Decay kinetics and quantum yields of fluorescence in photosystem I from Synechococcus elongatus with P700 in the reduced and oxidized state: Are the kinetics of excited state decay trap-limited or transfer-limited?
M. Byrdin et al., Decay kinetics and quantum yields of fluorescence in photosystem I from Synechococcus elongatus with P700 in the reduced and oxidized state: Are the kinetics of excited state decay trap-limited or transfer-limited?, BIOPHYS J, 79(2), 2000, pp. 992-1007
Transfer and trapping of excitation energy in photosystem I (PS I) trimers
isolated from Synechococcus elongafus have been studied by an approach comb
ining fluorescence induction experiments with picosecond time-resolved fluo
rescence measurements, both at room temperature (RT) and at low temperature
(5 K), Special attention was paid to the influence of the oxidation state
of the primary electron donor P700. A fluorescence induction effect has bee
n observed, showing a similar to 12% increase in fluorescence quantum yield
upon P700 oxidation at RT, whereas at temperatures below 160 K oxidation o
f P700 leads to a decrease in fluorescence quantum yield (similar to 50% at
5 K). The fluorescence quantum yield for open PS I (with P700 reduced) at
5 K is increased by similar to 20-fold and that for closed PS I (with P700
oxidized) is increased by similar to 10-fold, as compared to RT, Picosecond
fluorescence decay kinetics at RT reveal a difference in lifetime of the m
ain decay component: 34 +/- 1 ps for open PS I and 37 +/- 1 ps for closed P
S I, At 5 K the fluorescence yield is mainly associated with long-lived com
ponents (lifetimes of 401 ps and 1.5 ns in closed PS I and of 377 ps, 1.3 n
s, and 4.1 ns in samples containing similar to 50% open and 50% closed PS I
). The spectra associated with energy transfer and the steady-state emissio
n spectra suggest that the excitation energy is not completely thermally eq
uilibrated over the core-antenna-RC complex before being trapped. Structure
-based modeling indicates that the so-called red antenna pigments (A708 and
A720, i,e., those with absorption maxima at 708 nm and 720 nm, respectivel
y) play a decisive role in the observed fluorescence kinetics. The A720 are
preferentially located at the periphery of the PS I core-antenna-RC comple
x; the A708 must essentially connect the A720 to the reaction center. The e
xcited-state decay kinetics turn out to be neither purely trap limited nor
purely transfer (to the trap) limited, but seem to be rather balanced.