Department of Chemistry, Western University, London, Ontario, N6A 5B7 Canada.
Anal Chem. 2012 Aug 7;84(15):6798-804. doi: 10.1021/ac301298g. Epub 2012 Jul 18.
Electrospray ionization (ESI) of native proteins results in a narrow distribution of low protonation states. ESI for these folded species proceeds via the charged residue mechanism. In contrast, ESI of unfolded proteins yields a wide distribution of much higher charge states. The current work develops a model that can account for this effect. Recent molecular dynamics simulations revealed that ESI for unfolded polypeptide chains involves protein ejection from nanodroplets, representing a type of ion evaporation mechanism (IEM). We point out the analogies between this IEM, and the dissociation of gaseous protein complexes after collisional activation. The latter process commences with unraveling of a single subunit, in concert with Coulombically driven proton transfer. The subunit then separates from the residual complex as a highly charged ion. We propose that similar charge equilibration events accompany the IEM of unfolded proteins, thereby causing the formation of high ESI charge states. A bead chain model is used for examining how charge is partitioned as protein and droplet separate. It is shown that protein ejection from differently sized ESI droplets generates a range of protonation states. The predicted behavior agrees well with experimental data.
电喷雾电离（ESI）的天然蛋白质导致质子化状态的窄分布。这些折叠态物种的 ESI 通过带电残基机制进行。相比之下，未折叠蛋白质的 ESI 产生广泛分布的更高电荷态。当前的工作开发了一个可以解释这种效应的模型。最近的分子动力学模拟表明，未折叠多肽链的 ESI 涉及从纳米液滴中喷射蛋白质，代表一种离子蒸发机制（IEM）。我们指出了这种 IEM 与碰撞激活后气态蛋白质复合物解离之间的相似性。后一过程始于单个亚基的解开，与库仑驱动的质子转移协调进行。然后，亚基作为带高电荷的离子从剩余复合物中分离出来。我们提出，类似的电荷平衡事件伴随着未折叠蛋白质的 IEM，从而导致高 ESI 电荷态的形成。使用珠链模型来检查蛋白质和液滴分离时如何分配电荷。结果表明，从不同大小的 ESI 液滴中喷射蛋白质会产生一系列质子化状态。预测的行为与实验数据吻合良好。
