@article{7534,
  author = {P. Y. I. Shek and J. K. C. Lau and J. F. Zhao and J. Grzetic and U. H. Verkerk and J. Oomens and A. C. Hopkinson and K. W. M. Siu},
  title = {Fragmentations of protonated cyclic-glycylglycine and cyclic-alanylalanine},
  abstract = {Collision-induced dissociation has been used to study the fragmentations of two protonated diketopiperazines, protonated cyclic-glycylglycine and cyclic-alanylalanine. Protonated cyclo-AA lost CO and (CO + NH3) at low collision energies, channels attributed to dissociation of the O-protonated tautomer. Higher collision energies were required to dissociate protonated cyclo-GG, and the two lowest-energy products were the result of losses of one CO and two CO molecules. These occur from the higher-energy N-protonated tautomer, which is formed from the O-protonated tautomer by a 1,4-proton shift that has a high barrier (54.5 kcal mol(-1)) due to constraints imposed by the ring. Mechanistic schemes for four different dissociation channels, three from the N-protonated tautomer and one from the O-protonated tautomer, have been computed using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Comparison of the potential energy surfaces for the two protonated diketopiperazines reveals the factors behind this dichotomy of fragmentation pathways. The infrared multiple-photon dissociation spectrum of the [M+H-NH3-CO](+) ion (m/z 98) from protonated cyclo-M shows this product to be an oxazole, the lowest-energy isomer. (C) 2012 Elsevier B.V. All rights reserved.},
  year = {2012},
  journal = {International Journal of Mass Spectrometry},
  volume = {316},
  pages = {199-205},
  month = {Apr},
  isbn = {1387-3806},
  url = {http://www.sciencedirect.com/science/article/pii/S138738061200084X},
  doi = {10.1016/j.ijms.2012.02.011},
  language = {English},
}