Effect of quinone depletion on lifetime spectra in photosynthetic reaction centers

Abstract

It is shown that a well defined inhomogeneity consisting in a partial loss of the primary quinone (Q) may give rise to a spectral dependence of the lifetime of the radical pair state P + H −, P denoting the primary donor and H bacteriopheophytin. The lifetime spectrum follows from a complex decay pattern of H − due to the forward electron transfer on the 100 ps time scale and the recombination dynamics of the radical P + H − which is by two orders of magnitude slower. Since the underlying biexponential decays are detectable only in measurements with high sensitivity including sufficiently long delay times, the complex kinetics might be easily hidden in a single-exponential fitting function. Nevertheless, a simulation for 20% Q-depletion demonstrates, that at room temperature the monoexponentially fitted rate for P + H − → P + Q − may vary by a factor of ≈ 2 when probed at wavelengths between 786 and 812 nm. Thus, a sensitive determination of the occupancy of the primary quinone site is necessary when lifetime spectra in the 100 ps range are observed.