Cheng C, Giblin D, Gross M L
Department of Chemistry, Washington University, St. Louis, Missouri, USA.
J Am Soc Mass Spectrom. 1998 Mar;9(3):216-24. doi: 10.1016/S1044-0305(97)00283-3.
A strategy is described to locate the carbonyl position in oxofatty acids by utilizing charge-remote fragmentations of various molecular ions that are desorbed by fast atom bombardment (FAB). Oxofatty acids were cationized with alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) to form [M + 2Met-H]+ or alkaline earth metal ions (Mg2+, Ca2+, Sr2+ or Ba2+) to from [M + Met-H]+ in the gas phase. The cationized acids undergo charge-remote fragmentations upon high-energy activation, giving a product-ion pattern that has a gap corresponding to the oxo position and bordered by two high-intensity peaks. One of the peaks corresponds to an ion that is formed by the cleavage of the C-C bond beta to the oxo position and proximal to the charge (beta ion), whereas the other is formed from the cleavage of the C-C bond gamma to the oxo position and distal to the charge (gamma' ion). The oxo position is easily determined by identifying the gap and the beta and gamma' ions. Furthermore, there are two competing patterns of fragments in a CAD spectrum of an oxofatty acid or ester [M + Li]+ ion. These arise because Li+ attaches to either the oxo or the carboxylic end, as was confirmed by ab initio molecular orbital calculations. The results demonstrate that control of the fragmentation can be guided by an understanding of metal-ion affinities. Collisional activation of the anionic carboxylates gives results that are similar to those for positive ions, showing that the process is not related to the charge status. Collisional activation of [M + H]+ ions does not give structural information because the charge migrates, leading to charge-mediated fragmentations.
本文描述了一种通过利用快速原子轰击（FAB）解吸的各种分子离子的电荷远程裂解来确定氧代脂肪酸中羰基位置的策略。氧代脂肪酸用碱金属离子（Li⁺、Na⁺、K⁺、Rb⁺和Cs⁺）进行阳离子化，在气相中形成[M + 2Met - H]⁺，或用碱土金属离子（Mg²⁺、Ca²⁺、Sr²⁺或Ba²⁺）形成[M + Met - H]⁺。阳离子化的酸在高能活化时会发生电荷远程裂解，产生一种产物离子模式，该模式有一个与氧代位置相对应的间隙，并由两个高强度峰界定。其中一个峰对应于通过氧代位置β位且靠近电荷的C - C键裂解形成的离子（β离子），而另一个峰是由氧代位置γ位且远离电荷的C - C键裂解形成的（γ'离子）。通过识别间隙以及β和γ'离子，可以很容易地确定氧代位置。此外，氧代脂肪酸或酯[M + Li]⁺离子的碰撞诱导解离（CAD）光谱中有两种相互竞争的碎片模式。这是因为Li⁺附着在氧代端或羧基端，这一点通过从头算分子轨道计算得到了证实。结果表明，对裂解的控制可以通过对金属离子亲和力的理解来指导。阴离子羧酸盐的碰撞活化产生的结果与正离子相似，表明该过程与电荷状态无关。[M + H]⁺离子的碰撞活化不会给出结构信息，因为电荷会迁移，导致电荷介导的裂解。
