Fluctuations in the interactions between pigments due to transitions in the TLS is the main dephasing pathway in glasses below 10 K. The TLS transitions can both influence the dipole interactions between the pigments (low frequency transitions in TLS corresponding to large buy Entinostat displacements in the protein) as well as the site energies (high frequency, smaller displacement). At low temperatures, the coherent energy transfer is mainly limited by this coupling. Above 10 K, the contribution of the TLS tunneling is of minor significance to the dephasing mechanism that are dominated by other processes. With

these measurements, the earlier results from a preliminary study by Louwe and Aartsma (1994) were confirmed. Table 13 Frequency-dependent accumulated photon echo decay times of Prosthecochloris aestuarii at 1.4 K (Louwe and Aartsma 1997) λmax of DASa (nm) Decay time (ps) 827 385 826 110 824 30 818 5 aDAS spectra originate from a global analysis were the amplitudes of the different

decay components are plotted against the GSK1904529A chemical structure wavelength resulting in distinct bands Several years later, interesting features were seen in low-temperature two-photon-echo (2PE) signals of both Chlorobium tepidum and Prosthecochloris aestuarii (Prokhorenko et al. 2002). At 1.27 K, the 2PE signals show oscillations that increase in intensity when the excitation is tuned to the red edge of the absorption spectrum (up to 40% of the total amplitude for excitation at 832 nm). These oscillations last up to 300 ps and are ascribed to vibrational states of the BChl a molecule in the ground state. Fourier transforms of the 2PE traces show that the obtained frequencies match those from previous studies (Savikhin et al. 1997). In the same study, it was shown that the general theory to describe the results of photon-echo experiments did not account for the current results. The typical δ shape for dynamics in the Markov limit at initial time delays was not observed. Therefore, the dynamics

were described beyond the Markov limit where system–bath memory effects occur which, among others, result in the delayed growing in of coherence in the system. At that time, it was unclear whether this had a specific function in light harvesting. Vulto et al. used a similar approach PLEK2 as was used previously by Louwe et al. in the simulation of the static spectra (see “Exciton nature of the BChl a excitations in the FMO protein” and “Coupling strengths, linewidth and exciton energies”); however, to introduce dynamics, coupling of the electronic excitations to the vibrational modes in the system was included (Vulto et al. 1999). Homogeneous broadening within the system was not see more incorporated in the model. Owing to the weak coupling, the exciton-vibrational coupling can be treated as a perturbative term in the Hamiltonian.