二硫-水自由基阳离子复合物中二中心三电子键的质谱研究

Mass Spectrometric Study of Two-Center Three-Electron Bond in Disulfide-Water Radical Cation Complexes

含硫二中心三电子(2c3e)键的研究一直是蛋白质中自由基传递领域的重要课题。本研究采用串联质谱和同位素标记方法研究不同位阻[R S-S R+H_(2)O]^(+·)的质谱碎裂模式,以探索位阻不同时这些自由基阳离子配合物内2c3e键的结构性质。结果表明,空间位阻低的[R S-S R+H_(2)O]^(+·)(R=甲基,乙基或丙基)碎裂时主要产生[R SH+H_(2)O]^(+·)和[R SOH+H]+2种离子,而空间位阻高的[R S-S R+H_(2)O]^(+·)(R=异丙基或叔丁基)主要碎片离子是由母离子丢失H_(2)O而形成的[R S-S R]^(+·)。因此,在空间位阻低的[R S-S R+H_(2)O]^(+·)中2c3e键应以[S∴S]^(+)的形式存在,而在空间位阻高的[R S-S R+H_(2)O]^(+·)中2c3e键应以[S∴O]+的形式存在。由此可见,结构环境的不同对自由基结构的性质有重要影响,这有助于理解与2c3e键有关的化学反应和生物体中自由基的传递过程机理。

Two-center three-electron(2c3e)bonded disulfide complexes are of particular interest in biology and chemistry.This type of bonding is responsible for the stability of many ion-molecule clusters or long-lived radical intermediates widely encountered in many applications of organic chemistry,biology,and chemical catalysis.In this paper,disulfide-water radical cation complexes,derived from the gas-phase reaction between H_(2)O^(+·)and disulfides(R S-S R,R=methyl,ethyl,propyl,isopropyl or tert-butyl)were studied using tandem mass spectrometry and isotope-labelling experiment for exploring the characteristic of 2c3e bond inside these complexes.Interesting chemistry was revealed through the dissociation of[R S-S R+H_(2)O]^(+·)complexes.For example,[CH 3S-SCH 3+H_(2)O]^(+·)(m/z 112)would predominantly give product ions of m/z 66 and m/z 65 with 16%collision energy(CE),by the loss of CH 2=S(46 u)and CH 3S·(47 u),respectively,indicating the cleavage of the disulfide bond in the ionic complex of m/z 112.Similarly,when ethyl or propyl group replaced methyl group,[R S-S R+H_(2)O]^(+·)complexes would also produce the disulfide bond cleaved product.However,when isopropyl or tert-butyl group was in the disulfide,[R S-S R+H_(2)O]^(+·)would mainly generate[R S-S R]^(+·)by the loss of H_(2)O.It was proposed that[R S-S R+H_(2)O]^(+·)(R=methyl,ethyl,or propyl)with relative low steric hindrance mainly generated two ions of[R 1SH+H_(2)O]^(+·)and[R 2SOH+H]+,revealing that the bond strength between S and O was more stable than the bond strength between S and S in[R S-S R+H_(2)O]^(+·).While[R S-S R+H_(2)O]^(+·)(R=isopropyl and tert-butyl)with large steric hindrance mostly produced ions of[R S-S R]^(+·)by loss of H_(2)O,suggesting that the bond strength between S and S was more stable than the bond strength between S and O in[R S-S R+H_(2)O]^(+·).These results indicated that 2c3e bond in[R S-S R+H_(2)O]^(+·)with low steric hindrance should be the form of[S∴S]+,while 2c3e bond in[R S-S R+H_(2)O]^(+·)with large steric hindrance should be the form of[S∴O]+.The experimental results may shed light on advanced studies about the 2c3e bonded disulfide-water radical complexes,better for understanding the life process involving 2c3e bonded disulfide in peptides and proteins.