π–π interactions in a phenothiazine-based organic redox polymer lead to an ultra-high cycling stability of a lithium–organic battery.

Cryptochromes are blue-light photoreceptor proteins, which provide input to circadian clocks. The cryptochrome from Drosophila melanogaster (DmCry) modulates the degradation of Timeless and itself. It is unclear how light absorption by the chromophore and the subsequent redox reactions trigger these events. Here, we use nano- to millisecond time-resolved x-ray solution scattering to reveal the light-activated conformational changes in DmCry and the related (6-4) photolyase. DmCry undergoes a series of structural changes, culminating in the release of the carboxyl-terminal tail (CTT). The photolyase has a simpler structural response. We find that the CTT release in DmCry depends on pH. Mutation of a conserved histidine, important for the biochemical activity of DmCry, does not affect transduction of the structural signal to the CTT. Instead, molecular dynamics simulations suggest that it stabilizes the CTT in the resting-state conformation. Our structural photocycle unravels the first molecular events of signal transduction in an animal cryptochrome.

Until now, FMN/FAD radicals could not be stabilized in aqueous solution or other protic solvents because of rapid and efficient dismutation reactions. In this contribution, a novel system for stabilizing flavin radicals in aqueous solution is reported. Subsequent to trapping FMN in an agarose matrix, light-generated FMN radicals could be produced that were stable for days even under aerobic conditions, and their concentrations were high enough for extensive EPR characterization. All large hyperfine couplings could be extracted by using a combination of continuous-wave EPR and low-temperature ENDOR spectroscopy. To map differences in the electronic structure of flavin radicals, two exemplary proton hyperfine couplings were compared with published values from various neutral and anionic flavoprotein radicals: C(6)­H and C­(8α)­H 3 . It turned out that FMN•– in an aqueous environment shows the largest hyperfine couplings, whereas for FMNH• under similar conditions, hyperfine couplings are at the lower end and the values of both vary by up to 30%. This finding demonstrates that protein–cofactor interactions in neutral and anionic flavoprotein radicals can alter their electron spin density in different directions. With this aqueous system that allows the characterization of flavin radicals without protein interactions and that can be extended by using selective isotope labeling, a powerful tool is now at hand to quantify interactions in flavin radicals that modulate the reactivity in different flavoproteins.

Organic redox polymers are considered a “greener” alternative as battery electrode materials compared to transition metal oxides. Among these, phenothiazine-based polymers have attracted significant attention due to their high redox potential of 3.5 V vs Li/Li+ and reversible electrochemistry. In addition, phenothiazine units can exhibit mutual π-interactions, which stabilize their oxidized states. In poly­(3-vinyl-N-methylphenothiazine) (PVMPT), such π-interactions led to a unique charge/discharge mechanism, involving the dissolution and redeposition of the polymer during cycling, and resulted in an ultrahigh cycling stability. Herein, we investigate these π-interactions in more detail and what effect their suppression by molecular design has on battery performance. Our study includes a dimeric reference compound for PVMPT, polymers with bulky tolyl or mesityl substituents on the phenothiazine units to inhibit π-interactions and alternating copolymers with maleimide groups to increase spatial distancing between phenothiazine groups. UV/vis- and electron paramagnetic resonance (EPR)-spectroscopic as well as electrochemical measurements in composite electrodes demonstrate how the unique structure of PVMPT is instrumental in obtaining a high cycling stability in poly­(vinylene) derivatives of phenothiazine.