The efficiency of non-photochemical fluorescence quenching by cation radicals in photosystem II reaction centers

Abstract

In a direct experiment, the rate constants of photochemical k and non-photochemical k quenching of the chlorophyll fluorescence have been determined in spinach photosystem II (PS II) membrane fragments, oxygen-evolving PS II core, as well as manganese-depleted PS II particles using pulse fluorimetry. In the dark-adapted reaction center(s) (RC), the fluorescence decay kinetics of the antenna were measured at low-intensity picosecond pulsed excitation. To create a "closed" P680Q state, RCs were illuminated by high-intensity actinic flash 8 ns prior to the measuring flash. The obtained data were approximated by the sum of two decaying exponents. It was found that the antennae fluorescence quenching efficiency by the oxidized photoactive pigment of RC P680 was about 1.5 times higher than that of the neutral P680 state. These results were confirmed by a single-photon counting technique, which allowed to resolve the additional slow component of the fluorescence decay. Slow component was assigned to the charge recombination of P680Pheo in PS II RC. Thus, for the first time, the ratio k /k ≅ 1.5 was found directly. The mechanism of the higher efficiency of non-photochemical quenching comparing to photochemical quenching is discussed.

Ultrafast excitation quenching by the oxidized photosystem II reaction center

“…In comparison, the average fluorescence lifetime of dark-adapted PSII core complexes is ~130 ps [62]. Thus, the rate of excitation quenching by P + is higher than the rate of photochemical trapping in PSII with open RC, in agreement with previous reports [50,51]. From the average lifetime, one can calculate the quenching rate as kq = 1/τ−kd, where kd is the rate constant of excitation deactivation in the absence of quenching.…”

“…These values are similar to the measured average fluorescence lifetimes of PSII from T. vulcanus [62]. We can thus estimate that the rate of excitation quenching by P + is on average ~40% faster than the photochemical trapping, in line with the fluorescence measurements reported by Paschenko et al [51].…”

“…Within the 3-ns window after the pre-flash, we need to consider the formation of triplet excited states 3 Chl * and, subsequently, 3 Car * , both of which are efficient excitation quenchers [50,73]. Paschenko et al [51] could resolve fluorescence decay lifetimes of 80-90 ps (plus longer components of hundreds of ps) and assumed that the experimentally observed quenching is partly due to 3 Car * . The main pathway for the formation of triplets is by charge recombination of 3 [P + Pheo − ] creating 3 Chl * states [74] that rapidly migrate, via triplet-triplet energy transfer, to Cars in the core antenna [75].…”

“…It was therefore asserted that P + is a more efficient quencher than the open RC. In a more recent time-resolved fluorescence study, Paschenko et al [51] used saturating laser pulses to induce full oxidation of the RC and probed the fluorescence decay kinetics induced by excitation pulses delayed by several nanoseconds. They found that the fluorescence lifetimes of PSII core complexes or PSIIenriched membranes were shorter when the RCs were preoxidized compared to the lifetimes of the darkadapted open RCs, confirming that the rate of nonphotochemical quenching by P + is higher than the rate of photochemical trapping.…”