Cassady C J
Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio, USA.
J Am Soc Mass Spectrom. 1998 Jul;9(7):716-23. doi: 10.1016/S1044-0305(98)00034-8.
Gas-phase deprotonation and hydrogen/deuterium (H/D) exchange reactions for ions from three model dodecapeptides were studied by Fourier transform ion cyclotron resonance mass spectrometry. Molecular dynamics calculations were employed to provide information on conformations and Coulomb energies. The peptides, (KGG)4, (K2G4)2, and K4G8, each contain four high basicity lysine residues and eight low basicity glycine residues; however, in the present work only three lysine residues were protonated. Proton transfer reactions with a series of reference amines revealed apparent gas-phase acidities in a narrow range of 207.3-209.6 kcal/mol, with deprotonation efficiencies following the order [K4G8 + 3H]3+ > [(KGG)4 + 3H]3+ > [(K2G4)2 + 3H]3+. The three ions also react similarly with d4-methanol: each exchanged a maximum of 23-25 of their 25 labile hydrogens, with the first 15-17 exchanges occurring at rate constants of (1.6-2.6) x 10(-11) cm3 molecule-1 s-1. The experimental results agree with molecular modeling findings of similar conformations and Coulomb energies for the three peptide ions. The [M + 3H]3+ data are compared to data obtained previously in our laboratory for the "fully" protonated [M + 4H]4+ (Zhang, X.; Ewing, N. P.; Cassady, C. J. Int. J. Mass Spectrom. Ion Phys., in press). For (KGG)4 and (K2G4)2, there is a marked difference in H/D exchange reactivity between 3+ ions and 4+ ions. The 4+ ions, which have diffuse conformations, slowly exchange only 14 hydrogens, whereas their more compact 3+ counterparts exchange 23-25 hydrogens at a 5-times greater rate. In contrast, the 3+ and 4+ ions of K4G8 have similar compact conformations and exchange reactivity. The results indicate that a multiply hydrogen-bonded intermediate between the deuterating reagent and the peptide ion is necessary for facile H/D exchange. The slower, incomplete H/D exchange of [(KGG)4 + 4H]4+ and [(K2G4)2 + 4H]4+ is attributed to the inability of their protonated lysine n-butylamino groups (which extend away from the peptide backbone) to form this intermediate.
利用傅里叶变换离子回旋共振质谱研究了三种模型十二肽离子的气相去质子化和氢/氘（H/D）交换反应。采用分子动力学计算来提供有关构象和库仑能的信息。肽（KGG）4、（K2G4）2和K4G8各自包含四个高碱性赖氨酸残基和八个低碱性甘氨酸残基；然而，在本工作中只有三个赖氨酸残基被质子化。与一系列参考胺的质子转移反应显示，表观气相酸度在207.3 - 209.6 kcal/mol的狭窄范围内，去质子化效率顺序为[K4G8 + 3H]3+ > [(KGG)4 + 3H]3+ > [(K2G4)2 + 3H]3+。这三种离子与d4 - 甲醇的反应也相似：它们在其25个不稳定氢中最多交换23 - 25个，前15 - 17次交换的速率常数为(1.6 - 2.6)×10(-11) cm3分子-1 s-1。实验结果与三种肽离子相似构象和库仑能的分子建模结果一致。将[M + 3H]3+数据与我们实验室先前获得的“完全”质子化的[M + 4H]4+数据进行比较（Zhang, X.; Ewing, N. P.; Cassady, C. J. Int. J. Mass Spectrom. Ion Phys., 即将发表）。对于（KGG）4和（K2G4）2，3+离子和4+离子之间的H/D交换反应性存在显著差异。具有扩散构象的4+离子仅缓慢交换14个氢，而其更紧凑的3+对应物以快5倍的速率交换23 - 25个氢。相比之下，K4G8的3+和4+离子具有相似的紧凑构象和交换反应性。结果表明，氘代试剂与肽离子之间形成多重氢键中间体对于 facile H/D交换是必要的。[(KGG)4 + 4H]4+和[(K2G4)2 + 4H]4+较慢的、不完全的H/D交换归因于它们质子化的赖氨酸正丁氨基（远离肽主链）无法形成这种中间体。
