Recent advances in single-molecule spectroscopy studies on light-harvesting processes in oxygenic photosynthesis

Abstract

Photosynthetic light-harvesting complexes (LHCs) play a crucial role in concentrating the photon energy from the sun that otherwise excites a typical pigment molecule, such as chlorophyll- a , only several times a second. Densely packed pigments in the complexes ensure efficient energy transfer to the reaction center. At the same time, LHCs have the ability to switch to an energy-quenching state and thus play a photoprotective role under excessive light conditions. Photoprotection is especially important for oxygenic photosynthetic organisms because toxic reactive oxygen species can be generated through photochemistry under aerobic conditions. Because of the extreme complexity of the systems in which various types of pigment molecules strongly interact with each other and with the surrounding protein matrixes, there has been long-standing difficulty in understanding the molecular mechanisms underlying the flexible switching between the light-harvesting and quenching states. Single-molecule spectroscopy studies are suitable to reveal the conformational dynamics of LHCs reflected in the fluorescence properties that are obscured in ordinary ensemble measurements. Recent advanced single-molecule spectroscopy studies have revealed the dynamical fluctuations of LHCs in their fluorescence peak position, intensity, and lifetime. The observed dynamics seem relevant to the conformational plasticity required for the flexible activations of photoprotective energy quenching. In this review, we survey recent advances in the single-molecule spectroscopy study of the light-harvesting systems of oxygenic photosynthesis.

Access to the Antenna System of Photosystem I via Single-Molecule Excitation–Emission Spectroscopy

“…This suppression of the RC photoreaction enhances the fluorescence yield of PSI. Nevertheless, the protein dynamics coupled to the Chls modulate their site energies, resulting in the dynamic diversity of the fluorescence emission of PSI. − …”

“…Single-molecule spectroscopy (SMS) based on emission spectra is a powerful tool to study the effect of dynamic conformational fluctuation on the optical properties of photosynthetic proteins, which may be responsible for the ability to respond to changing environments. − , Rapid protein dynamics is considered to be responsible for the intermittent variation of fluorescence intensity, called “blinking”. The blinking effect has frequently been observed in the SMS measurement of PSI, even at low temperatures. , Previous SMS results showed that the excitation-energy transfer (EET) efficiency from the antenna system to different Red Chl pools is temporally fluctuating. ,, A recent SMS experiment on PSI explained the blinking based on a tentative model in which the amount of excitation energy flowing from the antenna Chls to either the RC or Red Chls pools is modulated by protein dynamics (see Supporting Information Text 1) .…”

“…Nevertheless, the protein dynamics coupled to the Chls modulate their site energies, resulting in the dynamic diversity of the fluorescence emission of PSI. 12−17 Single-molecule spectroscopy (SMS) based on emission spectra is a powerful tool to study the effect of dynamic conformational fluctuation on the optical properties of photosynthetic proteins, 15 which may be responsible for the ability to respond to changing environments. [6][7][8][9][10]18 Rapid protein dynamics is considered to be responsible for the intermittent variation of fluorescence intensity, called "blinking".…”

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes

“…Another method of overcoming inhomogeneous broadening, also briefly addressed below, is single pigment-protein complex spectroscopy (SPCS). This technique allows us (for very low-concentration samples) to probe photosynthetic complexes one by one [46][47][48]. In this case, data supposedly matching that of ensemble measurements (if needed) can be assembled by binning information for multiple individual complexes.…”

Access to the Antenna System of Photosystem I via Single-Molecule Excitation-Emission Spectroscopy

“…Nevertheless, the protein dynamics coupled to the Chls modulate their site energies, resulting in the dynamic diversity of the uorescence emission of PSI. (12)(13)(14)(15)(16) Single-molecule spectroscopy (SMS) based on emission spectra is a powerful tool to study the effect of dynamic conformational uctuation on the optical properties of photosynthetic proteins, (15) which may be responsible for the ability to respond to changing environments. (6-10, 17) Rapid protein dynamics is considered to be responsible for the intermittent variation of uorescence intensity called "blinking".…”