GLASS VERSUS PROTEIN DYNAMICS AT LOW-TEMPERATURE STUDIED BY TIME-RESOLVED SPECTRAL HOLE-BURNING

Low-temperature dynamics of doped organic glasses and photosynthetic p igment-protein complexes has been studied by time-resolved hole burnin g. 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-monome r subunits of light-harvesting complex 1 (LH1) of purple bacteria Rs.r ubrum, and the isolated photosystem II reaction center (PSII RC) of gr een plants. The ''effective'' homogeneous linewidth Gamma'(hom) has be en determined as a function of temperature T (1.2-4.2 K) and delay tim e t(d) (10(-6)-10(5) s). In contrast to the T-dependence of Gamma'(hom ), which follows a T-1.3-power law and is the same for both organic gl asses and photosynthetic proteins, the t(d)-dependence of Gamma'(hom) is different. Glasses exhibit spectral diffusion over a time span of a t least 10-15 decades leading to a 1/R distribution of relaxation rate s of the two-level systems (TLSs). Proteins appear to be crystalline-l ike, i.e. rigid, for short times (1 mu s less than or equal to t(d) le ss than or equal to 1 s) and glassy-like for long times (t(d) > 100 ms (-1) s). Only slow motions with rates less than or equal to 1-100 Hz, probably involving global motions of the protein, seem to be present i n the photosynthetic complexes studied. (C) 1998 Elsevier Science B.V. All rights reserved.