Delayed fluorescence from Rhodopseudomonas sphaeroides following single flashes

Area of Science:

• Photosynthesis Research
• Bioenergetics
• Bacteriochlorophyll Photochemistry

Background:

• Delayed fluorescence in Rhodopseudomonas sphaeroides chromatophores is a phenomenon linked to the back-reaction between oxidized reaction center bacteriochlorophyll (P+) and reduced electron acceptor (X-).
• Previous studies suggest a connection between delayed fluorescence decay and the P+X- state, but the precise mechanisms and energy storage involved require further elucidation.

Purpose of the Study:

• To investigate the rapid decay kinetics of delayed fluorescence in Rhodopseudomonas sphaeroides chromatophores following short excitation flashes.
• To correlate delayed fluorescence properties with changes in the electron acceptor state (X-) and explore the role of redox potential and flash repetition rate.
• To understand the energy storage mechanisms that enhance delayed fluorescence at higher flash rates.

Main Methods:

• Studied delayed fluorescence from Rhodopseudomonas sphaeroides chromatophores using short excitation flashes.
• Measured fluorescence decay kinetics and correlated them with optical absorbance changes at 450 and 770 nm.
• Varied flash repetition rates (2 s vs. 30 s) and redox potentials to observe effects on delayed fluorescence intensity and decay.

Main Results:

• Delayed fluorescence decay (τ1/2 ≈ 120 μs) was significantly faster than the decay of the P+X- state.
• Enhanced delayed fluorescence (11-18 fold) was observed at higher flash repetition rates (2 s) compared to lower rates (30 s), particularly at redox potentials < +240 mV.
• This enhancement correlated with optical absorbance changes and carotenoid spectral shifts, suggesting a link to the electron acceptor state and stored free energy.

Conclusions:

• The rapid decay of delayed fluorescence is not attributed to proton uptake or membrane potential changes but correlates with alterations in the electron acceptor state (X-).
• Reduction of P+X- to PX- stores free energy, which is utilized to promote delayed fluorescence on subsequent flashes at higher repetition rates.
• While membrane potential may play a role, it is not the sole determinant of enhanced delayed fluorescence, as indicated by partial inhibition with gramicidin D.