RATES OF PRIMARY ELECTRON-TRANSFER REACTIONS IN THE PHOTOSYSTEM-I REACTION-CENTER RECONSTITUTED WITH DIFFERENT QUINONES AS THE SECONDARY ACCEPTOR

Rates of sequential electron transfer reactions from the primary electron donor chlorophyll dimer (P700) to the electron acceptor chlorophyll a-686 (A(0)) and to the secondary acceptor quinone (Q(phi)) are measured by picosecond absorption spectroscopy in spinach photosystem I (PS I) particles. In the particles, 85-90% of antenna chlorophylls are extracted and the intrinsic phylloquinone (Qm) is removed and replaced by different quinones. (1) The excited single state of P700, monitored by the absorbance change at 695 nm, appeared with a time constant shorter than 1 ps after excitation with a 605 nm, 1 ps pulse at an intensity of 0.8 or 1.5 photons/reaction center. (2) The formation of a P700(+)A(0)(-) state, which was monitored by the absorbance change of A(0) at 685 nm, occurred with a time constant (tau(1)) of 8 ps. The P700(+)A(0)(-) state persisted more than 800 ps in the phylloquinone-depleted particles. (3) After the reconstitution of menaquinone-4, phylloquinone, or 2-methyl-1,4-naphthoquinone as Q(phi), the transition from the P700(+)A(0)(-)Q(phi) to P700(+)A(0)Q(phi)(-) state (monitored by the recovery of the A(0) absorption band) occurred with time constants (tau(2)) of 23, 23, or 34 ps in the particles with an antenna size of 30 chlorophylls/P700. (4) In the PS I particles with an antenna size of 16 chlorophylls/P700, a similar tau(1) of 11 ps and a tau(2) of 31 ps were observed when menaquinone was reconstituted as Q(phi). (5) Indication for the absorbancechange of the chlorophylls, which may function as the intermediate electron acceptor'between P700 and A(0) or between A(0) and Q(phi), was not obtained.