Understanding Native Electrospray of Artificial and Natural Polymers

Although electrospray was introduced more than thirty years ago, and is currently the most widely used ionization method for mass spectrometry, the underlying mechanisms are still not fully elucidated. Our objective is to better understand how (bio)polymers get charged and desolvated during electrospray. To conceive this project, we focused on several physical-chemical parameters underlying different stages of the electrospray process. In contrast to past studies, we will not limit ourselves to proteins: our model systems will include nucleic acids, and we will design new synthetic foldamers to test specific hypotheses. First, we will study how analytes and other solutes (electrolytes such as ammonium acetate, or neutral co-solutes such as supercharging agents) partition between the surface and the center of the droplets, and whether this partitioning influences the outcome of electrospray. To modulate the analyte partitioning, we will conceive foldamers decorated with various side chains on their surface (acidic or basic groups, neutral polar or neutral apolar groups). Surface tension measurements (also useful to estimate the Rayleigh limit charge) will give us access to the partition coefficients between bulk and surface. Second, we will investigate whether analyte properties such as its flexibility or the positioning of charge carriers do influence its preference for the charged residue (by staying at the droplet center) or for a chain extrusion process (which starts from the droplet surface). This preference should translate into differences in the extent of charging and differences in compactness, revealed by ion mobility spectrometry. Third, for analytes supposedly following the charged residue pathway and thus supposedly keeping their native conformation until the very late stages of desolvation and declustering, we will study the effect of charge density on the detected conformations. An expansion of the highest charges is indeed frequently observed, but in addition, a compaction of lowest charge states has been reported in some cases as well. We want to rationalize which charge density is optimal to preserve the native structures, and in light of the above-described studies, find out how to tune the experimental conditions to obtain this ideal charge density. The impact of the proposed work is both fundamental and applied, because it will allow to exploit, widely and wisely, the full potential of native mass spectrometry to infer structural information on structures originally present in solution, whether on proteins, nucleic acids, or synthetic macromolecules.