In search of a putative long-lived relaxed radical pair state in closed photosystem II: Kinetic modeling of picosecond fluorescence data.

The concept of a relaxed radical pair state in closed photosystem (PS) II centers (first quinone acceptor reduced) is critically examined on the basis of chlorophyll fluorescence decay data of the green alga Scenedesmus obliquus. Global analysis resulting in the decay-associated fluorescence spectra from closed PS II centers reveals a new PS II lifetime component (tau approximately 380 ps) in addition to two PS II components (tau approximately 1.3 and 2.1 ns) resolved earlier. Particular emphasis was given to resolve a potential long-lived ( approximately 10 ns) component of small amplitude; however, the longest lifetime found is only 2.1 ns. From comparison of experimental and simulated data we conclude that the maximum relative amplitude of such a potential long-lived component must be <0.1%. The PS II kinetics are analyzed in terms of a three-state model involving an antenna/reaction center excited state, a primary radical pair state, and a relaxed radical pair state. The rate constants for charge separation and presumed radical pair relaxation as well as those for the reverse processes are calculated. Critical examination of these results leads us to exclude the formation with high yield (> 15%) of a long-lived (tau >/= 3 ns) relaxed radical pair in closed PS II. If at all distinguishable kinetically and energetically from the primary radical pair, a relaxed radical pair would not live longer than 2-3 ns in green algae. The data suggest, however, that the concept of a long-lived relaxed radical pair state is inappropriate for intact PS II.