PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Temperature dependence of charge recombination in Photosystem I trimers embedded in trehalose glass. Rufat Agalarov^*,1, Ramakrishnan Balasubramanian¹, John Golbeck^{1, 2}, ¹ Department of Biochemistry and Molecular Biology, University Park, PA, USA² Department of Chemistry, Pennsylvania State University, University Park, PA, USA

ABSTRACT- The temperature dependence of charge recombination reactions in Photosystem I (PS I) was studied by transient absorption spectroscopy of PS I trimers embedded in trehalose glass. This technique makes it possible to separate the effects of temperature and solvent on electron transfer. It was found that the solvent plays a vital role in recombination reactions from the electron acceptors to the primary donor P700⁺. At room temperature, the monoexponential decay of P700⁺ in solution changes to multiexponential kinetics when PS I is embedded in trehalose glass. The method of maximum entropy (MEM) was applied to the kinetics to obtain a distribution of kinetic phases that appeared after embedding the PS I in the trehalose matrix. The appearance of new kinetic phases with rate constants k₁=1.85 s^-1, k₂=16 s^-1 and k₃=220 s^-1, and the acceleration of k₃ as the temperature was lowered to the value k₃=2160 s^-1 suggests a heterogeneous model for PS I. A study of background illumination on the suppression of the kinetic phases supports the model. These results indicate that electron transfer within PS I is influenced by protein motion and that the trehalose solid matrix results in its partial suppression. A comparison of charge recombination kinetics distribution in glycerol and trehalose shows that the trehalose effective suppresses protein motion at temperatures >200 K, while glycerol is most effective at temperatures <200 K. A significant change in the charge recombination from F_A/F_B to that of F_X and A₁ supposes that motion in the PsaC subunit modulates forward and backward electron transfer in PS I and might be a cause of heterogeneity.

KEY WORDS: electron transfer, Photosystem I, trehalose, heterogeneity