Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, P.O. Box 999, Mail Stop P8-19, 99352, Richland, WA, USA.
J Am Soc Mass Spectrom. 1993 Jul;4(7):536-45. doi: 10.1016/1044-0305(93)85015-P.
High mass-to-charge ratio ions (> 4000) from electrospray ionization (ESI) have been observed for several proteins, including bovine cytochrome c (M r 12,231) and porcine pepsin (M r 34,584), by using a quadrupole mass spectrometer with an m/z 45,000 range. The ESI mass spectrum for cytochrome c in an aqueous solution gives a charge state distribution that ranges from 12 + to 2 +, with a broad, low-intensity peak in the mass-to-charge ratio region corresponding to the M + H ion. the negative ion ESI mass spectrum for pepsin in 1% acetic acid solution shows a charge state distribution ranging from 7- to 2-. To observe the M - H ion, harsher desolvation and interface conditions were required. Also observed was the abundant aggregation of the protens with average charge states substantially lower than observed for their monomeric counterparts. The negative ion ESI mass spectrum for cytochrome c in 1-100 mM NH4OAc solutions showed greater relative abundances for the higher mass-to-charge ratio ions than in acuidic solutions, with an M - H ion relative abundance approximately 50% that of the most abundant charge state peak. The observation that protein aggregates are formed with charge states comparable to monomeric species (at fower mass-to-charge ratios) suggests that the high mass-to-charge ratio monomers may be formed by the dissociation of aggregate species. The observation of low charge state and aggregate molecular ions concurrently with highly charged species may serve to support a variation of the charged residue model, originally described by Dole and co-workers (Dole, M., et al. J. Chem. Phys. 1968, 49, 2240; Mack, L. L., et al. J. Chem. Phys. 1970, 52, 4977) which involves the Coulombically driven formation of either very highly solvated molecular ions or lower ananometer-diameter droplets.
采用质量范围为 45000 的四极杆质谱仪，已经观察到电喷雾电离（ESI）产生的高质荷比离子（>4000），其中包括牛细胞色素 c（Mr 12231）和猪胃蛋白酶（Mr 34584）。在水溶液中，细胞色素 c 的 ESI 质谱给出的电荷状态分布范围从 12+到 2+，在质荷比区域有一个宽的、低强度的峰，对应于[M+H]+离子。在 1%乙酸溶液中的胃蛋白酶的负离子 ESI 质谱显示电荷状态分布范围从 7-到 2-。为了观察[M-H]-离子，需要更苛刻的去溶剂化和界面条件。还观察到蛋白质的大量聚集，其平均电荷状态明显低于其单体对应物。在 1-100 mM NH4OAc 溶液中，细胞色素 c 的负离子 ESI 质谱显示，与在酸性溶液中相比，较高质荷比离子的相对丰度更大，[M-H]-离子的相对丰度约为最丰富电荷状态峰的 50%。观察到蛋白质聚集体的形成具有与单体物种相当的电荷状态（在较低的质荷比）表明，高质荷比的单体可能是通过聚集体物种的解离形成的。低电荷状态和聚集体分子离子与高电荷状态物种同时存在的观察结果可能支持最初由 Dole 和同事描述的带电残基模型的变化（Dole，M.，等人。J. Chem. Phys. 1968, 49, 2240; Mack, L. L.，等人。J. Chem. Phys. 1970, 52, 4977），该模型涉及库仑驱动形成非常高溶剂化的分子离子或更小的纳米级直径液滴。
