Structures of a Ions Derived from Protonated Pentaglycine and Pentaalanine: Results from IRMPD Spectroscopy and DFT Calculations

作者：Zhao Junfang; Lau Justin Kai Chi; Grzetic Josipa; Verkerk Udo H; Oomens Jos; Siu K W Michael; Hopkinson Alan C^*

来源：Journal of the American Society for Mass Spectrometry, 2013, 24(12): 1957-1968.

DOI：10.1007/s13361-013-0728-4

• Peptide fragment ions
• Fragmentation mechanisms
• 3,5-Dihydro-4H-imidazol-4-one ring structure
• Oxazolone structure
• a(4)(0) Structure

Infrared multiple-photon dissociation (IRMPD) spectroscopy and DFT calculations have been used to probe the most stable structures of a (3) (*) and a (4) (*) ions derived from both protonated pentaglycine (denoted G(5)) and pentaalanine (A(5)). The a (3) (*) and a (4) (*) ions derived from protonated A(5) feature a CHR=N-CHR%26apos;- group at the N-terminus and an oxazolone ring at the C-terminus, as proposed previously [J. Am. Soc. Mass Spectrom. 19, 1788-1798 (2008)]. The isomeric a (4) (*) ion derived from A(5) with a 3,5-dihydro-4H-imidazol-4-one ring structure was calculated to have a slightly better energy than the oxazolone, but the barrier to its formation is higher and there was no evidence of this ion in the IRMPD spectrum. By contrast, the a (4) (*) and [a (4) - H2O](+) (denoted a (4) (0) ) ions from G(5) gave strikingly similar IRMPD spectra and both have the 3,5-dihydro-4H-imidazol-4-one ring structure similar to that recently reported for the [GGGG + H - H2O](+) ion [Int. J. Mass Spectrom. 316-318, 268-272 (2012)]. In the absence of a solvent molecule, the pathway to the oxazolone is calculated to be lower than those to thermodynamically more stable products, the a (4) (0) and the a (4) (*) with the 3,5-dihydro-4H-imidazol-4-one ring structure. Incorporation of one water molecule is sufficient to reduce the barrier to formation of the a (4) (0) of G(5) to below that for formation of the oxazolone. On the equivalent potential energy surface for protonated A(5) the barrier to formation of the a (4) (0) ion is 12.3 kcal mol(-1) higher than that for oxazolone formation and the a (4) (0) ion is not observed experimentally.

• 出版日期2013-12