Quantifying Positional Isomers (QPI) by top-down mass spectrometry

Proteomics has exposed a plethora of post-translational modifications, but demonstrating functional relevance requires new approaches. Top-down proteomics can characterize co-occurring modifications in terms of localization, abundance and hierarchy. Here, we present a top-down MS analysis workflow for the discovery and quantification of proteoforms. Confident fragment assignment allows for localization of modification sites and quantification of all proteoforms, including positional isomers, as validated by investigating synthetic isoforms of ubiquitin and hyper-phosphorylated Bora.