Slow Charge Separation in a Minority of Reaction Centers Correlated with a Blueshift of the P₈₆₀-Band in Rb. Sphaeroides

Abstract

The fluorescence decay pattern of the primary donor state ¹P* in Rb. sphaeroides R26 as a function of quinone (Q_A) content and excitation (or detection) wavelength has been investigated with a 40ps instrumental response function. The open question, whether the fluorescence components between 40ps and 1ns originate from slow charge separation or from recombination of energetically unrelaxed P⁺H ^-_A -states (H_A denoting bacteriopheophytin) is addressed by comparing Q_A-containing reaction centers (RCs) with Q_A-depleted ones. The removal of Q_A increases the recombination fluorescence lifetime of P⁺H ^-_A from about 100ps to tens of nanoseconds. The key observation reported in this paper is that the fluorescence decay components in the range of 100ps to 1ns are not affected by the P⁺H ^-_A lifetime. Thus, these components reflect slow charge separation of a minority (<4% of the RCs). Moreover, this minority manifests itself in a blue-shifted Q_y-absorption band of the primary donor (P₈₆₀-band) detected in the time resolved fluorescence excitation spectra of the 100ps-lns components, and also affects the steady state fluorescence excitation spectrum. The fluorescence components faster than 40ps representing the sample majority as well as the delayed recombination fluorescence (>10ns) exhibit no shift. So far, all RC preparations investigated show these features of a blue shifted P₈₆₀-band and charge separation in the l00ps time-scale, indicating that the RCs of Rb. sphaeroides are intrinsically heterogeneous.

The fluorescence decay pattern of the primary donor state ¹P* in Rb. sphaeroides R26 as a function of quinone (Q_A) content and excitation (or detection) wavelength has been investigated with a 40ps instrumental response function. The open question, whether the fluorescence components between 40ps and 1ns originate from slow charge separation or from recombination of energetically unrelaxed P⁺H ^-_A -states (H_A denoting bacteriopheophytin) is addressed by comparing Q_A-containing reaction centers (RCs) with Q_A-depleted ones. The removal of Q_A increases the recombination fluorescence lifetime of P⁺H ^-_A from about 100ps to tens of nanoseconds. The key observation reported in this paper is that the fluorescence decay components in the range of 100ps to 1ns are not affected by the P⁺H ^-_A lifetime. Thus, these components reflect slow charge separation of a minority (<4% of the RCs). Moreover, this minority manifests itself in a blue-shifted Q_y-absorption band of the primary donor (P₈₆₀-band) detected in the time resolved fluorescence excitation spectra of the 100ps-lns components, and also affects the steady state fluorescence excitation spectrum. The fluorescence components faster than 40ps representing the sample majority as well as the delayed recombination fluorescence (>10ns) exhibit no shift. So far, all RC preparations investigated show these features of a blue shifted P₈₆₀-band and charge separation in the l00ps time-scale, indicating that the RCs of Rb. sphaeroides are intrinsically heterogeneous.