Trapping, loss and annihilation of excitations in a photosynthetic system: II. Experiments with the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulata

In this paper a number of experiments with the purple bacteria Rhodospirillum rubrum and Rhodopseudomonas capsulata is described in which the total fluorescence yield and/or the total fraction of reaction centers closed after a picosecond laser pulse were measured as a function of the pulse intensity. The conditions were such that the reaction centers were either all in the open or all in the closed state before the pulse arrived. These experiments are analysed using the theoretical formalism discussed in the preceding paper (Den Hollander, W.T.F., Bakker J.G.C., and Van Grondelle, R., Biochim. Biophys. Acta 725, 492–507). From the experimental results the number of connected photosynthetic units, λ, the rate of energy transfer between neighboring antenna molecules, k_h, and the rate of trapping by an open reaction center, k^o_t, can be estimated. For R. rubrum it is found that λ = 14?17, k_h = (1?2)·10¹² s^?1 and k^o_t = (4?6)·10¹¹ s^?1, for Rps. capsulata λ ≈ 30, k_h ≈ 4·10¹¹ s^?1 and k^o_t ≈ 3·10¹¹ s^?1. The findings are discussed in terms of current models for the structure of the antenna and the kinetic properties of the decay processes occurring in these purple bacteria.