Factors Affecting the Fragmentation of Peptide Ions: Metal Cationization and Fragmentation Timescale
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The factors affecting peptide fragmentation have been extensively studied in the literature in order to better predict the fragment ion spectra of peptides and proteins. While there are countless influences to consider, metal cation binding in the gas-phase is particularly interesting. Herein, a comparison of fragmentation patterns of a model peptide series with various charge carriers (H+, Li+, Na+, K+, and Cu+) will assist in determining the location of the preferred binding site of the metal cation and in assessing differences in the fragmentation pattern as a result of this binding site. An interesting observation from these studies reveals abundant x-type fragment ions occurring from the fragmentation of alkali-metal cationized peptides. As these fragment ions have been observed in previous studies by others but not addressed, the factors affecting the formation of these x-type fragment ions are explored. Additionally, a home-built 193-nm photodissociation tandem time-of-flight mass spectrometer is utilized to study how peptide fragmentation kinetics affect the fragmentation pattern observed. Initially, the fragmentation timescales of various peptides are investigated. Results indicate that longer fragmentation timescales (~10 microseconds) result in an increased number of identified peaks with internal and ammonia loss fragment ions being the most common in comparison to 'prompt' fragmentation timescales (~1 microsecond). Furthermore, b-type fragment ion formation is also favored at longer timescales for the arginine containing peptides investigated. The fragmentation pattern of several proline containing peptides is examined by collision-induced dissociation and 193-nm photodissociation. Unique fragment ions are observed with each occurring at a proline residue. Few differences are detected between CID and 193-nm photodissociation spectra, indicating that the proline residues direct fragmentation rather than the dissociation method. In an effort to improve the performance of the photodissociation tandem TOF instrument, the addition of a second source and a dual-stage reflectron are incorporated. The modifications result in improved mass range, signal-to-noise, and increased fragment ion collection efficiencies. High quality mass spectra are acquired across a range of mass-to-charge ratios from ~600 to 1900. Furthermore, the modifications continue to allow investigation of various fragmentation timescales with the addition of an additional timeframe of ~3 microseconds.