Elucidating Potential Energy Surfaces for Singlet O2 Reactions with Protonated, Deprotonated, and Di-Deprotonated Cystine Using a Combination of Approximately Spin-Projected Density Functional Theory and Guided-Ion-Beam Mass Spectrometry,The Journal of Physical Chemistry B

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Elucidating Potential Energy Surfaces for Singlet O2 Reactions with Protonated, Deprotonated, and Di-Deprotonated Cystine Using a Combination of Approximately Spin-Projected Density Functional Theory and Guided-Ion-Beam Mass Spectrometry
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2017-08-11 00:00:00 , DOI: 10.1021/acs.jpcb.7b05674
Wenchao Lu _{1,

2} , I-Hsien “Midas” Tsai ₃ , Yan Sun _{1,

2} , Wenjing Zhou ₁ , Jianbo Liu _{1,

2}

Affiliation

The reactivity of cystine toward electronically excited singlet O₂ (a¹Δ_g) has been long debated, despite the fact that most organic disulfides are susceptible to oxidation by singlet O₂. We report a combined experimental and computational study on reactions of singlet O₂ with gas-phase cystine at different ionization and hydration states, aimed to determine reaction outcomes, mechanisms, and potential energy surfaces (PESs). Ion–molecule collisions of protonated and di-deprotonated cystine ions with singlet O₂, in both the absence and the presence of a water ligand, were measured over a center-of-mass collision energy (E_col) range from 0.1 to 1.0 eV, using a guided-ion-beam scattering tandem mass spectrometer. No oxidation was observed for these reactant ions except collision-induced dissociation at high energies. Guided by density functional theory (DFT)-calculated PESs, reaction coordinates were established to unravel the origin of the nonreactivity of cystine ions toward singlet O₂. To account for mixed open- and closed-shell characters, singlet O₂ and critical structures along reaction coordinates were evaluated using broken-symmetry, open-shell DFT with spin contamination errors removed by an approximate spin-projection method. It was found that collision of protonated cystine with singlet O₂ follows a repulsive potential surface and possesses no chemically significant interaction and that collision-induced dissociation of protonated cystine is dominated by loss of water and CO. Collision of di-deprotonated cystine with singlet O₂, on the other hand, forms a short-lived electrostatically bonded precursor complex at low E_col. The latter may evolve to a covalently bonded persulfoxide, but the conversion is blocked by an activation barrier lying 0.39 eV above reactants. At high E_col, C–S bond cleavage dominates the collision-induced dissociation of di-deprotonated cystine, leading to charge-separated fragmentation. Cross section for the ensuing fragment ion H₂NCH(CO₂^–)CH₂SS^• was measured as a function of E_col, and the mechanism of charge-separated fragmentation was discussed. It was also found that the reaction of deprotonated cystine with singlet O₂ follows a similar mechanism as that of di-deprotonated cystine, but with an even higher activation barrier (0.72 eV).

中文翻译：

结合近似自旋投影密度泛函理论和引导离子束质谱技术，阐明质子化，去质子化和二去质子化的胱氨酸的单线态O ₂反应的势能面

胱氨酸的反应性朝向电子激发单Ô ₂（A ¹ Δ_克）已被长讨论，尽管事实上，大多数有机二硫化物是由单线ø易被氧化₂。我们报告了结合的实验和计算研究单线态O ₂与气相胱氨酸在不同电离和水合作用状态下的反应，旨在确定反应结果，机理和势能面（PESs）。在质子碰撞能量（E _col）下测量质子化和二去质子化的胱氨酸离子与单线态O _2的离子-分子碰撞，在不存在和存在水配体的情况下）范围从0.1到1.0 eV（使用引导离子束散射串联质谱仪）。除了在高能下碰撞诱发的离解，这些反应物离子均未观察到氧化。在密度泛函理论（DFT）计算的PESs的指导下，建立了反应坐标以阐明胱氨酸离子对单线态O ₂的非反应性起源。为了说明混合的开壳和闭壳特征，使用破碎对称性，开壳DFT评估了单线态O ₂和沿反应坐标的关键结构，并通过近似自旋投影方法消除了自旋污染误差。发现质子化胱氨酸与单线态O ₂碰撞遵循排斥势表面，没有化学上的显着相互作用，碰撞诱导的质子化胱氨酸解离主要是水和CO的损失。另一方面，双去质子化胱氨酸与单线态O _2的碰撞形成了短命的低E _col下的静电结合前驱物。后者可能演变成共价键合的过亚砜，但转化被反应物上方0.39 eV的活化势垒所阻止。在高E _col时，_CS键断裂主导了碰撞引发的双去质子化胱氨酸的解离，从而导致电荷分离的碎片化。随后的碎片离子H ₂ NCH（CO ₂^–）CH ₂ SS ^•是E _col的函数，并讨论了电荷分离碎裂的机理。还发现去质子化的胱氨酸与单线态O ₂的反应遵循与二去质子化的胱氨酸相似的机理，但是具有更高的活化屏障（0.72eV）。

更新日期：2017-08-11

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