Near edge X-ray absorption mass spectrometry of gas phase proteins: the influence of protein size

Multiply protonated peptides and proteins in the gas phase can respond to near edge X-ray absorption in three different ways: (i) non dissociative ionization and ionization accompanied by loss of small neutrals, both known to dominate for proteins with masses in the 10 kDa range. (ii) Formation of immonium ions, dominating for peptides in the 1 kDa range. (iii) Backbone scission leading to sequence ions which is typically weaker and has mainly been observed for peptides in the 1 kDa range. We have studied carbon 1s photoexcitation and photoionization for a series of peptides and proteins with masses covering the range from 0.5 kDa to more than 10 kDa. The gas phase protonated molecules were trapped in a radiofrequency ion trap and exposed to synchrotron radiation. Time of flight mass spectrometry was employed for investigation of the photoionization and photofragmentation processes. A smooth transition from the photofragmentation regime to the non-dissociative photoionization regime is observed. Mass spectra are most complex in the few kDa regime, where non-dissociative ionization, backbone scission and immonium ion formation coexist. The observed correlation between protein size and fragmentation, i.e. radiation damage, is of relevance for soft X-ray microscopy. The response of gas-phase proteins upon soft X-ray absorption depends strongly on the proteins size.