Pulse Q-band EPR and ENDOR spectroscopies of the photochemically generated monoprotonated benzosemiquinone radical in frozen alcoholic solution.

Quinones are essential cofactors in many physiological processes, among them proton-coupled electron transfer (PCET) in photosynthesis and respiration. A key intermediate in PCET is the monoprotonated semiquinone radical. In this work we produced the monoprotonated benzosemiquinone (BQH(center dot)) by UV illumination of BQ dissolved in 2-propanol at cryogenic temperatures and investigated the electronic and geometric structures of BQH(center dot) in the solid state (80 K) using EPR and ENDOR techniques at 34 GHz. The g-tensor of BQH(center dot) was found to be similar to that of the anionic semiquinone species (BQ(center dot-)) in frozen solution. The peaks present in the ENDOR spectrum of BQH(center dot) were identified and assigned by H-1/H-2 substitutions. The experiments reconfirmed that the hydroxyl proton (O-H) on BQH(center dot), which is abstracted from a solvent molecule, mainly originates from the central CH group of 2-propanol. They also showed that the protonation has a strong impact on the electron spin distribution over the quinone. This is reflected in the hyperfine couplings (hfc's) of the ring protons, which dramatically changed with respect to those typically observed for BQ(center dot-). The hfc tensor of the O-H proton was determined by a detailed orientation-selection ENDOR study and found to be rhombic, resembling those of protons covalently bound to carbon atoms in a pi-system (i.e., alpha-protons). It was found that the O-H bond lies in the quinone plane and is oriented along the direction of the quinone oxygen lone pair orbital. DFT calculations were performed on different structures of BQH(center dot) coordinated by four, three, or zero 2-propanol molecules. The O-H bond length was found to be around 1.0 angstrom, typical for a single covalent O-H bond. Good agreement between experimental and DFT results were found. This study provides a detailed picture of the electronic and geometric structures of BQH(center dot) and should be applicable to other naturally occurring quinones.