Chemistry Department and Henry Eyring Center for Theoretical Chemistry University of Utah, Salt Lake City, Utah 84112, USA.
J Phys Chem A. 2010 Jan 28;114(3):1309-23. doi: 10.1021/jp9057059.
Earlier work from this group has suggested that, in electron capture and electron-transfer mass spectrometry experiments on positively charged gas-phase samples of polypeptides, the initial electron attachment event most likely occurs at one of the peptide's positively charged sites (e.g., protonated side chains), although electron attachment can occur at a disulfide or amide site ca. 1-10% of the time. Focusing on the 90-99% dominant channel in which initial electron attachment occurs at a positive site, this paper addresses to what extent and over what distances electron transfer can take place from a positively charged site to a disulfide sigma* or amide pi* orbital, because it is thought that it is through such orbitals that disulfide or N-C(alpha) backbone bond cleavage occurs. Ab initio electronic structure calculations show that, as long as an SS sigma* (or OCN pi*) orbital experiences sufficient Coulomb stabilization from proximal positively charged groups, there are a myriad of excited Rydberg states located on positive sites that are able to induce such intrapeptide electron transfer. Computational data show that the transfer rates decay exponentially with distance for a given Rydberg orbital. An analytical model is developed that allows us to estimate the rates of Rydberg-to-valence and Rydberg-to-Rydberg electron transfers as functions of the Rydberg orbitals' n quantum numbers. This model suggests that transfer can occur over very long distances at rates that are more than competitive with the rates of radiationless relaxation within the manifold of Rydberg states (the latter processes eventually terminate the electron-transfer process an thus the disulfide or N-C(alpha) bond cleavages), and it gives formulas for how these rates depend on n (and thus the radial span of the Rydberg orbitals).
早期的工作表明，在对带正电荷的多肽气相样品进行电子俘获和电子转移质谱实验时，最初的电子附加事件很可能发生在多肽的一个正电荷部位（如质子化的侧链），尽管电子附加也可能在约 1-10%的时间发生在二硫键或酰胺部位。本文聚焦于初始电子附加发生在正电荷部位的占主导地位的 90-99%通道，研究电子从正电荷部位转移到二硫键 sigma或酰胺 pi轨道的程度和距离，因为人们认为正是通过这些轨道发生二硫键或 N-C(alpha)骨架键的断裂。从头算电子结构计算表明，只要 SS sigma*（或 OCN pi*）轨道从邻近的正电荷基团获得足够的库仑稳定化，就有无数位于正电荷部位的激发型 Rydberg 态能够诱导这种多肽内的电子转移。计算数据表明，对于给定的 Rydberg 轨道，转移速率随距离呈指数衰减。我们开发了一个分析模型，允许我们根据 Rydberg 轨道的 n 量子数来估计 Rydberg 到价电子和 Rydberg 到 Rydberg 电子转移的速率。该模型表明，转移可以在非常长的距离上以超过无辐射弛豫速率的速率发生（后者过程最终终止电子转移过程，从而终止二硫键或 N-C(alpha)键的断裂），并给出了这些速率如何依赖于 n（从而依赖于 Rydberg 轨道的径向范围）的公式。
