DIBROMOTHYMOQUINONE (DBMIB) REPLACES THE FUNCTION OF Q(A) AT 77 K IN THE ISOLATED PHOTOSYSTEM-II REACTION-CENTER (D1-D2-CYTOCHROME B(559)) COMPLEX - DIFFERENCE SPECTRUM OF THE P680(-)) STATE()(DBMIB()

Transient absorbance changes of the primary electron donor chlorophyll a (P680) and acceptor pheophytin a (H) were measured at 77 K by nanos econd laser spectroscopy in the D1-D2-cytochrome b(559) photosystem II reaction center complex containing dibromomethylisopropyl benzoquinon e (DBMIB). After the laser excitation of the reaction center in the pr esence of DBMIB, only the P680(+)- (DBMIB(-)) state was detected. P680 (+) mainly decayed with a t(1/e) of 11 ms, In the absence of DBMIB, th e excitation produced the P680(+)H(-) radical pair. The radical pair p roduced the triplet state (P680(T)) with a t(1/e) of 50 ns, and P680(T ) then decayed with a t(1/e) of 2.1 ms. It was concluded that H- was o xidized by DBMIB in a time range faster than the detecting time resolu tion (3.5 ns) even at 77 K. The rapid oxidation of H- by DBMIB was als o confirmed by the suppression of delayed fluorescence with a decay ty , of 50 ns. The P680(+)(DBMIB(-))/P680(DBMIB) difference spectrum exhi bited a Q(y) band with a peak at 682 nm with a shoulder at 673 nm, The spectral shape was almost temperature insensitive between 77 and 265 It. The feature of this spectrum in the wavelength range between 330 a nd 720 nm was compared with that of P680(T)/P680 or H-/H at 77 K.