Department of Chemistry, University of Toronto, Toronto, Canada.
J Mass Spectrom. 2014 Feb;49(2):161-7. doi: 10.1002/jms.3323.
The effect of N-methylation on sequence scrambling in the fragmentation of b5 ions has been investigated by studying a variety of peptides containing sarcosine (N-methylglycine). The product ion mass spectra for the b5 ions derived from Sar-A-A-A-Y-A and Sar-A-A-Y-A-A show only minor signals for non-direct sequence ions the major fragmentation reactions occurring from the unrearranged structures. This is in contrast to the b5 ions where the Sar residue is replaced by Ala and sequence scrambling occurs. The b5 ion derived from Y-Sar-A-A-A-A shows a product ion mass spectrum essentially identical to the spectrum of the b5 ion derived from Sar-A-A-A-Y-A, indicating that in the former case macrocyclization has occurred but the macrocyclic form shows a strong preference to reopen to put the Sar residue in the N-terminal position. Similar results were obtained in the comparison of b5 ions derived from A-Sar-A-A-Y-A and Sar-A-A-Y-A-A. The product ion mass spectra of the MH(+) ions of Y-Sar-A-A-A-A and A-Sar-A-A-Y-A show substantial signals for non-direct sequence ions indicating that fragmentation of the MH(+) ions channels extensively through the respective b5 ions and further fragmentation of these species.
已通过研究含有肌氨酸（N-甲基甘氨酸）的各种肽来研究 N-甲基化对 b5 离子碎裂中序列重排的影响。Sar-A-A-A-Y-A 和 Sar-A-A-Y-A-A 的 b5 离子衍生的产物离子质谱仅显示非直接序列离子的少量信号，主要的碎裂反应来自未重排的结构。这与 b5 离子形成鲜明对比，其中 Sar 残基被 Ala 取代并且发生序列重排。Y-Sar-A-A-A-A 衍生的 b5 离子显示出与 Sar-A-A-A-Y-A 衍生的 b5 离子的产物离子质谱基本相同，表明在前一种情况下发生了大环化，但大环形式强烈倾向于重新打开，将 Sar 残基置于 N-末端位置。在比较 A-Sar-A-A-Y-A 和 Sar-A-A-Y-A-A 衍生的 b5 离子时，得到了类似的结果。Y-Sar-A-A-A-A 和 A-Sar-A-A-Y-A 的 MH(+)离子的产物离子质谱显示出大量非直接序列离子的信号，表明 MH(+)离子的碎裂通道广泛地通过各自的 b5 离子，并进一步碎裂这些物质。
