Infrared Spectroscopy of Fragments of Protonated Peptides: Direct Evidence for Macrocyclic Structures of b5 Ions
Infrared Spectroscopy of Fragments of Protonated Peptides: Direct Evidence for Macrocyclic Structures of b5 Ions
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DOI：
10.1021/ja903390r
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发表时间：
2009-08-19
影响因子：
15
通讯作者：
Maitre, Philippe
中科院分区：
文献类型：
--
作者：
Erlekam, Undine;Bythell, Benjamin J.;Maitre, Philippe
b 离子在使用串联质谱法的肽测序中至关重要。通常认为这些离子以质子化恶唑酮衍生物的形式存在。最近的工作表明，中等大小的 b 离子可以通过恶唑酮结构的头尾环化来重排，产生大环质子化肽作为中间体。在这里，我们使用红外光谱和密度泛函理论计算表明，质子化 G(5)R 的 b(5) 离子在质谱仪中作为酰胺氧质子化环肽存在，而不是作为短暂的中间体存在。我们的 DFT 计算支持了这一分配，该计算表明，尽管环状形式引入了熵约束，但这种大环异构体在能量上优于开放恶唑酮形式。
b ions are of fundamental importance in peptide sequencing using tandem mass spectrometry. These ions have generally been assumed to exist as protonated oxazolone derivatives. Recent work indicates that medium-sized b ions can rearrange by head-to-tail cyclization of the oxazolone structures generating macrocyclic protonated peptides as intermediates. Here, we show using infrared spectroscopy and density functional theory calculations that the b(5) ion of protonated G(5)R exists in the mass spectrometer as an amide oxygen protonated cyclic peptide rather than fleetingly as a transient intermediate. This assignment is supported by our DFT calculations which show this macrocyclic isomer to be energetically preferred over the open oxazolone form despite the entropic constraints the cyclic form introduces.