Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor

Abstract

The mechanism and kinetics of electron transfer in isolated D1͞ D2-cyt b559 photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of Ϸ1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8 -12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the Ϸ200-ps component represents the electron transfer to the Q A acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are Ϸ3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor Chl acc D1. Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated RC* excited state is 170 -180 ns ؊1 , and the rate constant of secondary electron transfer is 120 -130 ns ؊1 .charge separation ͉ photosynthesis ͉ ultrafast spectroscopy ͉ D1͞D2-cytb559 ͉ femtosecond absorption P hotosystem (PS) II cores, whose structure has recently been determined to a resolution of 3.5-3.2 Å (1-3), consist of the antenna polypeptides CP43 and CP47, which carry 13 and 16 chlorophyll (Chl) a molecules, respectively. They contain furthermore the D1͞D2-cyt b559 reaction center (RC) polypeptides, which bind the pigments of the electron transfer chain [four Chls, two pheophytins (Pheo), and two quinones] and two additional antenna Chls (the so-called Chl z D1 and Chl z D2 molecules). The isolated RC (D1-D2-cyt b559 ) lacks the quinone acceptors and is thus only able to create a short-lived radical pair (RP) (see review in ref. 4).There exists presently no agreement on the mechanism of the primary events of energy and electron transfer in the isolated RC complex (see refs. 4-6 for recent reviews). Early studies suggested an apparent Ϸ3-ps charge separation lifetime in the RC at room temperature (7,8) in agreement with later studies (9, 10). Andrizhiyevskaya et al. (11) recently also proposed a model with an Ϸ3-ps charge separation. A somewhat slower charge separation of Ϸ8 ps has been reported by Wasielewski and coworkers (12), whereas more recent data from the same group were interpreted in terms of a 2-to 5-ps charge separation time (13). Substantially shorter charge separation times of 1 ps (14) and 0.4 ps (at 240 K) have been reported by Groot et al. (15). At the other extreme, a 1 order of magnitude longer charge separation time of Ϸ21 ps has been suggested by Klug and coworkers (16,17). Probably the largest ...

Spectroelectrochemical determination of the redox potential of pheophytina, the primary electron acceptor in photosystem II

“…The two bleaches at 517 and 547 nm arise from the Qx bands of Phe a, the bleach at 428 nm is the Soret band (18,33), and the 450 nm peak is due to the Phe a Ϫ absorption (33,37). Our result hence represents the lightinduced difference spectra due to the redox reaction of photoactive Phe a (38)(39)(40)(41)(42), which is in the D1 branch and denoted Phe a D1 , in the PS II core complexes. The spectral intensity at Ϫ500 mV is approximately half that at Ϫ350 mV.…”

Mediator-assisted water oxidation by the ruthenium “blue dimer”cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+

“…In the reaction center of PSII, light is harvested by an antenna array consisting of chlorophylls and organic pigments, which sensitize the lowest singlet excited state of chlorophyll P 680 or a neighboring pheophytin D1 (36). This excited state subsequently undergoes oxidative quenching, giving P 680 ϩ by electron transfer to quinone Q A .…”

From horse thief to professor: confessions of a plant physiologist

“…A host of further observations enforced the conclusion that drying activated mechanisms in mosses and lichens which convert the energy of light into heat before light can cause damage. This was not a trivial conclusion because it is known that light used for photosynthesis is converted into redox energy within picoseconds in special reaction centres of the photosynthetic apparatus (Holzwarth et al 2006). It meant that mechanisms capable of converting the energy of light into thermal energy must be even faster than the mechanisms permitting photosynthesis to occur.…”