碱金属离子与三肽复合物的气相裂解反应

Fragmentation Reaction of Complexes of Alkali Metal Cations with Tripeptides in Gas Phase

为了探索侧链R基团对碱金属离子与多肽复合物气相裂解反应的影响,采用电喷雾电离质谱法研究了碱金属离子Li^+,Na^+和K^+分别与甘氨酸三肽(GGG)、甘氨酰-苯丙氨酰-甘氨酸三肽(GFG)和甘氨酰-甘氨酰-苯丙氨酸三肽(GGF)形成的复合物的气相裂解反应.质谱定性实验结果表明,Li^+,Na^+和K^+与GGG,GFG或GGF在气相中可以形成稳定的复合物,配合比为1∶1或2∶1.竞争反应质谱图显示,GGG,GFG或GGF与碱金属离子形成的复合物的质谱峰丰度按Li^+,Na^+,K^+顺序依次下降,表明随着碱金属离子半径的增加,它们与三肽的结合强度依次减弱.碰撞诱导解离显示,母体离子[GGG+Na]^+,[GGF+Na]^+和[GFG+Na]^+的质心碰撞能量E(CM)50数值分别为1.94,1.76和1.63 e V.通过质谱滴定法测得[GGG+Na]^+,[GFG^+Na]^+和[GGF+Na]^+的结合常数lg Ka1分别为5.30,5.25和5.17.质谱法定量结果进一步确认复合物的稳定性顺序为[GGG+Na]^+>[GGF+Na]^+>[GFG+Na]^+,表明由于空间位阻的影响,侧链R基团含有苄基的GFG或GGF与Na^+的键合强度要小于侧链R全部为H的GGG.串级质谱分析结果显示,碱金属化的GGG断裂位点较多,可解离出丰富的金属化a2,b和y型碎片离子,而碱金属化的GGF和GFG解离出的金属化y型离子较多,b型离子其次,金属化a型离子几乎没有.此外,双碱金属化的GGF可解离出较多金属化y型离子.复合物[GGF+Na]^+的裂解曲线显示,当碰撞能量为25 e V时,[y2+Na-H]^+和[b2+Na+OH]^+为主要碎片离子,当碰撞能量>40 e V时,只有[b2+Na+OH]^+碎片离子占有优势数量.根据质子化三肽裂解机理可以推测,钠化GFG裂解后生成含噁唑酮的[b2+Na]^+离子,该离子经过一系列过渡态生成[a2+Na]^+(2-苄基-4-咪唑酮),而不是常见的亚胺离子.

For exploring effects of side chain R groups on dissociations of complexes of alkali metal ions with peptides in gas phase,the complexes of Li＋,Na＋,K＋,Rb＋,Cs＋ and the tripeptides Glycyl-glycyl-glycine（GGG）,Glycyl-phenylalanyl-glycine（GFG） and Glycyl-glycyl-phenylalanine（GGF） were chosen to probe the fragmentation reaction process by electrospray ionization mass spectrometry. The experimental results demonstrated that Li＋,Na＋or K＋and GGG,GFG or GGF can form complexes in a coordination ratio of 1 ∶ 1 or2 ∶ 1,respectively. The mass spectra for competition reactions of GGG,GFG or GGF with Li＋,Na＋or K＋showed peak intensity of alkali metallated-tripeptide complexes decrease in the order of Li＋,Na＋,K＋similarly,indicating that the binding strength of complexes descend with the ascend of the radii of alkali metal ion. Collision-induced dissociation（CID） results showed the center of mass frame collision energy E（CM）50values were 1. 94,1. 76 and 1. 63 e V for the precursor of [GGG ＋ Na]＋,[GGF＋Na]＋and [GFG ＋ Na]＋,respectively. The mass spectrometric titrations determined the lg Ka1 values were 5. 30,5. 25 and 5. 17,respectively. Quantitative analyses of mass spectra further confirmed the stability order [GGG＋Na]＋[GGF＋Na]＋[GFG＋Na]＋,indicating binding strength of complexes GFG or GGF with R side chain（benzyl group） were weaker than GGG with R side chain（only hydrogen atom） due to effects of steric hindrance. The tandem mass spectrometric analysis revealed that dissociation of soliated GGG produces most abundant soliated a2,bn,yn ion series,while soliated GGF or GFG ion gives abundant soliated ynand bnions,but rarely soliated an. Moreover,doubly alkali meallated GGF tends to dissociate to more metallated y ion series. The breakdown graph for the complex [GGF＋Na]＋showed the percentage of fragment ion [b2＋Na ＋OH]＋will go up greatly upon the increasing of collision energy. According to Siu ＇s fragmentation mechanisms of protonated tripeptide,it is inferred that soliated GFG will fragment to [b2＋Na]＋ion with an incipient oxazolone. Then [b2＋ Na]＋will continue to fragment to [a2＋Na]＋with a cyclic protonted 4-imidazolidone.