Femtosecond Relaxation Processes in Rhodobacter sphaeroides Reaction Centers

Area of Science:

• Photosynthetic energy transfer
• Bacteriochlorophyll spectroscopy
• Electron-vibrational dynamics

Background:

• Understanding energy relaxation in photosynthetic reaction centers is crucial for deciphering light-harvesting mechanisms.
• The YM210L mutant of Rhodobacter sphaeroides provides a model system to study these processes.
• Low-temperature studies offer insights into fundamental photophysical events.

Purpose of the Study:

• To investigate energy relaxation dynamics in the YM210L mutant of Rhodobacter sphaeroides at 90 K.
• To characterize the spectral dynamics of the excited bacteriochlorophyll dimer P*.
• To elucidate the mechanisms underlying the observed spectral shifts.

Main Methods:

• Difference femtosecond spectroscopy was employed to probe ultrafast dynamics.
• Stimulated emission was analyzed to observe spectral shifts and shape changes.
• Kinetic measurements at fixed wavelengths were used to assess wavelength-dependent phenomena.

Main Results:

• A dynamical long-wavelength shift of stimulated emission from the excited state of bacteriochlorophyll dimer P* was observed.
• This shift occurs simultaneously with P* formation, with a characteristic magnitude of ~30 nm and a time constant of ~200 fs.
• The spectral shift and its kinetics are consistent with electron-vibrational relaxation of the P* excited state.

Conclusions:

• The observed dynamical spectral shift is primarily attributed to electron-vibrational relaxation of the excited bacteriochlorophyll dimer P*.
• Vibrational and electronic relaxation time constants were determined to be approximately 100 fs and 50 fs, respectively.
• Alternative mechanisms, including energy redistribution or coherent mode excitation, were considered but electron-vibrational relaxation provides the best explanation.