Glass versus protein dynamics at low temperature studied by time-resolved spectral hole burning

Low-temperature dynamics of doped organic glasses and photosynthetic pigment-protein complexes has been studied by time-resolved hole burning. The systems investigated are: (1) bacteriochlorophyll-a (BChl-a) in a glass consisting of detergent (OG), buffer and glycerol, and BChl-a in 2-methyltetrahydrofuran (MTHF); (2) the B820-dimer and B777-monomer subunits of light-harvesting complex 1 (LH1) of purple bacteria Rs.rubrum, and the isolated photosystem II reaction center (PSII RC) of green plants. The "effective" homogeneous linevvidth Γ′_hom has been determined as a function of temperature T (1.2-4.2 K) and delay time td (10^-6-10⁵ s). In contrast to the T-dependence of Γ′_hom, which follows a T^1.3-power law and is the same for both organic glasses and photosynthetic proteins, the t_d-dependence of Γ′_hom is different. Glasses exhibit spectral diffusion over a time span of at least 10-15 decades leading to a 1/R distribution of relaxation rates of the two-level systems (TLSs). Proteins appear to be crystalline-like, i.e. rigid, for short times (1 μs ≤ t_d ≤ 1 s) and glassy-like for long times (t_d > 100 ms-1 s). Only slow motions with rates ≤ 1 - 100 Hz, probably involving global motions of the protein, seem to be present in the photosynthetic complexes studied.