Department of Chemistry, Seoul National University, Seoul 151-742, Korea.
J Am Soc Mass Spectrom. 2012 Aug;23(8):1326-35. doi: 10.1007/s13361-012-0406-y. Epub 2012 Jun 1.
Degree of ionization (DI) in matrix-assisted laser desorption ionization (MALDI) was measured for five peptides using α-cyano-4-hydroxycinnanmic acid (CHCA) as the matrix. DIs were low 10(-4) for peptides and 10(-7) for CHCA. Total number of ions (i.e., peptide plus matrix) was the same regardless of peptides and their concentration, setting the number of gas-phase ions generated from a pure matrix as the upper limit to that of peptide ions. Positively charged cluster ions were too weak to support the ion formation via such ions. The total number of gas-phase ions generated by MALDI, and that from pure CHCA, was unaffected by the laser pulse energy, invalidating laser-induced ionization of matrix molecules as the mechanism for the primary ion formation. Instead, the excitation of matrix by laser is simply a way of supplying thermal energy to the sample. Accepting strong Coulomb attraction felt by cations in a solid sample, we propose three hypotheses for gas-phase peptide ion formation. In Hypothesis 1, they originate from the dielectrically screened peptide ions in the sample. In Hypothesis 2, the preformed peptide ions are released as part of neutral ion pairs, which generate gas-phase peptide ions via reaction with matrix-derived cations. In Hypothesis 3, neutral peptides released by ablation get protonated via reaction with matrix-derived cations.
基质辅助激光解吸电离（MALDI）中的离解度（DI）使用α-氰基-4-羟基肉桂酸（CHCA）作为基质对五种肽进行了测量。DI 对肽为 10(-4)，对 CHCA 为 10(-7)。无论肽及其浓度如何，离子总数（即肽加基质）都是相同的，从而使纯基质产生的气相离子数量达到肽离子的上限。带正电荷的簇离子太弱，无法通过这些离子支持离子形成。MALDI 产生的总气相离子数和纯 CHCA 的总气相离子数不受激光脉冲能量的影响，从而否定了基质分子的激光诱导电离是主要离子形成的机制。相反，激光对基质的激发只是向样品提供热能的一种方式。我们接受阳离子在固体样品中感受到的强库仑吸引力，提出了三种关于气相肽离子形成的假设。在假设 1 中，它们源自样品中经介电屏蔽的肽离子。在假设 2 中，预形成的肽离子作为中性离子对的一部分释放出来，通过与基质衍生的阳离子反应生成气相肽离子。在假设 3 中，通过烧蚀释放的中性肽通过与基质衍生的阳离子反应而质子化。
