The Isoaspartome
Deamidation of asparagine (and to a lesser extent glutamine) residues occurs in almost all proteins all the time as a non-enzymatic, spontaneous, unimolecular reaction. It is the most common post-translational modification, but is understudied because the products of the reaction are hard to differentiate.
The two products, aspartic acid and isoaspartic acid, have the same mass and similar chemical reactivity making them difficult to differentiate. However, electron capture dissociation (ECD) cleaves the c_alpha to c_beta bond in these residues resulting in diagnostic marker peaks in the MS/MS spectra. For aspartic acid, it leaves a M-60 da peak, for isoaspartic acid, it leaves c+57, z-57 peaks.
To demonstrate this, cytochrome c was rapid aged at PH 11 for two weaks, resulting in the following ECD spectrum.
The following papers develop this work in detail:
Cournoyer, J. J.; Pittman, J. L.; Ivleva, V. B.; Fallows, E.; Waskell, L.; Costello, C. E.; O’Connor, P. B. Deamidation: Differentiation of aspartyl from isoaspartyl products in peptides by electron capture dissociation Protein Science 2005, 14, 452-463 Link.
O’Connor, P. B.; Cournoyer, J. J.; Pitteri, S. J.; Chrisman, P. A.; Mcluckey, S. A. Differentiation of aspartic and isoaspartic acids using electron transfer dissociation J. Am. Soc. Mass Spectrom. 2006, 15-19 Link.
Cournoyer, J. J.; Lin, C.; O’Connor, P. B. Detecting deamidation products in proteins by electron capture dissociation Anal. Chem. 2005, 77, in press Link.
