Electron and energy transfer in proteins are key processes in bioenergetics. Their understanding on a molecular
level can serve as an important guideline for the design of nanoscale assemblies. Energy transfer between pigment
molecules requires a match between their transition energies for energy emission and absorption. The tuning
of these pigment energies in proteins is achieved by pigment-protein interactions. In general, these interactions
are regarded as static properties determined by the three-dimensional structure of pigment-protein complexes.
Employing single-molecule fluorescence spectroscopy we demonstrate that protein dynamics, even at cryogenic
temperatures, significantly influences the transition energy of pigments and, as a consequence, modulates energy
transfer pathways. This variability of excitation energy transfer pathways introduced by protein dynamics might
be important for the extreme robustness of photosystems.