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Water nanodroplets charged with excess protons play a central role during electrospray ionization (ESI). In the current study molecular dynamics (MD) simulations were used for gaining insights into the nanodroplet behavior based on classical mechanics. The SPC/E water model was modified to permit the inclusion of protons as highly mobile point charges at minimum computational cost. A spherical trapping potential was assigned to every SPC/E oxygen, thereby allowing the formation of protonated water molecules. Within a tightly packed nanodroplet the individual potential wells merge to form a three-dimensional energy landscape that facilitates rapid proton hopping between water molecules. This approach requires short-range modifications to the standard Coulomb potential for modeling electrostatic proton−water interactions. Simulations on nanodroplets consisting of 1248 water molecules and 10 protons (radius, ca. 21 Å) result in a proton diffusion coefficient that is in agreement with the value measured in bulk solution. Radial proton distributions extracted from 1 ns MD runs exhibit a large peak around 14 Å, in addition to substantial population density closer to the droplet center. Similar radial distributions were found for nanodroplets charged with Na+ions. This behavior is dramatically different from that expected on the basis of continuum electrostatic theory, which predicts that excess charge should be confined to a thin layer on the droplet surface. One important contributor to this effect seems to be the ordering of water molecules at the liquid/vacuum interface. This ordering results in an electrical double layer, generating a potential gradient that tends to pull positive charge carriers (such as protons, but also others such as Na+ions) toward the droplet interior. This deviation from the widely assumed surface charge paradigm could have implications for the mechanism by which protonated analyte ions are formed during ESI. [ABSTRACT FROM AUTHOR]

The electrospray ionization (ESI) charge state distribution of proteins is highly sensitive to the protein structure in solution. Unfolded conformations generally form higher charge states than tightly folded structures. The current study employs a minimalist molecular dynamics model for simulating the final stages of the ESI process in order to gain insights into the physical reasons underlying this empirical relationship. The protein is described as a string of 27 beads (“residues”), 9 of which are negatively charged and represent possible protonation sites. The unfolded state of this bead string is a random coil, whereas the native conformation adopts a compact fold. The ESI process is simulated by placing the protein inside a solvent droplet with a 2.5 nm radius consisting of 1600 Lennard-Jones particles. In addition, the droplet contains 14 protons which are modeled as highly mobile point charges. Disintegration of the droplet rapidly releases the protein into the gas phase, resulting in average charge states of 4.8 and 7.4 for the folded and unfolded conformation, respectively. The protonation probabilities of individual residues in the folded state reveal a characteristic pattern, with values ranging from 0.2 to 0.8. In contrast, the protonation probabilities of the unfolded protein are more uniform and cover the range from 0.8 to 1.0. The origin of these differences can be traced back to a combination of steric and electrostatic effects. Residues exhibiting a small accessible surface area are less likely to capture a proton, an effect that is exacerbated by partial electrostatic shielding from nearby positive residues. Conversely, sites that are sterically exposed are associated with electrostatic funnels that greatly increase the likelihood of protonation. Unfolding enhances the steric and electrostatic exposure of protonation sites, thereby causing the protein to capture a greater number of protons during the droplet disintegration process. [ABSTRACT FROM AUTHOR]