The reaction catalyzed by E. coli ribonucleotide reductase (RNR) composed of α and β subunits that form an active α2β2 complex is a paradigm for proton-coupled electron transfer (PCET) processes in biological transformations. β2 contains the diferric tyrosyl radical (Y·) cofactor that initiates radical transfer (RT) over 35 Å via a specific pathway of amino acids (Y· ⇆ [W] ⇆ Y in β2 to Y ⇆ Y ⇆ C in α2). Experimental evidence exists for colinear and orthogonal PCET in α2 and β2, respectively. No mechanistic model yet exists for the PCET across the subunit (α/β) interface. Here, we report unique EPR spectroscopic features of Y·-β, the pathway intermediate generated by the reaction of 2,3,5-FY·-β2/CDP/ATP with wt-α2, YF-α2, or YF-α2. High field EPR (94 and 263 GHz) reveals a dramatically perturbed g tensor. [H] and [H]-ENDOR reveal two exchangeable H bonds to Y·: a moderate one almost in-plane with the π-system and a weak one. DFT calculation on small models of Y· indicates that two in-plane, moderate H bonds (r ∼1.8-1.9 Å) are required to reproduce the g value of Y· (wt-α2). The results are consistent with a model, in which a cluster of two, almost symmetrically oriented, water molecules provide the two moderate H bonds to Y· that likely form a hydrogen bond network of water molecules involved in either the reversible PCET across the subunit interface or in H release to the solvent during Y oxidation.
Fullerene C 60 and its derivatives are widely used in molecular electronics, photovoltaics, and battery materials, because of their exceptional suitability as electron acceptors. In this context, single-electron transfer on C 60 generates the C 60 • − radical anion. However, the short lifetime of free C 60• − hampers its investigation and application. In this work, we dramatically stabilize the usually short-lived C 60• − species within a self-assembled M 2 L 4 coordination cage consisting of a triptycene-based ligand and Pd(II) cations. The electron-deficient cage strongly binds C 60 by providing a curved inner π-surface complementary to the fullerene's globular shape. Cyclic voltammetry revealed a positive potential shift for the first reduction of encapsulated C 60 , which is indicative of a strong interaction between confined C 60• − and the cationic cage. Photochemical one-electron reduction with 1benzyl-1,4-dihydronicotinamide allows selective and quantitative conversion of the confined C 60 molecule in millimolar acetonitrile solution at room temperature. Radical generation was confirmed by nuclear magnetic resonance, electron paramagnetic resonance, ultraviolet−visible−near-infrared spectroscopy and electrospray ionization mass spectrometry. The lifetime of C 60• − within the cage was determined to be so large that it could still be detected after one month under an inert atmosphere.
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