Protein dynamics in photosystem II complexes of green plants studied by time-resolved hole-burning

We have studied the protein dynamics of three subcore reaction-center complexes of photosystem II: the isolated reaction center (RC), the core antenna CP47, and the CP47 RC complex by means of time-resolved hole-burning in the red wing of their Q_y-absorption bands. The dependence of the "effective" homogeneous line width Γ′_hom on temperature T between 1.2 and 4.2 K suggests that optical dephasing in these proteins is determined by two-level systems (TLSs), as in doped organic glasses. By contrast, the increase of Γ′_hom as a function of delay time t_d (between burning and probing the hole) from 10^-5 to 10⁵ s, caused by spectral diffusion (SD), differs from that in glasses and is characteristic for each complex. CP47 RC does not undergo any SD over 10 decades in time for T ≤ 4.2 K, i.e., Γ′_hom = constant at a given temperature. Although CP47 and the RC do not show SD for t_d ≤ 1 s, they do for longer delay times, with Γ′_hom increasing logarithmically with t_d. The onset and amount of SD appear to be correlated with the mass of the protein. We conclude that only slow motions, related to TLSs located at the more flexible surface of the protein (with a broad and continuous distribution of rates R < 1-3 Hz), contribute to SD at long delay times and that the whole protein, or a substantial part of it, is involved in SD. Fast, local fluctuations associated with a rigid, crystalline-like inner protein core are responsible for "pure" dephasing at short t_d.