Department of Chemistry, University of Toronto, M5S 1A1, Toronto, Ontario, Canada.
J Am Soc Mass Spectrom. 1996 Mar;7(3):233-42. doi: 10.1016/1044-0305(95)00677-X.
The unimolecular and low energy collision-induced fragmentation reactions of the MH(+) ions of N-acetyl-tri-alanine, N-acetyl-tri-alanine methyl ester, N-acetyl-tetra-alanine, tetra-alanine, penta-alanine, hexa-glycine, and Leu-enkephalin have been studied with a particular emphasis on the formation and fragmentation of B n (n=3,4,5) ions. In addition, the metastable ion fragmentation reactions of protonated tetra-glycine, penta-glycine, and Leu-enkephalin amide have been studied. B n ions are prominent stable species in all spectra. The B n ions fragment, in part, by elimination of CO to form A n ions; this reaction occurs on the metastable ion time scale with a substantial release of kinetic energy (T 1/2=0. 3-0. 5 eV) that indicates that a stable configuration of the B n ion fragments by way of a reacting configuration that is higher in energy than the fragmentation products, A n + CO. Ab initio calculations strongly suggest that the stable configuration of the B3 and B4 ions is a protonated oxazolone formed by interaction of the developing charge with the next-nearest carbonyl group as HX is lost from the protonated species H-(Yyy) n -X · H(+). The higher B n ions also fragment, in part, to form the next-lower B ion, presumably in its stable protonated oxazolone form. This reaction is rationalized in terms of the three-dimensional structure of the B n ions and it is proposed that the neutral eliminated is an α-lactam.
已研究了 MH(+)离子的单分子和低能碰撞诱导碎裂反应，这些 MH(+)离子来自 N-乙酰-三丙氨酸、N-乙酰-三丙氨酸甲酯、N-乙酰-四丙氨酸、四丙氨酸、五丙氨酸、六甘氨酸和亮氨酸脑啡肽。特别强调了 B n (n=3,4,5)离子的形成和碎裂。此外，还研究了质子化四甘氨酸、五甘氨酸和亮氨酸脑啡肽酰胺的亚稳离子碎裂反应。在所有光谱中，B n 离子都是突出的稳定物种。B n 离子部分通过消除 CO 形成 A n 离子碎裂；该反应在亚稳离子时间尺度上发生，伴随着大量的动能释放（T 1/2=0. 3-0. 5 eV），这表明 B n 离子碎片通过反应性配置进行稳定配置，该配置的能量高于碎裂产物 A n + CO。从头算计算强烈表明，B3 和 B4 离子的稳定构型是通过发展电荷与下一个羰基基团相互作用形成的质子化恶唑酮，因为 HX 从质子化物种 H-(Yyy) n -X·H(+)中丢失。较高的 B n 离子也部分碎裂形成下一较低的 B 离子，可能以其稳定的质子化恶唑酮形式存在。该反应可以根据 B n 离子的三维结构来合理化，并提出被消除的中性物质是α-内酰胺。
