Direct EPR detection of a hydrogen-bonded complex between a semiquinone radical anion and a protonated amino acid, and electron transfer driven by hydrogen bonding

Strong hydrogen bonding in a complex of a semiquinone radical anion and protonated histidine (His·2 H⁺), detected for the first time by EPR spectroscopy during photoinduced electron transfer from 10,10′-dimethyl-9,9′-biacridine (D) to 1-(p-tolylsulfinyl)-2,5- benzoquinone in the presence of His·2 H⁺ (picture, top), was revealed by the presence in the EPR spectrum (picture, bottom) of superhyperfine splitting caused by the NH₃⁺ group. (Chemical Equation Presented).