Direct observation of solvation dynamics and dielectric relaxation in the photosynthetic light-harvesting-2 complex of Rhodopseudomonas acidophila

We studied the fluorescence dynamics of the light-harvesting-2 complex of Rhodopseudomonas acidophila both with and without an externally applied electric field. The fluorescence emission spectrum of the complex shows (small) spectral shifts of the average emission wavelength on a time scale of similar to300 ps. The shifts are temperature dependent and result from a combination of the (a) dielectric relaxation, or solvation, of the excited states by the protein, which causes a red shift of the emission, and (b) nonradiative loss processes occurring in a subset of the ensemble of complexes. These two processes compete with delocalization and energy transfer around the ring and are shown to be sensitive to an external electric field. The experiments illustrate how nature has prevented the above-mentioned loss processes in vivo by having fast energy transfer around a symmetric structure. Local protein relaxation is slow for the delocalized (or fast-hopping) excited states. However, once localized, localization and (dielectric) relaxation act in a cooperative way. Therefore, at low temperatures (<100 K), the nonradiative loss processes are more prominent. In a more general way, the experiments demonstrate how, by looking at the effects of an external electric field on the fluorescence dynamics, one can identify the nature of the relaxation processes; that is, the electric field is a probe for solvation and relaxation processes. These first experiments demonstrate the promising potential of the approach presented here for studies on ultrafast solvation processes.