Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 2. Heterodimer Special Pair

Ultrafast singlet excited state energy transfer occurs from the monomeric chromophores to the primary electron donor or special pair in photosynthetic reaction centers. The mechanism of this process has been investigated using reaction centers whose special pair absorption is severely perturbed by removal of the Mg atom of one of the bacteriochlorophylls (the heterodimer mutant (M)H202L). This was achieved by observing the rise time kinetics and induced anisotropy in the spontaneous fluorescence from the heterodimer special pair following excitation of the monomeric chromophores using fluorescence up-conversion at 85 K. The rise is characterized by two time constants. By scanning the excitation wavelength across the absorption bands of the monomeric bacteriochlorophyll and bacteriopheophytin, it is demonstrated that the two components correspond to energy transfer along the L (functional) and M (nonfunctional) branches of chromophores. This contrasts with the more symmetric native special pair, where energy transfer along the two branches has been shown to have comparable rates (Stanley et al. J. Phys. Chem. 1996, 100, 12052-12059). Thus, the electronic asymmetry of the heterodimer special pair alters the interactions on the functional and nonfunctional sides of the reaction center that are responsible for singlet energy transfer.