Mechanistic Study of Enhanced Protonation by Chromium(III) in Electrospray Ionization: A Superacid Bound to a Peptide.

Addition of trivalent chromium, Cr(III), to solutions undergoing electrospray ionization (ESI) enhances protonation and leads to formation of [M + 2H] for peptides that normally produce [M + H]. This effect is explored using electronic structure calculations at the density functional theory (DFT) level to predict the energetics of various species that are potentially important to the mechanism. Gas- and solution-phase reaction free energies for glycine and its anion reacting with [Cr(III)(HO)] and for dehydration of these species have been predicted, where glycine is used as a simple model for a peptide.

The use of chromium(III) complexes to enhance peptide protonation by electrospray ionization mass spectrometry.

The addition of trivalent chromium, Cr(III), reagents to peptide solutions can increase the intensity of doubly protonated peptides, [M + 2H] , through electrospray ionization (ESI). Three model heptapeptides were studied: neutral (AAAAAAA), acidic (AAEEEAA), and basic (AAAKAAA). The neutral and acidic peptides form almost no 2+ ions in the absence of Cr(III).

Optimization of electrospray ionization conditions to enhance formation of doubly protonated peptide ions with and without addition of chromium(III).

Rationale: Production of multiply protonated ions by electrospray ionization (ESI) is important to the analysis of peptides by mass spectrometry. For small neutral and acidic peptides, addition of chromium(III) greatly increases the intensity of doubly protonated ions. The current study examines instrumental and solution parameters that maximize peptide ion charge by ESI.