A reaction cycle for redox-mediated, Ni-catalyzed aryl etherification is proposed under both photoredox and electrochemically mediated conditions. We demonstrate that a self-sustained Ni(I/III) cycle is operative in both cases by chemically synthesizing and characterizing a common paramagnetic Ni intermediate and establishing its catalytic activity. Furthermore, deleterious pathways leading to off-cycle Ni(II) species have been identified, allowing us to discover optimized conditions for achieving self-sustained reactivity at a ∼15-fold increase in the quantum yield and a ∼3-fold increase in the faradaic yield. These results highlight the importance of leveraging insight of complete reaction cycles for increasing the efficiency of redox-mediated reactions.

Protein hydration is essential to its structure, dynamics, and function, but water-protein interactions have not been directly observed in real time at physiological temperature to our awareness. By using a tryptophan scan with femtosecond spectroscopy, we simultaneously measured the hydration water dynamics and protein side-chain motions with temperature dependence. We observed the heterogeneous hydration dynamics around the global protein surface with two types of coupled motions, collective water/side-chain reorientation in a few picoseconds and cooperative water/sidechain restructuring in tens of picoseconds. The ultrafast dynamics in hundreds of femtoseconds is from the outer-layer, bulk-type mobile water molecules in the hydration shell. We also found that the hydration water dynamics are always faster than protein sidechain relaxations but with the same energy barriers, indicating hydration shell fluctuations driving protein side-chain motions on the picosecond time scales and thus elucidating their ultimate relationship.hydration shell dynamics | protein side-chain motion | water-driven relaxation | coupled fluctuation | tryptophan scan W ater-protein interactions are critical to protein structural stability and flexibility, functional dynamics, and biological activities (1, 2). Various methods such as neutron scattering (3), NMR (4), laser spectroscopy (5, 6), and molecular dynamics (MD) simulations (7) have been used to reveal protein surface hydration and coupled water-protein dynamics on different time and length scales. Hydration water molecules often participate in various protein functions and their motions even directly "control" protein fluctuations (2, 8). Frauenfelder et al. recently proposed a unified model for protein dynamics (8): large-scale protein motions are slaved to the fluctuations of bulk solvent and controlled by solvent viscosity while internal protein motions are slaved to the fluctuations of the hydration shell and controlled by hydration water. However, direct measurements of such coupled fluctuations at physiological temperature are challenging as a result of the ultrafast nature of water motions, and therefore most studies are indirect or at low temperature (3, 4). Here, we used a tryptophan (W) scan to probe global surface hydration (9) and used femtosecond spectroscopy to follow hydration water motions and local side-chain fluctuations in real time. With temperature dependence, we systematically measured their dynamics and thus finally elucidate their ultimate relationship. Results and DiscussionTryptophan Scan and Femtosecond Fluorescence Spectroscopy. We used DNA polymerase IV (Dpo4) (10), a hyperthermal enzyme without a single tryptophan residue, as a model protein, and designed 10 tryptophan mutants, one at a time, to probe four different domains (Fig. 1A). We performed systematic measurements of tryptophan fluorescence intensity changes with wavelength and time and thereby constructed 3D fluorescence profiles (SI Appendix, SI Note 1). One example of an R176W mutant is ...

Self-sustained Ni I/III cycles are established as ap otentially general paradigm in photoredox Ni-catalyzed carbonheteroatom cross-coupling reactions through as trategy that allows us to recapitulate photoredox-like reactivity in the absence of light across aw ide range of substrates in the amination, etherification, and esterification of aryl bromides, the latter of whichh as remained, hitherto,e lusive under thermal Ni catalysis.M oreover,t he accessibility of esterification in the absence of light is especially notable because previous mechanistic studies on this transformation under photoredox conditions have unanimously invoked energytransfer-mediated pathways.

The quantum efficiency in photoredox catalysis is the crucial determinant of energy intensity and, thus, is intrinsically tied to the sustainability of the overall process. Here, we track the formation of different transient species of a catalytic photoredox hydroamidation reaction initiated by the reaction of an Ir(III) photoexcited complex with 2-cyclohexen-1-yl(4-bromophenyl)carbamate. We find that the back reaction between the amidyl radical and Ir(II) photoproducts generated from the quenching reaction leads to a low quantum efficiency of the system. Using transient absorption spectroscopy, all of the rate constants for productive and nonproductive pathways of the catalytic cycle have been determined, enabling us to establish a kinetically competent equilibrium involving the crucial amidyl radical intermediate that minimizes its back reaction with the Ir(II) photoproduct. This strategy of using an off-pathway equilibrium allows us to improve the overall quantum efficiency of the reaction by a factor of 4. Our results highlight the benefits from targeting the back-electron transfer reactions of photoredox catalytic cycles to lead to improved energy efficiency and accordingly improved sustainability and cost benefits of photoredox synthetic methods.

Photoredox-mediated nickel-catalyzed cross-couplings have evolved as a new effective strategy to forge carbon− heteroatom bonds that are difficult to access with traditional methods. Experimental mechanistic studies are challenging because these reactions involve multiple highly reactive intermediates and perplexing reaction pathways, engendering competing, but unverified, proposals for substrate conversions. Here, we report a comprehensive mechanistic study of photoredox nickel-catalyzed C−S cross-coupling based on time-resolved transient absorption spectroscopy, Stern−Volmer quenching, and quantum yield measurements. We have (i) discovered a self-sustained productive Ni(I/III) cycle leading to a quantum yield Φ > 1; (ii) found that pyridinium iodide, formed in situ, serves as the dominant quencher for the excited state photocatalyst and a critical redox mediator to facilitate the formation of the active Ni(I) catalyst; and (iii) observed critical intermediates and determined the rate constants associated with their reactivity. Not only do the findings reveal a complete reaction cycle for C−S cross-coupling, but the mechanistic insights have also allowed for the reaction efficiency to be optimized and the substrate scope to be expanded from aryl iodides to include aryl bromides, thus broadening the applicability of photoredox C−S cross-coupling chemistry.