Zhao Junfang, Song Tao, Xu Minjie, Quan Quan, Siu K W Michael, Hopkinson Alan C, Chu Ivan K
Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario, CanadaM3J 1P3.
Phys Chem Chem Phys. 2012 Jun 28;14(24):8723-31. doi: 10.1039/c2cp40708f. Epub 2012 May 21.
Dissociation of peptide radical ions involves competition between charge-induced and radical-induced reactions that can be preceded by isomerization. The isomeric radical cations of the peptide methyl ester G˙GR-OMe and GG˙R-OMe provide very similar collision-induced dissociation (CID) spectra, suggesting that isomerization occurs prior to fragmentation. They undergo characteristic radical-induced bond cleavage of the peptide N-terminal amide bond resulting in the y2(+) ion, and of the arginine side-chain's Cα-Cβ bond giving protonated allylguanidine {CH2[double bond, length as m-dash]CHCH2NHC(NH2)2, m/z 100}. The absence of a y2(+) fragment ion in the CID of the radical cationic tripeptide ACH3G˙R and of an m/z 100 ion in the spectrum of G˙ACH3R (where ACH3 is an α-aminoisobutyric acid residue, which cannot form an α-carbon-centered radical through hydrogen atom transfer) establishes the importance of hydrogen atom migration along the peptide backbone prior to specific radical-induced fragmentations. Herein we use density functional theory (DFT) at the B3LYP/6-31++G(d,p) level to evaluate the barriers for interconversion between the α-carbon-centered radicals and for dissociation. The radical cations G˙GR and GG˙R have their radicals located on the α-carbon atoms of the peptide backbone and their charge densities largely sequestered on the guanidine groups of the side-chain of arginine residues. This is in contrast to the isomeric radical cations of [GGG]˙(+), in which the charge resides necessarily on the peptide backbone. The lower charge densities on the backbones of G˙GR and GG˙R result in greater structural flexibility, decreasing the barrier for interconversion between these α-carbon-centered radicals to 36.2 kcal mol(-1) (cf. 44.7 kcal mol(-1) for [GGG]˙(+)). The total absence of charge, assessed by examining intramolecular hydrogen atom transfers among the three α-carbon centers of the isomeric neutral α-carbon-centered triglycine radicals [GGG-H]˙, leads to an additional but slight reduction in enthalpy, to approximately 34 kcal mol(-1).
肽自由基离子的解离涉及电荷诱导反应和自由基诱导反应之间的竞争，这些反应之前可能会发生异构化。肽甲酯G˙GR-OMe和GG˙R-OMe的异构自由基阳离子提供非常相似的碰撞诱导解离（CID）光谱，表明异构化在碎片化之前发生。它们经历肽N端酰胺键的特征性自由基诱导的键断裂，产生y2(+)离子，以及精氨酸侧链的Cα-Cβ键断裂，产生质子化的烯丙基胍{CH2[双键，长度为m破折号]CHCH2NHC(NH2)2，m/z 100}。在自由基阳离子三肽ACH3G˙R的CID中没有y2(+)碎片离子，以及在G˙ACH3R的光谱中没有m/z 100离子（其中ACH3是α-氨基异丁酸残基，不能通过氢原子转移形成以α-碳为中心的自由基），这确立了在特定自由基诱导的碎片化之前氢原子沿肽主链迁移的重要性。在此，我们使用B3LYP/6-31++G(d,p)水平的密度泛函理论（DFT）来评估以α-碳为中心的自由基之间相互转化的势垒和解离势垒。自由基阳离子G˙GR和GG˙R的自由基位于肽主链的α-碳原子上，它们的电荷密度主要隔离在精氨酸残基侧链的胍基上。这与[GGG]˙(+)的异构自由基阳离子形成对比，在[GGG]˙(+)中电荷必然位于肽主链上。G˙GR和GG˙R主链上较低的电荷密度导致更大的结构灵活性，将这些以α-碳为中心的自由基之间相互转化的势垒降低到36.2 kcal mol(-1)（相比之下，[GGG]˙(+)为44.7 kcal mol(-1)）。通过检查异构中性以α-碳为中心的三甘氨酸自由基[GGG-H]˙的三个α-碳中心之间的分子内氢原子转移来评估的完全没有电荷，导致焓进一步但轻微降低，至约34 kcal mol(-1)。
