ANNIHILATION PROCESSES IN THE ISOLATED D1-D2-CYT-B559 REACTION-CENTERCOMPLEX OF PHOTOSYSTEM-II - AN INTENSITY-DEPENDENCE STUDY OF FEMTOSECOND TRANSIENT ABSORPTION
Mg. Muller et al., ANNIHILATION PROCESSES IN THE ISOLATED D1-D2-CYT-B559 REACTION-CENTERCOMPLEX OF PHOTOSYSTEM-II - AN INTENSITY-DEPENDENCE STUDY OF FEMTOSECOND TRANSIENT ABSORPTION, Journal of physical chemistry, 100(22), 1996, pp. 9537-9544
The excitation intensity dependence of the kinetics of the primary pro
cesses and of the yield of radical pair formation in the isolated D1-D
2-cyt-b559 reaction center of photosystem II has been studied by femto
second transient absorption spectroscopy. It is shown that the kinetic
s is strongly dependent on the excitation intensity. The radical pair
yield as a function of excitation intensity is compared with the theor
etical annihilation curve and a good agreement between theory and expe
riment is observed, indicating that the intensity effects on the kinet
ics and radical pair yield arise primarily from annihilation processes
. Sufficiently annihilation-free measurements require excitation inten
sities that give rise to less than or equal to 0.06 absorbed photons/R
C while maintaining a high signal/noise ratio of greater than or equal
to 100:1 at most detection wavelengths in order to resolve the comple
x kinetics. It is shown that such low excitation intensities give rise
to absorption changes that are at the edge of the capabilities of pre
sent femtosecond absorption equipment. We also compare the excitation
conditions that have been used so far by other research groups for pub
lished transient absorption data on the isolated D1-D2-cyt-b559 comple
x. This comparison shows that essentially all published data have been
obtained under conditions where annihilation or quenching effects are
expected to significantly distort the kinetics and time-resolved spec
tra and to reduce the radical pair yield.