Global and target analysis of fluorescence measurements on photosystem 2 reaction centers upon red excitation

We measured the excited state dynamics of the photosystem 2 reaction center (RC) by fluorescence measurements with good temporal and spectral resolution and a high signal to noise. The fluorescence decay is highly multi-exponential. Upon excitation at 681 nm the data are well described with lifetimes of 6, 34, 160 ps and 7 ns. The 6 ps Decay Associated Spectrum (DAS) is red shifted with respect to the long lived DAS whereas the 34 ps DAS is blue shifted. The 34 ps component is dominant over the 6 ps component. More importantly, we show that the observed intermediate lifetimes (30-50 ps) cannot be ascribed to radical-pair relaxation only, since they correspond with significant spectral evolution of the fluorescence. From a target analysis with a minimal model, involving two radiative and three dark compartments, we conclude that these data are consistent with a kinetic scheme including fast charge separation (1 ps), moderately slow energy transfer within the RC, and radical pair relaxation