Limitations of atomistic modeling to reveal ejection of proteins from charged nanodroplets

Molecular dynamics using atomistic modeling is frequently used to extract the mechanisms of macroion release from electrosprayed droplets. However, atomistic modeling is currently feasible for only the smallest window of droplet sizes that omits the droplet history. The relevance of the observations made in this narrow window to the actual droplet evolution has not been addressed. Here, we perform a systematic study of desolvation mechanisms of poly(ethylene glycol) (PEG), protonated peptides of different compositions and protonated proteins in order to examine whether atomistic modeling can establish the extrusion mechanism of proteins from droplets. Atomistic modeling of PEG charging shows that above a critical droplet size charging occurs transiently by transfer of ions from the solvent to the macroion, while below the critical size, the capture of the ion from PEG has a lifetime sufficient for extrusion of the charged PEG from an aqueous droplet. This is the first report of the role of the droplet curvature in the charging of macroions. Modeling of the process in droplets of various sizes allow us to extrapolate the charging mechanisms in systems that cannot be modeled atomistically yet. Simulations even with highly hydrophobic peptides show that partial extrusion of a peptide from the droplet surface is rare relative to desolvation by drying-out of the protonated peptide. Differently from what has been presented in the literature we argue that atomistic simulations have not sufficiently established extrusion mechanism of proteins from droplets and their charging mechanism. Moreover, we argue that release of highly charged proteins can occur in earlier stage of a droplet's lifetime than that that is atomistically modeled. In this earlier stage, we emphasize the key role of jets emanating from a droplet at the point of charge-induced instability in the release of proteins.