School of Chemistry, University of Melbourne, Victoria 3010, Australia.
Rapid Commun Mass Spectrom. 2010 Jun 30;24(12):1779-90. doi: 10.1002/rcm.4576.
A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments is used to examine the gas-phase fragmentation reactions of the three isomeric phenylalanine derivatives, alpha-phenylalanine, beta(2)-phenylalanine and beta(3)-phenylalanine. Under collision-induced dissociation (CID) conditions, each of the protonated phenylalanine isomers fragmented differently, allowing for differentiation. For example, protonated beta(3)-phenylalanine fragments almost exclusively via the loss of NH(3), only beta(2)-phenylalanine via the loss of H(2)O, while alpha- and beta(2)-phenylalanine fragment mainly via the combined losses of H(2)O + CO. Density functional theory (DFT) calculations were performed to examine the competition between NH(3) loss and the combined losses of H(2)O and CO for each of the protonated phenylalanine isomers. Three potential NH(3) loss pathways were studied: (i) an aryl-assisted neighbouring group; (ii) 1,2 hydride migration; and (iii) neighbouring group participation by the carboxyl group. Finally, we have shown that isomer differentiation is also possible when CID is performed on the protonated methyl ester and methyl amide derivatives of alpha-, beta(2)- and beta(3)-phenylalanines.
采用电喷雾电离（ESI）、多级和高分辨质谱实验相结合的方法，研究了三种苯丙氨酸异构体（α-苯丙氨酸、β（2）-苯丙氨酸和β（3）-苯丙氨酸）在气相中的碎裂反应。在碰撞诱导解离（CID）条件下，每种质子化的苯丙氨酸异构体的碎裂方式不同，从而可以进行区分。例如，质子化的β（3）-苯丙氨酸几乎只通过失去 NH3 进行碎裂，只有β（2）-苯丙氨酸通过失去 H2O 进行碎裂，而α-和β（2）-苯丙氨酸主要通过 H2O+CO 的联合损失进行碎裂。进行了密度泛函理论（DFT）计算，以研究对于每种质子化的苯丙氨酸异构体，NH3 损失与 H2O 和 CO 的联合损失之间的竞争。研究了三种可能的 NH3 损失途径：（i）芳基辅助的邻基；（ii）1,2 氢迁移；和（iii）羧基的邻基参与。最后，我们表明，当对α-、β（2）-和β（3）-苯丙氨酸的质子化甲酯和甲基酰胺衍生物进行 CID 时，也可以进行异构体区分。
