ON THE MAXIMUM CHARGE-STATE AND PROTON-TRANSFER REACTIVITY OF PEPTIDEAND PROTEIN IONS FORMED BY ELECTROSPRAY-IONIZATION

A relatively simple model for calculation of the energetics of gas-pha se proton transfer reactions and the maximum charge state of multiply protonated ions formed by electrospray ionization is presented. This m odel is based on estimates of the intrinsic proton transfer reactivity of sites of protonation and point charge Coulomb interactions. From t his model, apparent gas-phase basicities (GB(app)) Of multiply protona ted ions are calculated. Comparison of this value to the gas-phase bas icity of the solvent from which an ion is formed enables a maximum cha rge state to be calculated. For 13 commonly electrosprayed proteins, o ur calculated maximum charge states are within an average of 6% of the experimental values reported in the literature. This indicates that t he maximum charge state for proteins is determined by their gas-phase reactivity. Similar results are observed for peptides with many basic residues. For peptides with few basic residues, we find that the maxim um charge state is better correlated to the charge state in solution. For low charge state ions, we find that the most basic sites Arg, Lys, and His are preferentially protonated. A significant fraction of the less basic residues Pro, Trp, and Gin are protonated in high charge st ate ions. The calculated GB(app) Of individual protonation sites varie s dramatically in the high charge state ions. From these values, we ca lculate a reduced cross section for proton transfer reactivity that is significantly lower than the Langevin collision frequency when the GB (app) of the ion is approximately equal to the GB of the neutral base.