Terahertz Absorption change in Photosynthetic Reaction Center upon photoactivation

Photosynthetic reaction center is a membrane protein which plays an important role in converting solar energy into chemical energy. Upon continuous illumination a charge separation occurs within the protein which is stable for several seconds [1]. The stability of this state is very important to protect the protein against the release of excess heat and an instable radical due to charge recombination. Absorption of terahertz radiation corresponds to the excitation of large vibrations in the protein and is thus suitable for studying its different dynamical states. Here we recorded terahertz absorption spectra of the dark and illuminated state of reaction centre from the purple membrane bacteria Rhodobacter sphaeroides to investigate the stability of the charge separated state. The difference terahertz absorption spectra show an overall increase in absorption for the illuminated state in the 30-130 cm -1 spectral region. By simulating and fitting a theoretical terahertz spectrum of reaction center from an elastic network model we were able to investigate the effect of charge separation and conformational changes within the interaction network. We find that a more loose interaction network in the photoactivated state of the protein is the most probable cause of the absorption increase.