Events near the reaction center in O2 evolving PS II enriched thylakoid membranes: The presence of an electric field during the S2 state in a population of centers

Abstract

Flash-induced absorption changes at 515 nm observed as a function of flash number are examined in relation to the flash-induced fluorescence yields in inside-out thylakoids. After partial dissipation of the delocalized transmembrane electric field by adding gramicidin, the analysis of period 4 oscillations and of the kinetics in the 10 ms-1 s range suggest that the variation of the absorption changes at 515 nm as a function of flash number is the result of at least two processes:1) an electric field increase related to the S2 state and 2) the fact that the field generated by the water protons inside the membrane decreases when these protons are released outside the membrane. The former field correlates with the flash-induced fluorescence yield increase induced by the donor side of Photosystem II. Both measurements show similar oscillations as a function of flash number, with maxima on the 1st, 5th and 9th flash. These oscillations, after a shift of two flashes, appear to be different from those of the O2 yield observed under similar conditions. It is proposed that, in a population of centers the electric field during the S2 state reflects the presence of a stabilized positive equivalent in the protein close to the Mn complex.

Modulation of Flash-Induced Photosystem II Fluorescence by Events Occurring at the Water Oxidizing Complex

“…The mechanism proposed in this work to account for a period four oscillation in the fluorescence is in contradiction with the study of Delrieu and Rosengard (10). They ascribed the oscillation pattern of the fluorescence yield measured 80 ms after exitation flashes in thylakoids to flash-dependent changes in the S 2 population.…”

“…A similarity between oscillation patterns of flash-induced absorption changes at 515 nm (electrochromic band shifts of carotenoids) and flash-induced Chl a fluorescence yield measured in thylakoid membranes at 80 and 800 ms after excitation flashes has been found (10). The absorption changes as a function of flash number have been suggested to reflect an appearance of a surplus of charge in the S 2 state (11).…”

“…A thorough analysis of the fluorescence yield oscillation pattern and the flash-induced absorption changes at 515 nm led Delrieu and Rosengard (10) to the conclusion that the physical processes underlying these two data sets are different. Electric field localized in the vicinity of the Mn cluster of the WOC and Y z , was suggested to influence the fluorescence yield by a change in the free energy of the PSII charge separation reactions (17,18).…”

“…In contrast, the absorption changes at 515 nm were suggested to be affected by electric fields far from the Mn cluster. Delrieu and Rosengard (10) concluded that S 2 associated charge surplus stably accumulated near the Mn cluster in a population of centers was responsible for the modulation of the flash-induced Chl a fluorescence yield measured on a long time scale (t > 80 ms).…”