Charge separation in the reaction center of photosystem II studied as a function of temperature

Abstract

In photosystem II of green plants the key photosynthetic reaction consists of the transfer of an electron from the primary donor called P680 to a nearby pheophytin molecule. We analyzed the temperature dependence of this reaction by subpicosecond transient absorption spectroscopy over the temperature range 20-240 K using isolated photosystem II reaction centers from spinach. After excitation in the red edge of the Qy absorption band, the decay of the excited state can conveniently be described by two kinetic components that both accelerate with temperature. This temperature behavior differs remarkably from that observed in purple bacterial reaction centers. We attribute the first component, which accelerates from 2.6 ps at 20 K to 0.4 ps at 240 K, to charge separation after direct excitation of P680, and explain its temperature dependence by an intermediate that lies in energy above the singlet-excited P680 and that possibly has charge-transfer character. The second component accelerates from 120 ps at 20 K to 18 ps at 240 K and is attributed to charge separation after direct excitation of the "trap" state near-degenerate with P680 and subsequent slow energy transfer from this trap state to P680. We suggest that the slow energy transfer from the trap state to P680 plays an important role in the kinetics of radical pair formation at room temperature.

Figure 1

Temperature = 20 K, λ_exc = 685 nm. ΔOD spectra at time delays of 0.4 ps (solid line), 1.0 ps (dashed line), 4.75 ps (dotted line), 21 ps (chain-dash line), 55 ps (chain-dotted line), 155 ps (dashed line), and 750 ps (solid line)

Figure 2

DAS at T = 20 K of the 2.6-ps (dotted line), 120-ps (dashed line), and 2-ns (solid line) components. (Inset) DAS spectrum of the component that follows the excitation pulse instantaneously and which is responsible for a ΔOD of 14.10⁻³ at the maximum of the instrument response. At all temperatures such a component was included in the fit.

Figure 3

DAS if T = 77 K, 0.7 ps (dotted line), 34 ps (dashed line) (a), T = 110 K, 0.5 ps (dotted line), 37 ps (dashed line) (b), T = 150 K, 0.5 ps (dotted line), 33 ps (dashed line) (c), and T = 240 K, 0.4 ps (dotted line), 18 ps (dashed line) (d). The long-lived spectra are indicated by the solid lines.

Figure 4

Energy level diagram of the states of the PSII RC involved in energy transfer and charge separation. C670, C681, and P680 denote (groups of) pigments or states absorbing at that particular wavelength; arrows denote energy transfer and charge separation. All reactions are reversible, but those that are on the time domain of interest effectively unidirectional are indicated as such. Note that due to inhomogeneous broadening the energy levels are actually broader as depicted here (≈5 nm FWHM) and therefore overlap extensively. C670 represents one or two pigments that transfer slowly [≈15 ps (13, 31, 37)] to the other pigments and can be inferred at T ≥ 240 K to equilibrate with the states absorbing around 680 nm in about 10 ps; C681 or the trap-state is degenerate with P680 and transfers to P680 directly in ≈35 ps or slower at low temperature; P680 is the primary electron donor; and X may represent the higher excitonic multimer levels or a multimer state with charge transfer character that mediates electron transfer from P680*. The 2.4 → 0.4-ps component is essentially ascribed to equilibration between P680 and X and its decay into P⁺I⁻, the 120 → 18-ps component to the C681 to P680 transfer which at higher temperature is accelerated through the C681 → C670 → P680 decay channel (see Discussion).