e., dephasing) at T ≤ 4.2 K. Thus, motions involving the entire complex (or a part of it) take place in these protein systems, even at liquid-helium temperature. It is further ABT-737 in vitro striking that the slopes in Fig. 7 seem to be correlated with the mass or size of the protein, and not with the number of pigments in these proteins (1 in B777, 8 in RC, 16 in CP47 and ~24 in CP47–RC). The results of Fig. 7 indicate that at low temperature and short delay times (t d < ms), there is no SD, but only ‘pure’ dephasing, i.e. local, fast fluctuations remain. At longer times, very slow

motions (with cut-off frequencies of 1–100 Hz) take place, probably at the protein–glass interface (Creemers and Völker 2000; Den Hartog et al. 1999b). If we assume that the amount of SD is proportional Wortmannin ic50 to the pigment–protein interaction (\( \propto \left( r^n \right)^ – 1 \) for multipolar types with n ≥ 3) and to the number of TLSs present at the surface of the protein \( \left( \propto r^2 \right), \) then SD \( \approx \textd\Upgamma_\hom ^’ /\textdt_\textd \propto \left( r^n – 2 \right)^ – 1 \propto r^ – 1 \) (for n = 3; Den Hartog et al. 1999b). SD should thus increase with decreasing r, i.e. with decreasing size of the protein (or with its mass, for constant

density). In conclusion, the heavier the protein, the smaller the amount of SD. The nature of the protein motions involved, however, is still unknown and, as mentioned above, it is a matter of controversy whether TLSs check details are a useful concept for explaining the dynamics Celecoxib of proteins at low temperatures. (For recent reviews, see Berlin et al. (2006, 2007), where an anomalous power law in waiting time was observed for heme proteins at low temperature.) More time-resolved HB experiments on larger complexes, combined with different solvents, and at higher temperatures may shed some light on these unsolved issues. Hidden spectral bands made visible: hole depth as a function of wavelength

The advantages of HB, as compared to ultrafast time-resolved techniques, are the high spectral resolution (of a few MHz) and the wavelength and burning-fluence selectivity. These properties make HB an attractive tool for disentangling spectral bands ‘hidden’ in strongly heterogeneously broadened and overlapping absorption bands. The disentanglement can be achieved by measuring the hole depth, in addition to the hole width, as a function of excitation wavelength, at constant (and low) burning-fluence density (Pt/A) and at liquid-helium temperature. Such ‘action’ spectra were first reported by the group of G. Small for LH1 and LH2 (Reddy et al. 1992, 1993; Wu et al. 1997a, b, c) and, subsequently, by A. Freiberg and co-workers for the same systems (Freiberg et al. 2003, 2009 and references therein; Timpmann et al.