Based on new experimental data of linear and nonlinear spectroscopic investigations of LHC2 complexes, both the energy transfer and energy relaxation of excitons were studied in the limiting cases of strong pigment-pigment and pigment-protein interactions, respectively. Consequences of the structural basis and the dominance of strong pigment-pigment interactions in monomers were independently established by different optical measurements. In this model, the energy relaxation of excitons was determined by dynamical processes of exciton interactions with the radiation fíeld and the long-wave protein vibrations, respectively, in particular with the mode having the wave number of 43 cm'f
The exciton equilibration in the light-harvesting complex of photosystem 2 at room temperature was studied with pump-probe experiments under quasi-stationary conditions. The measurements were performed with two dye lasers, which had a pulse diuation of 10 ns and a spectral bandwidth of less than 0.01 nm. The relative probe beam transmission ffom 640-690 nm was recorded at different excitation wavelengths, varied from 645 to 685 nm. Two maxima were observed at 650 nm (Chl b absorption region) and at 680 nm (Chl a absorption region). The relative probe beam transmission in the Chl a region showed a course nearly independent of the pump beam wavelengťh in contrast to a weak dependence in the Chl b region. Measurements of the relative probe beam transmission as a function of pump beam intensity revealed a dominating exciton-exciton annihilation in the Chl a absorption region. In the Chl b region exciton-exciton annihilation was only marginal.
The radiant energy induced degradation at 293 K of isolated Dl/D2/cyt b559 complexes from spinách was investigated under aerobic conditions by measurements of absorption and emission spectra between 1.1 and 270 K. In the 1.1 K absorption spectrum a bleaching and slight blue shift at 679 nm and a blue shift of the 672 nm band was observed. In the emission spectrum deconvoluted into Gaussian bands, the major fluorescence band referred to as F682 was most susceptible to photodegradation: (7) At all temperatures F682 was blue shifted with respect to the control. (2) The half-width of F682 was temperature independent at T < 10 K and became significantly increased in damaged samples. (3) The temperature dependence of rp6g2 in the range of 10 K < T < 270 K was less pronounced in damaged samples. Our results support the idea of a multistep model of photodegradation with several intermediates in the pathway from the undamaged control into strongly damaged complexes. The temperature dependence of rp6g2 i® explained by contributions owing to static and dynamical inhomogeneous broadening. The static inhomogeneous broadening increases with progressing degradation. Furthermore, our results suggest that aerobic photodegradation of the Dl/D2/cyt 3559 complexes also causes modifications of the electron-phonon interactions. The frequency of the phonon mode dominating the electron phonon coupling decreases in damaged samples.
Using intense ns-laser pulses up to 2 x 10** photons per cm^ per pulse the intensity dependence of fluorescence yield (O) of different photosystem 2 (PS2) subunits in spinách on the intensity of excitation pulse (/) was measured. A sti ong drop of O was observed with increasing laser pulse I. Measurements of the light-harvesting complex of PS2, LHC2, were taken at excitation wavelengths of 645-700 nm. A shift of the í) vs. I curves to lower I with increasing absorption cross section was observed. Measurements of PS2 and thylakoid membrane ífagments taken at a fixed excitation wavelength of 645 nm showed the same O V5, I dependences as LHC2. This suggests that LHC2-pigment proteins dominate the exciton concentration in the photosynthetic apparatus. A smáli drop of O measured at CP29 and CC2 particles was possibly caused by smaller domains. Smaller drop of O measured at D1D2 particles was similar to that of free chlorophyll (Chl), indicating that bimolecular annihilation was absent. Experimental results could be described using a simple two-level model including exciton-exciton-annihilation and depletion of Chl ground States. Good fits of measured O-intensity ciu-ves were possible for LHC2 with all the ušed excitation wavelengths. Outputs of these approximations are statements about excited statě concentration and time dependence of their decay as a function of excitation irradiance.