title: Unified Model of Nanosecond Charge Recombination in Closed Reaction Centers from Rhodobacter sphaeroides: Role of Protein Polarization Dynamics.
abstract: 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.
authors.affiliation: Department of Physics, Adam Mickiewicz University , ul. Umultowska 85, 61-614 Poznań, Poland.
authors.forename: K
authors.initials: K
authors.lastname: Dubas
authors.affiliation: Department of Experimental Physics, Wroclaw University of Technology , Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland.
authors.forename: M
authors.initials: M
authors.lastname: Baranowski
authors.affiliation: Department of Experimental Physics, Wroclaw University of Technology , Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland.
authors.forename: A
authors.initials: A
authors.lastname: Podhorodecki
authors.affiliation: School of Biochemistry, Medical Sciences Building, University of Bristol , University Walk, Bristol, BS8 1TD, U.K.
authors.forename: M R
authors.initials: MR
authors.lastname: Jones
authors.affiliation: Department of Physics, Adam Mickiewicz University , ul. Umultowska 85, 61-614 Poznań, Poland.
authors.forename: K
authors.initials: K
authors.lastname: Gibasiewicz
mesh_terms: D004579:Electron Transport; D010674:Pheophytins / Q000737:chemistry / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; C006678:bacteriopheophytin