P+HA- charge recombination reaction rate constant in Rhodobacter sphaeroides reaction centers is independent of the P/P+ midpoint potential.
The kinetics of the P+HA- (oxidized donor, reduced bacteriopheophytin acceptor) recombination reaction was measured in a series of reaction center mutants of Rhodobacter sphaeroides with altered P/P+ midpoint potentials between 410 and 765 mV. The time constant for P+HA- recombination was found to range between 14 and 26 ns and was essentially independent of P/P+ midpoint potential. Previous work has shown that the time constant for initial electron transfer in these mutants at room temperature is also only weakly dependent on the P/P+ midpoint potential, ranging from about 2.5 ps to about 50 ps. These results, taken together, imply that heterogeneity in the P/P+ midpoint potential within the reaction center population is not likely the dominant cause of the substantial kinetic complexity observed in the decay of the excited singlet state of P on the picosecond to nanosecond time scale. In addition, the pathway of P+HA- decay appears to be direct or via P+BA- rather than proceeding back through P, even in the highest-potential mutant, as is evident from the fact that the rate of P+HA- recombination is unaltered by pushing P+HA- much closer to P in energy. Finally, the midpoint potential independence of the P+HA- recombination rate constant suggests that the slow rate of P+HA- recombination arises from an inherent limitation in the maximum rate of this process rather than because it occurs in the inverted region of a classical Marcus rate vs free energy curve.
Tang
C K
CK
Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe 85287-1604, USA.
Williams
J C
JC
Taguchi
A K
AK
Allen
J P
JP
Woodbury
N W
NW
D004579:Electron Transport; D007700:Kinetics; D010084:Oxidation-Reduction; D010674:Pheophytins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D011809:Quinones / Q000737:chemistry; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013696:Temperature; D013816:Thermodynamics
D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; C006678:bacteriopheophytin