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]²⁺ , 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). Twenty Cr(III) complexes were used as potential enhanced protonation reagents, including 11 complexes with nonlabile ligands and nine with labile ligands. The complexes that provide the most abundant [M + 2H]²⁺ , the greatest [M + 2H]²⁺ to [M + H]⁺ ratio, and the cleanest mass spectra are [Cr(H₂ O)₆ ](NO₃ )₃ ·3H₂ O and [Cr(THF)₃ ]Cl₃ . Anions in Cr(III) reagents can also affect the intensity of [M + 2H]²⁺ and the [M + 2H]²⁺ to [M + H]⁺ ratio through cation-anion interactions. The influence of anions on the extent of peptide protonation follows the trend ClO₄ ^-  ˃ SO₄ ^2-  ˃ Br^-  ˃ Cl^-  ˃ F^-  ≈ NO₃ ^- . Solvent effects and complexes with varying number of water ligands were investigated to study the importance of water in enhanced protonation. Aqueous solvent systems and Cr(III) complexes that have at least one bound water ligand in solution must be used for successful increase in the intensity of [M + 2H]²⁺ , which indicates that water is involved in the mechanism of Cr(III)-induced enhanced protonation. The ESI source design is also important because no enhanced protonation was observed using a Z-spray source. The current results suggest that this Cr(III)-induced effect occurs during the ESI desolvation process.