Analysis of bromotryptophan and hydroxyproline modifications by high-resolution, high-accuracy precursor ion scanning utilizing fragment ions with mass-deficient mass tags.

Protein modifications are often detected by precursor ion scanning. When quadrupole TOF mass spectrometers are used for precursor ion scanning with high-resolution, high-accuracy fragment ion selection, "reporter" ions are required to have a unique mass within +/-0.04 Da or less instead of +/-0.5 Da on triple quadrupole mass spectrometers, the traditional instrument used for precursor ion scanning. Thus, characteristic fragment ions can be utilized even if other fragment ions have the same nominal mass as long as the characteristic fragment ions are slightly mass deficient as compared to the other fragments, i.e., when they have an inherent mass-deficient mass tag. Here, the immonium ions of bromotryptophan and hydroxyproline are described as two fragment ions characteristic for tryptophan-brominated and proline-hydroxylated peptides, respectively. The "reporter" ion of trytophan-brominated peptides is highly mass deficient due to the presence of bromine, thereby allowing the selective detection of these species and the distinction from other dipeptidic a-, b-, and y-fragment ions by high-resolution, high-accuracy precursor ion scanning. This strategy also enables the differentiation between precursors giving rise to the oxygen-containing immonium ion of hydroxyproline and precursors of the immonium ions of near-ubiquitous leucine/isoleucine. Both immonium ions have the same nominal mass of 86 Da, but the exact masses differ by less than 0.04 Da. High-resolution, high-accuracy precursor ion scanning enabled the identification of proline-hydroxylated and tryptophan-brominated species and the directed analysis of species carrying these modifications in a highly complex Conus textile conotoxin mixture. This lead to the characterization of one novel C. textile conotoxin containing a bromotryptophan residue and one novel C. textile conotoxin carrying two hydroxyproline residues.