Lioe Hadi, O'Hair Richard A J, Reid Gavin E
School of Chemistry, University of Melbourne, Melbourne, Victoria, Australia.
J Am Soc Mass Spectrom. 2004 Jan;15(1):65-76. doi: 10.1016/j.jasms.2003.09.011.
The gas phase reactions of protonated tryptophan have been examined in a quadrupole ion trap using a combination of collision induced dissociation, hydrogen-deuterium exchange, regiospecific deuterium labeling and molecular orbital calculations (at the B3LYP/6-31G* level of theory). The loss of ammonia from protonated tryptophan is observed as the primary fragmentation pathway, with concomitant formation of a M + H - NH(3) ion by nucleophilic attack from the C3 position of the indole side chain. Hydrogen-deuterium exchange and regiospecific deuterium labeling reveals that scrambling of protons in the C2 and C4 positions of the indole ring, via intramolecular proton transfer from the thermodynamically preferred site of protonation at the amino nitrogen, precedes ammonia loss. Molecular orbital calculations have been employed to demonstrate that the activation barriers to intramolecular proton transfer are lower than that for NH(3) loss.
利用碰撞诱导解离、氢氘交换、区域特异性氘标记和分子轨道计算（在B3LYP/6-31G*理论水平）相结合的方法，在四极离子阱中研究了质子化色氨酸的气相反应。观察到质子化色氨酸失去氨是主要的碎片化途径，同时通过吲哚侧链C3位置的亲核攻击形成[M + H - NH(3)] (+)离子。氢氘交换和区域特异性氘标记表明，在氨损失之前，通过从氨基氮上热力学上优先的质子化位点进行分子内质子转移，吲哚环C2和C4位置的质子发生了重排。已采用分子轨道计算来证明分子内质子转移的活化能垒低于氨损失的活化能垒。
