ESEEM study of the plastoquinone anion radical (QA.--) in 14N- and 15N-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, QA- anion radical by electron spin-echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of the ESEEM data of QA- in 14N- and 15N-labeled PSII demonstrates the existence of a protein nitrogen nucleus coupled to the QA-. The 14N coupling is characterized by a quadrupolar coupling constant e2qQ/4h = 0.82 MHz, an asymmetry parameter eta = 0.45, and hyperfine coupling constant A approximately 2.1 MHz. The 15N hyperfine coupling is characterized by T = 0.41 MHz and alpha iso approximately 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 QA- in PSII. Based on a comparison of the 14N ESEEM with 14N-NQR and 14N-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.