Unified Model of Nanosecond Charge Recombination in Closed Reaction Centers from Rhodobacter sphaeroides: Role of Protein Polarization Dynamics

Ongoing questions surround the influence of protein dynamics on rapid processes such as biological electron transfer. Such questions are particularly addressable in light-activated systems. In Rhodobacter sphaeroides reaction centers, charge recombination or back electron transfer from the reduced bacteriopheophytin, HA–, to the oxidized dimeric bacteriochlorophyll, P+, may be monitored by both transient absorption spectroscopy and transient fluorescence spectroscopy. Signals measured with both these techniques decay in a similar three-exponential fashion with lifetimes of ∼0.6–0.7, ∼2–4, and ∼10–20 ns, revealing the complex character of this electron transfer reaction. In this study a single kinetic model was developed to connect lifetime and amplitude data from both techniques. The model took into account the possibility that electron transfer from HA– to P+ may occur with transient formation of the state P+BA–. As a result it was possible to model the impact of nanosecond protein relaxation on the free energy levels of both P+HA– and P+BA– states relative to that of the singlet excited state of P, P*. Surprisingly, whereas the free energy gap between P* and P+HA– increased with time in response to protein reorganization, the free energy gap between P* and P+BA– decreased. This finding may be accounted for by a gradual polarization of the protein environment which stabilizes the state P+HA– and destabilizes the state P+BA–, favoring productive charge separation over unproductive charge recombination.