Department of Chemistry, University of Guelph, Guelph, Ontario, N1G 2W1, Canada.
J Am Soc Mass Spectrom. 2012 May;23(5):858-68. doi: 10.1007/s13361-012-0343-9. Epub 2012 Feb 14.
In our recent work towards the nontarget identification of products of nucleic acid (NA) damage in urine, we have found previous work describing the dissociation of NA bases not adequate to fully explain their observed reactivity. Here we revisit the gas-phase chemistry of protonated uracil (U) during collision induced dissociation (CID) using two modern tandem mass spectrometry techniques; quadrupole ion trap (QIT) and quadrupole time of flight (Q-TOF). We present detailed mechanistic proposals that account for all observed products of our experiments and from previous isotope labeling data, and that are supported by previous ion spectroscopy results and theoretical work. The diverse product-ions of U cannot be explained adequately by only considering the lowest energy form of protonated U as a precursor. The tautomers adopted by U during collisional excitation make it possible to relate the complex reactivity observed to reasonable mechanistic proposals and feasible product-ion structures for this small highly conjugated heterocycle. These reactions proceed from four different stable tautomers, which are excited to a specific activated precursor from which dissociation can occur via a charge-directed process through a favorable transition state to give a stabilized product. Understanding the chemistry of uracil at this level will facilitate the identification of new modified uracil derivatives in biological samples based solely on their reactivity during CID. Our integrated approach to describing ion dissociation is widely applicable to other NA bases and similar classes of biomolecules.
在我们最近针对非靶向识别尿液中核酸 (NA) 损伤产物的工作中，我们发现之前描述 NA 碱基离解的工作不足以完全解释它们观察到的反应性。在这里，我们使用两种现代串联质谱技术——四极离子阱 (QIT) 和四极杆飞行时间 (Q-TOF)——重新研究了质子化尿嘧啶 (U) 在碰撞诱导解离 (CID) 过程中的气相化学。我们提出了详细的机理提案，这些提案解释了我们实验和以前同位素标记数据中观察到的所有产物，并且得到了以前离子光谱结果和理论工作的支持。U 的各种产物离子不能仅通过考虑作为前体的质子化 U 的最低能量形式来充分解释。在碰撞激发过程中 U 采用的互变异构体使得有可能将观察到的复杂反应性与合理的机理建议和这种小的高共轭杂环的可行产物离子结构联系起来。这些反应从四个不同的稳定互变异构体开始，这些互变异构体被激发到特定的活化前体，通过有利的过渡态通过电荷导向过程发生解离，从而给出稳定的产物。在这种水平上理解尿嘧啶的化学将有助于仅基于它们在 CID 过程中的反应性，在生物样品中鉴定新的修饰尿嘧啶衍生物。我们用于描述离子解离的综合方法广泛适用于其他 NA 碱基和类似的生物分子类别。
