Uncovering the Pathway of Serine Octamer Magic Number Cluster Formation during Electrospray Ionization: Experiments and Simulations.

Electrospray ionization (ESI) of serine (Ser) solution generates SerH as an abundant magic number cluster. ESI clustering of most other solutes yields nonspecific stoichiometries. It is unclear why SerH dominates in the case of Ser, and how SerH forms during ESI. Even the location of SerH formation is contentious (in solution, in ESI droplets, or elsewhere). Here we unravel key aspects of the l-SerH formation pathway. Harsh ion sampling conditions promote the collision-induced dissociation (CID) of regular ESI analytes. Unexpectedly, SerH was seemingly resistant against CID during ion sampling, despite its extremely low tandem mass spectrometry (MS/MS) stability. This unusual behavior reveals that SerH forms during ion sampling. We propose the following pathway: (1) Nonspecific Ser clusters are released when ESI droplets evaporate to dryness. These initial clusters cover a wide size range, from a few Ser to hundreds or thousands of monomers. (2) The clusters undergo dissociation during ion sampling, mostly via successive loss of neutral monomers. For any source activation voltage, there is a subpopulation of clusters for which this CID cascade tends to terminate at the octamer level, culminating in SerH-dominated product distributions. Mobile proton molecular dynamics simulations were used to model the entire pathway. SerH structures formed in these simulations were consistent with ion mobility experiments. The most compact structures resembled the model of [Scutelnic, V. 2018, 140, 7554-7560], with numerous intermolecular salt bridges and H-bonds. Our findings illustrate how the interplay of association and dissociation reactions across phase boundaries can culminate in magic number clusters.