ESEEM STUDY OF THE PLASTOQUINONE ANION-RADICAL (Q(A)(CENTER-DOT-)) INN-14-LABELED AND N-15-LABELED PHOTOSYSTEM-II TREATED WITH CN

The nonheme iron of the photosystem II reaction center was converted t o its low-spin state (S = 0) by treatment with CN-. This allowed the s tudy of the plastoquinone, Q(A)(-) anion radical by electron spin-echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of t he ESEEM data of Q(A)(-) in N-14- and N-15-labeled PSII demonstrates t he 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 co upling constant A similar to 2.1 MHz. The N-15 hyperfine coupling is c haracterized by T = 0.41 MHz and alpha(iso) similar to 3.3 MHz. The po ssible origins of the nitrogen hyperfine coupling are discussed in ter ms of the amino acids thought to be close to the Q(A)(-) in PSII. Base d 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 o f the peptide backbone of Ala261 of the polypeptide D2, although the i ndole nitrogen of Trp254 and the imino nitrogen of His215 of D2 also r emain candidates.