Electrosprayed Serine Octamer Magic Number Cluster: Formation via Gas phase Assembly and Collision-Induced Dissociation

Vida Alinezhad, Western University

Abstract

Magic number clusters refer to ion species that exhibit unusually high intensities in electrospray ionization (ESI) mass spectrometry (MS) experiments. The serine octamer cluster [Ser₈+H]⁺ is of special interest since its formation mechanism and structure remain controversial, although a recently proposed model in the literature represents a possible candidate structure. Nonetheless, the formation mechanism of [Ser₈+H]⁺ remains unexplored. Some hypotheses suggest that it may pre-exist in bulk solution, or that [Ser₈+H]⁺ can be a collision-induced dissociation (CID) product of large clusters formed in evaporating ESI droplets. We used ESI-MS, mobile proton molecular dynamics (MPMD) simulations, and density functional theory (DFT) to probe the mechanism of ESI-induced [Ser₈H]⁺ formation. The results obtained rule out existence of [Ser₈+H]⁺ in bulk. Additionally, our data reveal that initial Ser clusters formed in shrinking ESI droplets are unstable and undergo CID. Ser monomers released during these dissociation events undergo low-temperature reclustering during free jet expansion. Subsequent CID events culminate in Ser₈H⁺-dominated mass spectra.