含酪氨酸三肽自由基离子的形成、异构和解离反应机理研究  

Formation,isomerization,and dissociation of tyrosine-containing tripeptide radical cations

作　　者：路时芳 曹洁 贾贺园 孙烁琪 陈世稆 Shifang Lu;Jie Cao;Heyuan Jia;Shuoqi Sun;Shilv Chen(Key Laboratory of Cluster Science(Beijing Institute of Technology),Ministry of Education,School of Chemistry and Chemical Engineering,Beijing Institute of Technology,Beijing 100081,China)

机构地区：[1]北京理工大学化学与化工学院,原子分子簇科学教育部重点实验室,北京100081

出　　处：《中国科学：化学》2020年第9期1177-1186,共10页SCIENTIA SINICA Chimica

基　　金：国家自然科学基金(编号:21371025)资助项目。

摘　　要：本文使用铜-配体-三肽组成的三元复合物[Cu(L)Peptide]2+(配体L=4′-氯-2,2′:6′,2′′-三联吡啶,缩写4Cl-tpy;Peptide=丙氨酰-酪氨酰-甘氨酸(AYG)或甘氨酰-酪氨酰-甘氨酸(GYG))通过碰撞诱导解离(collision-induced dissociation,CID)的方法,成功制备了AYG和GYG两个系列自由基离子([AYG]·+,[AYG·]+;[GYG]·+,[G·YG]+和[GYG·]+).通过对比AYG不同位点自由基离子的CID谱图以及对比AYG和GYG两个系列自由基离子的CID谱图,结合密度泛函理论(density functional theory,DFT)计算,阐明AYG自由基离子的气相异构/解离反应机理,并解释了AYG和GYG自由基离子不同解离行为的内在原因.研究表明,(1)[AYG]·+和[AYG·]+的解离路径基本相同,二者的主要裂解产物均为[x2+H]·+离子,说明二者在解离之前已完全异构.(2)[A·YG]+和[G·YG]+的解离路径相同,二者均断裂酰胺C−N键,产生[b2−H]·+碎片离子,说明[A·YG]+不能异构为[AYG]·+和[AYG·]+.(3)DFT计算给出了AYG三个自由基离子的稳定性顺序([A·YG]+>[AYG·]+>[AYG]·+)和它们的异构/解离能垒.[AYG·]+®[AYG]·+的异构能垒(47.4 kcal/mol)低于[AYG·]+的解离能垒(72.7 kcal/mol),但高于[AYG]·+的解离能垒(44.3 kcal/mol),由此得到[AYG·]+的气相裂解反应机理为[AYG·]+®[AYG]·+的异构过程,再经历[AYG]·+®[x2+H]·+的解离反应,断裂[AYG]·+N端Cα−C键而产生[x2+H]·+碎片离子.(4)[GYG]·+与[AYG]·+的气相裂解特征不同,[GYG]·+是断裂第二个N−Cα键(从N端氨基酸残基数起)产生[z2−H]·+碎片离子.Two series of tripeptide radical cations([AYG]·+,[AYG·]+;[GYG]·+,[G·YG]+and[GYG·]+)were successfully prepared via multistage collision-induced dissociation(CID)of a ternary complex of copper-ligand-tripeptide[Cu(L)Peptide]2+(L=4′-chloro-2,2′:6′,2′′terpyridine(4Cl-tpy);Peptide=alanyl-tyrosyl-glycine(AYG)or glycyl-tyrosyl-glycine(GYG)tripeptide).The gas-phase isomerization and dissociation mechanism of isomeric AYG radical ions,and the different dissociation behaviors between AYG and GYG radical ions,have been obtained by observing their gas-phase dissociation reactions combined with density functional theory(DFT)calculation.Studies have revealed that:(1)[AYG·]+and[AYG]·+produced almost identical CID spectra(their main product ions are[x2+H]·+ions),suggesting that interconversion occurs prior to dissociation.(2)The dissociation paths of[A·YG]+and[G·YG]+are the same(both of which produces[b2−H]·+fragment ions by cleavage of amide C−N bond,indicating that[A·YG]+cannot isomerize to[AYG·]+and[AYG]·+.DFT calculation gives the gas-phase stability order of these three peptide radical ions([A·YG]+>[AYG·]+>[AYG]·+)and their isomerization/dissociation energy barriers.Isomerization energy barrier(47.4 kcal/mol)of[AYG·]+®[AYG]·+is lower than dissociation energy barrier(72.7 kcal/mol)of[AYG·]+,but higher than that(44.3 kcal/mol)of[AYG]·+,thereby the gas-phase dissociation mechanism of[AYG·]+undergoes a two-step reaction:the first step is the isomerization process of[AYG·]+®[AYG]·+,and the second step is the dissociation reaction of[AYG]·+®[x2+H]·+,which produces[x2+H]·+fragment ions by cleavage of N-terminal Cα−C bond in[AYG]·+.(4)The gas-phase dissociation characteristics of[GYG]·+and[AYG]·+are dramatically different.[GYG]·+mainly produces[z2−H]·+by cleavage of the second N−Cαbond(counting from the N-terminal amino acid residue).

关 键 词：电喷雾串联质谱(ESI-MS/MS) 密度泛函理论计算 三肽自由基离子 气相异构和解离反应 

分 类 号：O641.4[理学—物理化学]