气相中碱金属离子与丝氨酸、 亮氨酸和赖氨酸五肽复合物的裂解反应

为了探索金属离子对含有不同侧链的多肽气相解离的影响, 采用质谱法研究了碱金属离子Li⁺, Na⁺, K⁺, Rb⁺和Cs⁺分别与丝氨酸、 亮氨酸和赖氨酸五肽(分别简写为S5, L5和K5)形成的复合物的裂解反应. 质谱定性结果表明, 5种碱金属离子均可以在气相中与丝氨酸、 亮氨酸和赖氨酸五肽形成配合比为1∶1 和2∶1的非共价复合物; 竞争反应结果表明, 随着碱金属离子半径的增加, 它们与3种五肽的结合能力逐渐减弱. 质谱定量结果表明, K⁺与丝氨酸、 亮氨酸和赖氨酸五肽复合物的结合常数分别为8.94×10⁴, 2.83×10⁴和2.50×10³ L/mol, 表明K⁺与五肽复合物的结合强度按照丝氨酸、 亮氨酸和赖氨酸的顺序依次减小. 含不同侧链碱金属离子-五肽复合物的碰撞诱导解离结果表明, 复合物的碎裂主要发生在骨架上, 丝氨酸五肽复合物最易碎裂, 亮氨酸五肽复合物其次, 赖氨酸五肽复合物则较难碎裂, 且3种复合物的侧链断裂情况也呈现明显差异. 此外, 研究了Na⁺与亮氨酸五肽复合物所产生的碎片离子, 分析了不同离子之间的来源关系, 并以Dunbar的复合物理论模型为依据, 推测在碎裂过程中, 碱金属离子可能向五肽的碳端或氮端偏移. 质谱碎片分析结果表明, 在2∶1的非共价复合物中, 第一个碱金属离子与五肽上4个酰胺键的羰基结合, 第二个碱金属离子与五肽的羧基氧原子结合.

Fragmentation Reactions of Complexes of Alkali Metal Ions with Pentaserine,Pentaleucine and Pentalysine in Gas Phase†

For exploring the effects of alkali metal ions on the dissociations of peptides with different side chains in the gas phase, the complexes of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ with pentapeptides, Ser-Ser-Ser-Ser-Ser(S5), Leu-Leu-Leu-Leu-Leu(L5) and Lys-Lys-Lys-Lys-Lys(K5), were chosen to investigate the fragmentation reaction pathways by mass spectrometry. The experimental results indicated that alkali metal ions and S5, L5, K5 can form 1∶1 and 2∶1 non-covalent complexes in the gas phase, and the binding strength between alkali metal ions and pentapeptides descend with the increasing of the radii of alkali metal ions. Quantitative analyses of mass spectra demonstrated that the binding constants for the complexes of K⁺ with S5, L5 and K5 are 8.94×10⁴, 2.83×10⁴ and 2.50×10³ L/mol, respectively, revealing that the binding strength of complexes decrease in order of S5, L5 and K5. The analyses of the fragment ions arising from the complexes of alkali metal ions with pentapeptides showed that the dissociations mainly occur in the backbones of the peptides. Among three pentapeptides, the complexes of pentaserines are the easiest to fragment, pentaleucines take second place, while pentalysines are the hardest. The dissociations also occur in the side chains, and there are significant differences among the three complexes. Moreover, the relationships among various fragment ions arising from the complex of Na⁺ with L5 were tested. Based on the theoretical model of Dunbar, it was proposed that in the fragmentation process, the alkali metal ion may shift to the N- or C-terminal of the pentapeptide. In 2∶1 complex, the first alkali metal ion may conjugate to the carbonyl groups of four amides, while the second one may conjugate to the oxygen atoms of carboxyl group of the pentapeptide.