The dynamics of water evaporation from partially solvated cytochrome c in the gas phase

The study of evaporation of water from biological macromolecules is important for the understanding of electrospray mass spectrometry experiments. In electrospray ionization (ESI), electrically charged nanoscale droplets are formed from solutions of, for example, proteins. Then evaporation of the solvent leads to dry protein ions that can be analyzed in the mass spectrometer. In this work the dynamics of water evaporation from native cytochrome c covered by a monolayer of water is studied by molecular dynamics (MD) simulations at constant energy. A model of the initial conditions of the process is introduced. The temperature of the protein drops by about 100 K during the 400 picoseconds of the simulations. This sharp drop in temperature causes the water evaporation rate to decrease by about an order of magnitude, leaving the protein with 50% to 90% of the original water molecules, depending on the initial temperature of the simulation. The structural changes of the protein upon desolvation were considered through calculations of the radius of gyration and the root mean square (RMS) of the protein. A variation of 0.4 Å in the radius of gyration, together with an RMS value of less than 3 Å, indicates only minor changes in the overall shape of the protein structure. The water coordination number of the solvation shell is much smaller than that for bulk water. The mobility of water is high at the beginning of the simulations and drops as the simulation progresses and the temperature decreases. Incomplete desolvation of protein ions was also observed in recent experiments.