The photosynthetic reaction center is the protein responsible for the primary steps that generate most of the energy in the biosphere. The reaction center (RC) is a membrane spanning protein that contains bacteriochlorophylls (BChl), bacteriopheophytins (BPheo) and ubiquinones. The primary donor, P, consists of a pair of BChl molecules. The fact that P is a dimer is apparently not an accident of nature. The role of P is to trap excitation energy and to promote ultrafast electron trnasfer. As a dimer P apparently uses the coupling of som intradimer modes to the electronic transition. This can be seen in the resonance Raman spectrum, the time-resolved stimulated impulsive Raman and the very broad phonon sideband observed in holeburning experiments. These three aspects of the excited state are discussed below in the presentation on excited state dynamics. The first presentation introduces the overall structure and the electron transfer reactions that follow only one side of the protein. THe question of how the system manages to promote efficient forward electron transfer with a minimum of reverse electron transfer is addressed using electron tranfer theory.
Electron transfer theory
Electron transfer theory is derived using an application of time-dependent perturbation theory that is analogous to the absorption of light. The difference for the non-radiative process is that the hamiltonisn is not an interaction of a transition dipole with an electric field, but rather it is the Born-Oppenheimer breakdown operator. The B-O breakdown involves the same terms as we have seen in vibronic coupling. The derivative with respect to a nuclear coordinate causes the coupling between two states. The sample given in the presentation is the coupling of the donor/acceptor state DA and the charge separated state D+A-. However, this operator is only one possible representation (diabatic representation). We normally transform to the adiagbatic representation since that one is easier to work with. In the adiabatic representation the electronic coupling is simply the overlap of the reactant (DA) and product (D+A-) states. Actually, we discussing this for charge separation, but the same holds true for charge shift or charge recombination.
Marcus theory activation energy
This presentation gives a derivation of the activation energy used in Marcus theory. Marcus theory is useful because of its relative simpliciy. It reduces a complex problem down to two important parameters, the driving force, epsilon, and the reorganization energy, lamda.
Spectroscopic probes of charge separated intermediates
The reactino scheme shows us that charge is rapidly separated
