The gas phase structures of several proteins have been studied by electron transfer dissociation (ETD) with and without prior collisional heating after electrospraying these proteins from native-like solutions into a quadrupole ion trap mass spectrometer. Without prior collisional heating, we find that ETD fragmentation is mostly limited to regions of the protein that are not spanned by the salt bridges known to form in solution. When protein ions are collisionally heated before ETD, new product ions are observed, and in almost all cases, these new ions arise from protein regions that are spanned by the salt bridges. Together these results confirm the existence of salt bridges in protein ions and demonstrate that a sufficient amount energy is required to disrupt these salt bridges in the gas phase. More interestingly, we also show that different salt bridges require different collisional activation voltages to be disrupted, suggesting that they have variable stabilities in the gas phase. These stabilities appear to be influenced by the gas-phase basicities of the involved residues and the presence of nearby charged residues. We also find that higher collisional activation voltages are needed to enable the formation of new product from sites spanned by multiple salt bridges.

中文翻译：

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来自碰撞活化和电子转移解离的气相蛋白质盐桥稳定性

在将这些蛋白质从类天然溶液中电喷雾到四极杆离子阱质谱仪中后，通过电子转移解离 (ETD) 研究了几种蛋白质的气相结构，在有和没有预先碰撞加热的情况下。在没有预先碰撞加热的情况下，我们发现 ETD 断裂主要限于蛋白质的区域，这些区域未被已知在溶液中形成的盐桥跨越。当蛋白质离子在 ETD 之前受到碰撞加热时，会观察到新的产物离子，并且在几乎所有情况下，这些新离子都来自被盐桥跨越的蛋白质区域。这些结果共同证实了蛋白质离子中盐桥的存在，并证明需要足够的能量来破坏气相中的这些盐桥。更有趣的是，我们还表明，不同的盐桥需要不同的碰撞激活电压才能被破坏，这表明它们在气相中具有不同的稳定性。这些稳定性似乎受到相关残留物的气相碱性和附近带电残留物的影响。我们还发现需要更高的碰撞激活电压才能从多个盐桥跨越的位点形成新产品。