*Article* **Fenton-Chemistry-Based Oxidative Modification of Proteins Reflects Their Conformation**

**Thomas Nehls †, Tim Heymann †, Christian Meyners , Felix Hausch and Frederik Lermyte \***

Clemens-Schöpf-Institute, Department of Chemistry, Technical University of Darmstadt, Alarich-Weiss-Straße 4, 64287 Darmstadt, Germany; thomas.nehls@tu-darmstadt.de (T.N.); tim.heymann@tu-darmstadt.de (T.H.); christian\_stephan.meyners@tu-darmstadt.de (C.M.); felix.hausch@tu-darmstadt.de (F.H.)

**\*** Correspondence: frederik.lermyte@tu-darmstadt.de

† These authors contributed equally to this work.

**Abstract:** In order to understand protein structure to a sufficient extent for, e.g., drug discovery, no single technique can provide satisfactory information on both the lowest-energy conformation and on dynamic changes over time (the 'four-dimensional' protein structure). Instead, a combination of complementary techniques is required. Mass spectrometry methods have shown promise in addressing protein dynamics, but often rely on the use of high-end commercial or custom instruments. Here, we apply well-established chemistry to conformation-sensitive oxidative protein labelling on a timescale of a few seconds, followed by analysis through a routine protein analysis workflow. For a set of model proteins, we show that site selectivity of labelling can indeed be rationalised in terms of known structural information, and that conformational changes induced by ligand binding are reflected in the modification pattern. In addition to conventional bottom-up analysis, further insights are obtained from intact mass measurement and native mass spectrometry. We believe that this method will provide a valuable and robust addition to the 'toolbox' of mass spectrometry researchers studying higher-order protein structure.

**Keywords:** mass spectrometry; protein folding; protein–ligand interactions; protein dynamics; FK506-binding protein; FKBP12; FKBP51
