Allen Julia M, Racine Alawee H, Berman Ashley M, Johnson Jeffrey S, Bythell Benjamin J, Paizs Béla, Glish Gary L
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina, USA.
J Am Soc Mass Spectrom. 2008 Dec;19(12):1764-70. doi: 10.1016/j.jasms.2008.09.022. Epub 2008 Sep 30.
It has been determined experimentally that a(3) ions are generally not observed in the tandem mass spectroscopic (MS/MS) spectra of b(3) ions. This is in contrast to other b(n) ions, which often have the corresponding a(n) ion as the base peak in their MS/MS spectra. Although this might suggest a different structure for b(3) ions compared to that of other b(n) ions, theoretical calculations indicate the conventional oxazolone structure to be the lowest energy structure for the b(3) ion of AAAAR, as it is for other b(n) ions of this peptide. However, it has been determined theoretically that the a(3) ion is lower in energy than other a(n) ions, relative to the corresponding b ions. Furthermore, the a(3) --> b(2) transition structure (TS) is lower in energy than other a(n) --> b(n-1) TSs of AAAAR, compared with the corresponding b ions. Consequently, it is suggested that the b(3) ion does fragment to the a(3) ion, but that the a(3) ion then immediately fragments (to b(2) and a(3)) because of the excess internal energy arising from its relatively low energy and the facile a(3) --> b(2) reaction. That is why a(3) ions are not observed in the MS/MS spectra of b(3) ions.
通过实验已确定，在b(3)离子的串联质谱（MS/MS）谱中通常观察不到a(3)离子。这与其他b(n)离子形成对比，其他b(n)离子在其MS/MS谱中常常以相应的a(n)离子作为基峰。尽管这可能表明b(3)离子与其他b(n)离子相比具有不同的结构，但理论计算表明，对于AAAAR的b(3)离子而言，传统的恶唑酮结构是能量最低的结构，就像该肽的其他b(n)离子一样。然而，从理论上已确定，相对于相应的b离子，a(3)离子的能量比其他a(n)离子更低。此外，与相应的b离子相比，AAAAR的a(3)→b(2)过渡结构（TS）的能量比其他a(n)→b(n - 1) TSs更低。因此，有人提出b(3)离子确实会裂解为a(3)离子，但由于a(3)离子相对较低的能量以及容易发生的a(3)→b(2)反应产生的过量内能，a(3)离子随后会立即裂解（为b(2)和a(3)）。这就是为什么在b(3)离子的MS/MS谱中观察不到a(3)离子的原因。
