Spectral diffusion induced by proton dynamics in pigment-protein complexes.

Area of Science:

• Biophysics
• Photosynthesis research
• Spectroscopy

Background:

• Photosystem I (PSI) is crucial for light energy conversion in photosynthesis.
• Understanding the photophysics of PSI is key to optimizing light-harvesting complexes.
• Fluorescence emission provides insights into the electronic and structural dynamics of PSI.

Purpose of the Study:

• To investigate the impact of solvent deuteration on the fluorescence emission of individual photosystem I complexes.
• To elucidate the role of exchangeable protons in spectral diffusion within the PSI chromophore binding pocket.

Main Methods:

• Single-molecule fluorescence spectroscopy on isolated photosystem I complexes from Synechocystis PCC 6803.
• Comparison of fluorescence emission in protonated (H2O) and deuterated (D2O) buffer solutions.
• Analysis of zero-phonon lines and broad intensity distributions in fluorescence spectra.

Main Results:

• Solvent deuteration significantly reduced spectral diffusion rates in PSI fluorescence.
• An increase in the number and resolution of zero-phonon lines was observed upon deuteration.
• Changes in spectral diffusion were linked to exchangeable protons from amino acids and structural water, not chlorophyll a.

Conclusions:

• Exchangeable protons in the PSI chromophore binding pocket play a critical role in spectral diffusion.
• Deuteration of these protons effectively suppresses spectral diffusion, leading to sharper fluorescence features.
• This study highlights the importance of the local environment and proton dynamics in PSI photophysics.