Backbone z-type fragment ions formed by electron-transfer dissociation (ETD) of doubly protonated peptides AAHAL, AHDAL, and AHADL were subjected to collisional activation and their dissociation products were studied by ETD-CID-MS³ and MS⁴. Electron structure theory calculations were performed to elucidate ion structures and reaction mechanisms. All z ions showed competitive eliminations of C₃H₇ and C₄H₈ from the C-terminal Leu side chain. The energetics and kinetics of these dissociations were studied computationally for the z₄ ion from AAHAL, and optimized structures are reported for several intermediates and transition states. RRKM calculations on the combined B3LYP and PMP2/6-311++G(2d,p) potential energy surface provided unimolecular rate constants that closely reproduced the experimental branching ratios for C₃H₇ and C₄H₈ eliminations. Mechanisms were also studied for the loss of CO₂ from z ions generated by ETD of AHDAL and AHADL and for a specific radical-induced Asp-C_α-CO backbone cleavage. CID of the z ions under study did not produce any fragment ions that would indicate cascade backbone dissociations triggered by the radical sites. In contrast, the majority of backbone dissociations occurred at bonds that were remote from the radical sites (spin-remote dissociations) and were triggered by proton migrations that were analogous to those considered for standard peptide ion fragmentations.

骨干ž双质子化肽的AAHAL，AHDAL和AHADL进行碰撞活化和它们的离解产物通过电子转移解离（ETD）形成型碎片离子通过ETD-CID-MS研究³和MS ⁴。进行电子结构理论计算以阐明离子结构和反应机理。所有z离子均显示出从C末端Leu侧链竞争性消除C ₃ H ₇和C ₄ H ₈。对于z ₄，通过计算研究了这些解离的能量学和动力学报道了来自AAHAL的离子，并针对几种中间体和过渡态报告了优化的结构。在结合的B3LYP和PMP2 / 6-311 ++ G（2d，p）势能面上的RRKM计算提供了单分子速率常数，该常数紧密地再现了C ₃ H ₇和C ₄ H ₈消除的实验支化比。机制也进行了研究对CO的损失₂从Ž由AHDAL和AHADL的ETD产生的离子以及用于特定的自由基诱导的Asp-C _α -CO骨架裂解。z的CID研究中的离子没有产生任何碎片离子，这些碎片离子表明由自由基位点触发的级联主链解离。相反，大多数主链解离发生在远离自由基位点的键处（自旋-远程解离），并且是由质子迁移触发的，质子迁移与标准肽离子碎片化所考虑的类似。