsion between the monomers is not the largest contribution to the overall potential energy, it dominates the potential energy difference between different charge states.