Are the Radical Centers in Peptide Radical Cations Mobile? The Generation, Tautomerism, and Dissociation of Isomeric α-Carbon-Centered Triglycine Radical Cations in the Gas Phase

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• 作者：Ivan K. Chu ; Junfang Zhao ; Minjie Xu ; Shiu On Siu ; Alan C. Hopkinson ; K. W. Michael Siu
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：June 25, 2008
• 年：2008
• 卷：130
• 期：25
• 页码：7862 - 7872
• 全文大小：2331K
• 年卷期：v.130,no.25(June 25, 2008)
• ISSN：1520-5126

文摘

The mobility of the radical center in three isomeric triglycine radical cations—[G^•GG]⁺, [GG^•G]⁺, and [GGG^•]⁺—has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y^•GG]⁺, [GY^•G]⁺, and [GGY^•]⁺; CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G^•GG]⁺, [GG^•G]⁺, and [GGG^•]⁺, respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (≥44.7 kcal/mol), in support of the thesis that isomerically pure [G^•GG]⁺, [GG^•G]⁺, and [GGG^•]⁺ can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006−3012]. The CID spectra of [G^•GG]⁺, [GG^•G]⁺, and [GGG^•]⁺ were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.