Energy and Electron Transfer in Photosystem II Reaction Centers with Modified Pheophytin Composition

Energy and electron transfer in Photosystem II reaction centers in which the photochemically inactive pheophytin had been replaced by 131-deoxo-131-hydroxy pheophytin were studied by femtosecond transient absorption-difference spectroscopy at 77 K and compared to the dynamics in untreated reaction center preparations. Spectral changes induced by 683-nm excitation were recorded both in the QY and in the QX absorption regions. The data could be described by a biphasic charge separation. In untreated reaction centers the major component had a time constant of 3.1 ps and the minor component 33 ps. After exchange, time constants of 0.8 and 22 ps were observed. The acceleration of the fast phase is attributed in part to the redistribution of electronic transitions of the six central chlorin pigments induced by replacement of the inactive pheophytin. In the modified reaction centers, excitation of the lowest energy QY transition produces an excited state that appears to be localized mainly on the accessory chlorophyll in the active branch (BA in bacterial terms) and partially on the active pheophytin HA. This state equilibrates in 0.8 ps with the radical pair . BA is proposed to act as the primary electron donor also in untreated reaction centers. The 22-ps (pheophytin-exchanged) or 33-ps (untreated) component may be due to equilibration with the secondary radical pair . Its acceleration by HB exchange is attributed to a faster reverse electron transfer from BA to . After exchange both and are nearly isoenergetic with the excited state.