The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site‐directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in‐cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide‐functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm, and shows high stability against reduction. Using this label, the guanine‐nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.

The reversibility of current Li-O2 batteries suffers from high charging overpotentials. To address this problem, the use of redox mediators has been proposed, which are supposed to improve the sluggish reaction kinetics of the oxygen evolution reaction via a solution mediated oxidation of lithium peroxide. In this study, we present a new thin layer cell for battery related differential electrochemical mass spectrometry (DEMS) experiments, which exhibits a high electrode surface area to electrolyte volume ratio which is closer to the situation in batteries other approaches/cells with their usually large electrolyte excess. The confined volume also allows a better distinction between the mediating activity of a redox system and a near continuous electrochemical reaction of this species. One further benefit of the new thin layer cell is that experiments can easily be performed under different O2-partial pressures. This new set-up allows the highly sensitive detection of volatile species formed during the OER. Therefore, small changes in the number of electrons transferred per oxygen molecule are observable. These changes help to identify side reactions and possible decomposition of the reaction products. During our experiments, we investigated the impact of TTF, TMPD, Fc and TEMPO on the oxidation of Li2O2. Within our experiments, we are able to precisely determine the potential at which the catalytic activity of the redox mediation starts. A comparison between the potential at which we observe the activity of the redox mediator to the half wave potential of the redox system could be explained with an outer sphere electron transfer for the oxidation of Li2O2 by a redox mediator. This observation is confirmed by a theoretical treatment of the redox mediation mechanism. Moreover, insights into the number of transferred electrons per oxygen molecule during the activity of the different redox mediators reveal the presence of side reactions. This finding is also underlined by an unexpected shift of the CO2 evolution onset for the redox mediator containing electrolytes. Our experiments also reveal that a Li-O2 cell, which contains a redox mediator, undergoes less fluctuation in its reversibility compared to a cell without a redox mediator.

Understanding the mechanism underlying the oxygen reduction and evolution in aprotic solvents is crucial for developing secondary metal–air batteries. Despite much scientific effort, the mechanism of the oxygen reduction in aprotic solvents in the presence of Li+ ions is still not fully understood. In this work, rotating ring–disk electrode and differential electrochemical mass spectrometry experiments have been employed to investigate the influence of the oxygen partial pressure on the oxygen reduction and evolution reaction at gold, glassy carbon, and platinum electrodes. A further aim was to elucidate the different pathways leading to peroxide formation and to analyze their importance for the overall reaction. As expected, the electrochemical reaction order for the superoxide formation is unity. Despite that, the reaction order for the reaction path leading to peroxide is below unity, indicating the participation of an adsorption step. This is further indicated by the finding that the amount of peroxide deposited on the surface changes only very slightly upon increasing the oxygen concentration. While the oxygen reduction at glassy carbon and platinum takes place via the parallel formation of superoxide and peroxide, there is a distinct transition between superoxide and peroxide formation at the gold electrode. This transition occurs close to a potential where the rate of superoxide formation is mainly limited by diffusion. By investigating the rotation dependence of the collection efficiency, it can be shown that the peroxide formation at gold indeed proceeds via a direct reduction step without the formation of a soluble intermediate. This finding adds to the current mechanistic picture which only differentiates between the electrochemical and the chemical formation of lithium peroxide. For platinum and glassy carbon, this direct reduction without the formation of soluble intermediates cannot be noted. Based on these observations, a reaction scheme is presented including rate constants for the different reaction pathways.

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Differential electrochemical mass spectrometry and classical electrochemical methods reveal that electrochemically produced K2O2 can be reversibly reoxidized to O2.