Dynamics of the Excited State in Photosynthetic Bacterial Reaction Centers

Abstract

In the initial charge-separation reaction of photosynthetic bacterial reaction centers, a dimer of strongly interacting bacteriochlorophylls (P) transfers an electron to a third bacteriochlorophyll (BL). It has been suggested that light first generates an exciton state of the dimer and that an electron then moves from one bacteriochlorophyll to the other within P to form a charge-transfer state (PL +PM –), which passes an electron to BL. This scheme, however, is at odds with the most economical analysis of the spectroscopic properties of the reaction center and particularly with the unusual temperature dependence of the long-wavelength absorption band. The present paper explores this conflict with the aid of a simple model in which exciton and charge-transfer states are coupled to three vibrational modes. It then uses a similar model to show that the main experimental evidence suggesting the formation of PL +PM – as an intermediate could reflect pure dephasing of vibrational modes that modulate stimulated emission.

Dynamic Stark Effect in Two-Dimensional Spectroscopy Revealing Modulation of Ultrafast Charge Separation in Bacterial Reaction Centers by an Inherent Electric Field

“…As can be seen in Figure d, the conversion point of the Marcus curve moved in the direction of lower temperature as a result of the positive Stark shift. This behavior could explain the long-lasting question over why primary ET at 10 K is twice as fast as that at RT, a severe deviation from the Marcus theory. , This scheme was consistent with the view of Parson that the P A + P B – state should be above the lowest exciton transition in the wild-type RC (which M1 represents).…”

“…Despite very extensive investigation of both wild-type and mutated purple bacterial RCs, − many aspects of the fundamental mechanisms of primary charge separation are still unclear. One important problem is deviation of the experimentally determined rate of the primary P* → P + B A – step at cryogenic temperature from that predicted by activation energy theory. ,, Proposed explanations include vibrational mode coupling that helps to overcome the activation barrier or a reaction-diffusion model in which primary ET is determined by protein dynamics rather than by barrier crossing. However, neither of these explanations takes into account the real starting event of primary ET, namely the photoinduced charge separation within the P BChl dimer to create a CT state.…”