Jing Xinyao, Edwards Kyle C, Vincent John B, Cassady Carolyn J
Department of Chemistry and Biochemistry, The University of Alabama, Tuscaloosa, Alabama.
J Mass Spectrom. 2018 Dec;53(12):1198-1206. doi: 10.1002/jms.4297.
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). 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] , the greatest [M + 2H] to [M + H] ratio, and the cleanest mass spectra are Cr(H O) ·3H O and [Cr(THF) ]Cl . Anions in Cr(III) reagents can also affect the intensity of [M + 2H] and the [M + 2H] to [M + H] ratio through cation-anion interactions. The influence of anions on the extent of peptide protonation follows the trend ClO ˃ SO ˃ Br ˃ Cl ˃ F ≈ NO . 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] , 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.
向肽溶液中添加三价铬（Cr(III)）试剂，可以通过电喷雾电离（ESI）增加双质子化肽[M + 2H]⁺的强度。研究了三种模型七肽：中性（AAAAAAA）、酸性（AAEEEAA）和碱性（AAAKAAA）。在没有Cr(III)的情况下，中性和酸性肽几乎不形成2⁺离子。使用了20种Cr(III)配合物作为潜在的增强质子化试剂，包括11种具有非活性配体的配合物和9种具有活性配体的配合物。能提供最丰富的[M + 2H]⁺、最大的[M + 2H]⁺与[M + H]⁺比值以及最纯净质谱图的配合物是Cr(H₂O)₆₃·3H₂O和[Cr(THF)₆]Cl₂。Cr(III)试剂中的阴离子也可以通过阳离子 - 阴离子相互作用影响[M + 2H]⁺的强度以及[M + 2H]⁺与[M + H]⁺的比值。阴离子对肽质子化程度的影响遵循ClO₄⁻ ˃ SO₄²⁻ ˃ Br⁻ ˃ Cl⁻ ˃ F⁻ ≈ NO₃⁻的趋势。研究了溶剂效应以及具有不同数量水配体的配合物，以探究水在增强质子化中的重要性。必须使用水性溶剂体系和溶液中至少有一个结合水配体的Cr(III)配合物，才能成功提高[M + 2H]⁺的强度，这表明水参与了Cr(III)诱导的增强质子化机制。ESI源设计也很重要，因为使用Z - spray源未观察到增强质子化现象。目前的结果表明，这种Cr(III)诱导效应发生在ESI去溶剂化过程中。
