Despite the continuous progress in the research and development of Ti 3 C 2 T x (MXene) electrodes for high-power batteries and supercapacitor applications, the role of the anions in the electrochemical energy storage and their ability to intercalate between the MXene sheets upon application of positive voltage have not been clarified. A decade after the discovery of MXenes, the information about the possibility of anion insertion into the restacked MXene electrode is still being questioned. Since the positive potential stability range in diluted aqueous electrolytes is severely limited by anodic oxidation of the Ti, the possibility of anion insertion was evaluated in concentrated aqueous electrolyte solutions and aprotic electrolytes as well. To address this issue, we have conducted in situ gravimetric electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurements in highly concentrated LiCl and LiBr electrolytes, which enable a significant extension of the operation range of the MXene electrodes toward positive potentials. Also, halogens are among the smallest anions and should be easier to intercalate between MXene layers, in comparison to multiatomic anions. On the basis of mass change variations in the positive voltage range and complementary density functional theory calculations, it was demonstrated that insertion of anionic species into MXene, within the range of potentials of interest for capacitive energy storage, is not likely to occur. This can be explained by the strong negative charge on Ti 3 C 2 T x sheets terminated by functional groups.

Metallochaperones are responsible for shuttling metal ions to target proteins. Thus, a metallochaperone's structure must be sufficiently flexible both to hold onto its ion while traversing the cytoplasm and to transfer the ion to or from a partner protein. Here, we sought to shed light on the structure of Atox1, a metallochaperone involved in the human copper regulation system. Atox1 shuttles copper ions from the main copper transporter, Ctr1, to the ATP7b transporter in the Golgi apparatus. Conventional biophysical tools such as X-ray or NMR cannot always target the various conformational states of metallochaperones, owing to a requirement for crystallography or low sensitivity and resolution. Electron paramagnetic resonance (EPR) spectroscopy has recently emerged as a powerful tool for resolving biological reactions and mechanisms in solution. When coupled with computational methods, EPR with site-directed spin labeling and nanoscale distance measurements can provide structural information on a protein or protein complex in solution. We use these methods to show that Atox1 can accommodate at least four different conformations in the apo state (unbound to copper), and two different conformations in the holo state (bound to copper). We also demonstrate that the structure of Atox1 in the holo form is more compact than in the apo form. Our data provide insight regarding the structural mechanisms through which Atox1 can fulfill its dual role of copper binding and transfer. Statement (50-75 Words)Using EPR spectroscopy and computations, we show that the human copper metallochaperone Atox1 can accommodate at least six different conformational states. We also show that, upon binding Cu(I), the metallochaperone's structure becomes more compact and rigid. This research highlights how the structure of Atox1 enables it to fulfill its dual role of Cu(I) coordination and transfer, and points to the importance of the flexibility of this structure in controlling the copper transfer mechanism.

Atox1 is a human copper metallochaperone that is responsible for transferring copper ions from the main human copper transporter, hCtr1, to ATP7A/B in the Golgi apparatus. Atox1 interacts with the Ctr1 C-terminal domain as a dimer, although it transfers the copper ions to ATP7A/B in a monomeric form. The copper binding site in the Atox1 dimer involves Cys12 and Cys15, while Lys60 was also suggested to play a role in the copper binding. We recently showed that Atox1 can adopt various conformational states, depending on the interacting protein. In the current study, we apply EPR experiments together with hybrid quantum mechanics–molecular mechanics molecular dynamics simulations using a recently developed semiempirical density functional theory approach, to better understand the effect of Atox1’s conformational states on copper coordination. We propose that the flexibility of Atox1 occurs owing to protonation of one or more of the cysteine residues, and that Cys15 is an important residue for Atox1 dimerization, while Cys12 is a critical residue for Cu­(I) binding. We also show that Lys60 electrostatically stabilizes the Cu­(I)–Atox1 dimer.

Among the examined organic electrodes for aqueous mono and multivalent ions batteries, polyimide is considered a promising candidate because of its high capacity and good cyclability in different electrolyte solutions. While most of the studies so far were focused on the energetic performance of polyimide anodes, much less is known about their charge storage mechanism and particularly how such electrodes are affected by the solvation properties of the inserted cations. Using in situ EQCM-D, a direct assessment of the cationic fluxes and their hydration shells inserted/extracted to/from PI electrodes upon potential application was performed for a large variety of mono and multivalent cations. Our observations demonstrated a pronounced withdrawal of water molecules from the polymeric electrodes during insertion of chaotropic cations and significantly less water withdrawal upon insertion of kosmotropic cations. These findings are well correlated with the capacity and the rate capability of the polyimide electrodes in the examined systems and shed light on their charge storage mechanism.

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