Computer Simulation of Gas-Phase Neutralization of Electrospray-Generated Protein Macroions

The process of neutralizing hydrated multicharged gas-phase protein ions with small counterions was simulated using a molecular dynamics (MD) technique. Hen egg white lysozyme (HEWL) molecules with different numbers of positive charges, both dry and solvated by up to 1500 water molecules, were first equilibrated. Simulations revealed that the hydration layer over a highly charged protein surface adapted a spiny structure with water protrusions composed of oriented water dipoles. MD simulations of the neutralization process showed that the impact of a small dehydrated single-charged counterion with a dehydrated HEWL ion bearing eight uncompensated charges resulted in a short local increase in temperature by 600-1000 K, which quickly (in 3-5 ps) dissipated over the whole protein molecule, increasing its average temperature by 20-25 K. When the protein ion was solvated, no drastic local increase in the temperature of the protein atoms was observed, because the impact energy was dissipated among the water molecules near the collision site.