ESEEM study of the plastoquinone anion radical (Q(A)(center dot-)) in N-14- and N-15-labeled photosystem II treated with CN

The nonheme iron of the photosystem II reaction center was converted to its low-spin state (S = 0) by treatment with CN-. This allowed the study of the plastoquinone, Q(A)(-) anion radical by electron spin-echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of the ESEEM data of Q(A)(-) in N-14- and N-15-labeled PSII demonstrates the existence of a protein nitrogen nucleus coupled to the Q(A)(-). The N-14 coupling is characterized by a quadrupolar coupling constant e(2)qQ/4h = 0.82 MHz, an asymmetry parameter eta = 0.45, and hyperfine coupling constant A similar to 2.1 MHz. The N-15 hyperfine coupling is characterized by T = 0.41 MHz and alpha(iso) similar to 3.3 MHz. The possible origins of the nitrogen hyperfine coupling are discussed in terms of the amino acids thought to be close to the Q(A)(-) in PSII. Based on a comparison of the N-14 ESEEM with N-14-NQR and N-14-ESEEM data from the literature, the most likely candidate is the amide nitrogen of the peptide backbone of Ala261 of the polypeptide D2, although the indole nitrogen of Trp254 and the imino nitrogen of His215 of D2 also remain candidates.