DISTINGUISHING N-TERMINUS AND C-TERMINUS IONS FOR MASS-SPECTROMETRY SEQUENCING OF PROTEINS WITHOUT PRIOR DEGRADATION

Identifying complementary pairs of fragment ions, b-type vs y-type, is a key step in mass spectral sequencing of proteins without degradatio n prior to ionization. Observations that b-type ions tend to be less s table than y-type have been extended to carbonic anhydrase (29 kDa) as a model protein. Infrared multiphoton dissociation (IRMPD) of 24 +, 2 7 +, 30 + molecular ions shows y-type ions from complementary pairs re quire, on average, 27% longer irradiation times for onset of abundance decrease, while in nozzle/skimmer dissociation of all charge states, y-type ions require a 20% higher potential difference. In some cases, b-ion dissociation appears to accompany its formation, as its rate of increase with increasing energy deposition is far less than that of it s y-complement.