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 compl exes 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 th e red wing of their Q(y)-absorption bands. The dependence of the "effective " homogeneous line width Gamma'(hom) on temperature T between 1.2 and 4.2 K suggests that optical dephasing in these proteins is determined by two-lev el systems (TLSs), as in doped organic glasses. By contrast, the increase o f Gamma'(hom) as a function of delay time t(d) (between burning and probing the hole) from 10(-5) to 10(5) s, caused by spectral diffusion (SD), diffe rs from that in glasses and is characteristic for each complex. CP47 RC doe s not undergo any SD over 10 decades in time for T less than or equal to 4. 2 K, i.e., Gamma'(hom) = constant at a given temperature. Although CP47 and the RC do not show SD for t(d) less than or equal to 1 s, they do for long er delay times, with Gamma'(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 f lexible 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 fluct uations associated with a rigid, crystalline-like inner protein core are re sponsible for "pure" dephasing at short t(d).