Dibromothymoquinone (DBMIB) Replaces the Function of QA at 77 K in the Isolated Photosystem II Reaction Center (D1-D2-Cytochrome b559) Complex: Difference Spectrum of the P680+(DBMIB-) State

Transient absorbance changes of the primary electron donor chlorophyll a (P680) and acceptor pheophytin a (H) were measured at 77 K by nanosecond laser spectroscopy in the Dl-D2-cytochrome bSS9 photosystem II reaction center complex containing dibromomethylisopropyl benzoquinone (DBMIB). After the laser excitation of the reaction center in the presence 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, the excitation produced the P680+H~ radical pair. The radical pair produced the triplet state (P680T) with a t 1/e of 50 ns, and P680T then decayed with a tVt of 2.1 ms. It was concluded that H~ was oxidized by DBMIB in a time range faster than the detecting time resolution (3.5 ns) even at 77 K. The rapid oxidation of H~ by DBMIB was also confirmed by the suppression of delayed fluorescence with a decay t1/e of 50 ns. The P680+(DBMIB")/P680(DBMIB) difference spectrum exhibited a Q, band with a peak at 682 nm with a shoulder at 673 nm. The spectral shape was almost temperature insensitive between 77 and 265 K. The feature of this spectrum in the wavelength range between 330 and 720 nm was compared with that of P680 T/P680 or H"/H at 77 K.